From 211a216706df4f4188b32fd40a89ed7af1f92d52 Mon Sep 17 00:00:00 2001
From: Jeff Davis <jeff@j-davis.com>
Date: Mon, 1 May 2023 15:38:29 -0700
Subject: [PATCH v20 4/6] Introduce collation provider "builtin".
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Three locales are offered by the builtin provider: C, C.UTF-8, and
PG_UNICODE_FAST.

The builtin "C" locale is equal in semantics and implementation to
the libc "C" locale (neither of which actually use libc).

The builtin "C.UTF-8" locale offers similar semantics to the libc
"C.UTF-8" locale, which is collation according to code point combined
with simple Unicode character semantics. Unlike the libc "C.UTF-8"
locale, the builtin "C.UTF-8" is available on all platforms with
consistent behavior, and benefits from additional optimizations.

The builtin "PG_UNICODE_FAST" locale offers collation according to
code point order and more complete Unicode character semantics. As the
SQL standard requires, it offers full case mappings that may increase
the length of a string, such as "ß" changing to "SS" when uppercased.

Discussion: https://postgr.es/m/ab925f69-5f9d-f85e-b87c-bd2a44798659@joeconway.com
Discussion: https://postgr.es/m/dd9261f4-7a98-4565-93ec-336c1c110d90@manitou-mail.org
---
 doc/src/sgml/charset.sgml                    |  88 ++++--
 doc/src/sgml/ref/create_collation.sgml       |  11 +-
 doc/src/sgml/ref/create_database.sgml        |   8 +-
 doc/src/sgml/ref/createdb.sgml               |   2 +-
 doc/src/sgml/ref/initdb.sgml                 |  17 +-
 src/backend/catalog/pg_collation.c           |   5 +-
 src/backend/commands/collationcmds.c         |  93 ++++--
 src/backend/commands/dbcommands.c            | 121 ++++++--
 src/backend/regex/regc_pg_locale.c           |  41 ++-
 src/backend/utils/adt/formatting.c           | 149 ++++++++++
 src/backend/utils/adt/pg_locale.c            | 167 +++++++++--
 src/backend/utils/init/postinit.c            |  51 +++-
 src/bin/initdb/initdb.c                      |  58 ++--
 src/bin/initdb/t/001_initdb.pl               |  57 +++-
 src/bin/pg_dump/pg_dump.c                    |  49 ++--
 src/bin/pg_upgrade/t/002_pg_upgrade.pl       |  70 +++--
 src/bin/psql/describe.c                      |   4 +-
 src/bin/scripts/createdb.c                   |  18 +-
 src/bin/scripts/t/020_createdb.pl            |  78 +++++
 src/include/catalog/pg_collation.dat         |   6 +-
 src/include/catalog/pg_collation.h           |   3 +
 src/include/utils/pg_locale.h                |  10 +-
 src/test/icu/t/010_database.pl               |  22 +-
 src/test/regress/expected/collate.out        |  24 +-
 src/test/regress/expected/collate.utf8.out   | 288 +++++++++++++++++++
 src/test/regress/expected/collate.utf8_1.out |   8 +
 src/test/regress/parallel_schedule           |   4 +-
 src/test/regress/sql/collate.sql             |  10 +
 src/test/regress/sql/collate.utf8.sql        | 123 ++++++++
 29 files changed, 1422 insertions(+), 163 deletions(-)
 create mode 100644 src/test/regress/expected/collate.utf8.out
 create mode 100644 src/test/regress/expected/collate.utf8_1.out
 create mode 100644 src/test/regress/sql/collate.utf8.sql

diff --git a/doc/src/sgml/charset.sgml b/doc/src/sgml/charset.sgml
index 4fc143025e..7fe4a9bc39 100644
--- a/doc/src/sgml/charset.sgml
+++ b/doc/src/sgml/charset.sgml
@@ -342,22 +342,14 @@ initdb --locale=sv_SE
    <title>Locale Providers</title>
 
    <para>
-    <productname>PostgreSQL</productname> supports multiple <firstterm>locale
-    providers</firstterm>.  This specifies which library supplies the locale
-    data.  One standard provider name is <literal>libc</literal>, which uses
-    the locales provided by the operating system C library.  These are the
-    locales used by most tools provided by the operating system.  Another
-    provider is <literal>icu</literal>, which uses the external
-    ICU<indexterm><primary>ICU</primary></indexterm> library.  ICU locales can
-    only be used if support for ICU was configured when PostgreSQL was built.
+    A locale provider specifies which library defines the locale behavior for
+    collations and character classifications.
    </para>
 
    <para>
     The commands and tools that select the locale settings, as described
-    above, each have an option to select the locale provider.  The examples
-    shown earlier all use the <literal>libc</literal> provider, which is the
-    default.  Here is an example to initialize a database cluster using the
-    ICU provider:
+    above, each have an option to select the locale provider. Here is an
+    example to initialize a database cluster using the ICU provider:
 <programlisting>
 initdb --locale-provider=icu --icu-locale=en
 </programlisting>
@@ -370,12 +362,74 @@ initdb --locale-provider=icu --icu-locale=en
    </para>
 
    <para>
-    Which locale provider to use depends on individual requirements.  For most
-    basic uses, either provider will give adequate results.  For the libc
-    provider, it depends on what the operating system offers; some operating
-    systems are better than others.  For advanced uses, ICU offers more locale
-    variants and customization options.
+    Regardless of the locale provider, the operating system is still used to
+    provide some locale-aware behavior, such as messages (see <xref
+    linkend="guc-lc-messages"/>).
    </para>
+
+   <para>
+    The available locale providers are listed below.
+   </para>
+
+   <sect3 id="locale-provider-builtin">
+    <title>Builtin</title>
+    <para>
+     The <literal>builtin</literal> provider uses built-in operations. Only
+     the <literal>C</literal> and <literal>C.UTF-8</literal> locales are
+     supported for this provider.
+    </para>
+    <para>
+     The <literal>C</literal> locale behavior is identical to the
+     <literal>C</literal> locale in the libc provider. When using this locale,
+     the behavior may depend on the database encoding.
+    </para>
+    <para>
+     The <literal>C.UTF-8</literal> locale is available only for when the
+     database encoding is <literal>UTF-8</literal>, and the behavior is based
+     on Unicode. The collation uses the code point values only. The regular
+     expression character classes are based on the "POSIX Compatible"
+     semantics, and the case mapping is the "simple" variant.
+    </para>
+   </sect3>
+   <sect3 id="locale-provider-icu">
+    <title>ICU</title>
+    <para>
+     The <literal>icu</literal> provider uses the external
+     ICU<indexterm><primary>ICU</primary></indexterm>
+     library. <productname>PostgreSQL</productname> must have been configured
+     with support.
+    </para>
+    <para>
+     ICU provides collation and character classification behavior that is
+     independent of the operating system and database encoding, which is
+     preferable if you expect to transition to other platforms without any
+     change in results. <literal>LC_COLLATE</literal> and
+     <literal>LC_CTYPE</literal> can be set independently of the ICU locale.
+    </para>
+    <note>
+     <para>
+      For the ICU provider, results may depend on the version of the ICU
+      library used, as it is updated to reflect changes in natural language
+      over time.
+     </para>
+    </note>
+   </sect3>
+   <sect3 id="locale-provider-libc">
+    <title>libc</title>
+    <para>
+     The <literal>libc</literal> provider uses the operating system's C
+     library. The collation and character classification behavior is
+     controlled by the settings <literal>LC_COLLATE</literal> and
+     <literal>LC_CTYPE</literal>, so they cannot be set independently.
+    </para>
+    <note>
+     <para>
+      The same locale name may have different behavior on different platforms
+      when using the libc provider.
+     </para>
+    </note>
+   </sect3>
+
   </sect2>
 
   <sect2 id="icu-locales">
diff --git a/doc/src/sgml/ref/create_collation.sgml b/doc/src/sgml/ref/create_collation.sgml
index 5cf9777764..85f18cbbe5 100644
--- a/doc/src/sgml/ref/create_collation.sgml
+++ b/doc/src/sgml/ref/create_collation.sgml
@@ -96,6 +96,11 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
        <replaceable>locale</replaceable>, you cannot specify either of those
        parameters.
       </para>
+      <para>
+       If <replaceable>provider</replaceable> is <literal>builtin</literal>,
+       then <replaceable>locale</replaceable> must be specified and set to
+       either <literal>C</literal> or <literal>C.UTF-8</literal>.
+      </para>
      </listitem>
     </varlistentry>
 
@@ -129,9 +134,9 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
      <listitem>
       <para>
        Specifies the provider to use for locale services associated with this
-       collation.  Possible values are
-       <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
-       (if the server was built with ICU support) or <literal>libc</literal>.
+       collation.  Possible values are <literal>builtin</literal>,
+       <literal>icu</literal><indexterm><primary>ICU</primary></indexterm> (if
+       the server was built with ICU support) or <literal>libc</literal>.
        <literal>libc</literal> is the default.  See <xref
        linkend="locale-providers"/> for details.
       </para>
diff --git a/doc/src/sgml/ref/create_database.sgml b/doc/src/sgml/ref/create_database.sgml
index 72927960eb..1f5cdf1271 100644
--- a/doc/src/sgml/ref/create_database.sgml
+++ b/doc/src/sgml/ref/create_database.sgml
@@ -162,6 +162,12 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
         linkend="create-database-lc-ctype"/>, or <xref
         linkend="create-database-icu-locale"/> individually.
        </para>
+       <para>
+        If <xref linkend="create-database-locale-provider"/> is
+        <literal>builtin</literal>, then <replaceable>locale</replaceable>
+        must be specified and set to either <literal>C</literal> or
+        <literal>C.UTF-8</literal>.
+       </para>
        <tip>
         <para>
          The other locale settings <xref linkend="guc-lc-messages"/>, <xref
@@ -243,7 +249,7 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
       <listitem>
        <para>
         Specifies the provider to use for the default collation in this
-        database.  Possible values are
+        database.  Possible values are <literal>builtin</literal>,
         <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
         (if the server was built with ICU support) or <literal>libc</literal>.
         By default, the provider is the same as that of the <xref
diff --git a/doc/src/sgml/ref/createdb.sgml b/doc/src/sgml/ref/createdb.sgml
index e4647d5ce7..d3e815f659 100644
--- a/doc/src/sgml/ref/createdb.sgml
+++ b/doc/src/sgml/ref/createdb.sgml
@@ -171,7 +171,7 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
-      <term><option>--locale-provider={<literal>libc</literal>|<literal>icu</literal>}</option></term>
+      <term><option>--locale-provider={<literal>builtin</literal>|<literal>libc</literal>|<literal>icu</literal>}</option></term>
       <listitem>
        <para>
         Specifies the locale provider for the database's default collation.
diff --git a/doc/src/sgml/ref/initdb.sgml b/doc/src/sgml/ref/initdb.sgml
index cd75cae10e..08a1c2538f 100644
--- a/doc/src/sgml/ref/initdb.sgml
+++ b/doc/src/sgml/ref/initdb.sgml
@@ -286,6 +286,11 @@ PostgreSQL documentation
         environment that <command>initdb</command> runs in. Locale
         support is described in <xref linkend="locale"/>.
        </para>
+       <para>
+        If <option>--locale-provider</option> is <literal>builtin</literal>,
+        <option>--locale</option> must be specified and set to
+        <literal>C</literal> or <literal>C.UTF-8</literal>.
+       </para>
       </listitem>
      </varlistentry>
 
@@ -314,8 +319,18 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry id="app-initdb-builtin-locale">
+      <term><option>--builtin-locale=<replaceable>locale</replaceable></option></term>
+      <listitem>
+       <para>
+        Specifies the locale name when the builtin provider is used. Locale support
+        is described in <xref linkend="locale"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="app-initdb-option-locale-provider">
-      <term><option>--locale-provider={<literal>libc</literal>|<literal>icu</literal>}</option></term>
+      <term><option>--locale-provider={<literal>builtin</literal>|<literal>libc</literal>|<literal>icu</literal>}</option></term>
       <listitem>
        <para>
         This option sets the locale provider for databases created in the new
diff --git a/src/backend/catalog/pg_collation.c b/src/backend/catalog/pg_collation.c
index 7bad94f908..01e91000af 100644
--- a/src/backend/catalog/pg_collation.c
+++ b/src/backend/catalog/pg_collation.c
@@ -68,7 +68,10 @@ CollationCreate(const char *collname, Oid collnamespace,
 	Assert(collname);
 	Assert(collnamespace);
 	Assert(collowner);
-	Assert((collcollate && collctype) || colllocale);
+	Assert((collprovider == COLLPROVIDER_LIBC &&
+			collcollate && collctype && !colllocale) ||
+		   (collprovider != COLLPROVIDER_LIBC &&
+			!collcollate && !collctype && colllocale));
 
 	/*
 	 * Make sure there is no existing collation of same name & encoding.
diff --git a/src/backend/commands/collationcmds.c b/src/backend/commands/collationcmds.c
index 27564e569a..0fa073496e 100644
--- a/src/backend/commands/collationcmds.c
+++ b/src/backend/commands/collationcmds.c
@@ -68,7 +68,7 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 	DefElem    *versionEl = NULL;
 	char	   *collcollate;
 	char	   *collctype;
-	char	   *colllocale;
+	const char *colllocale;
 	char	   *collicurules;
 	bool		collisdeterministic;
 	int			collencoding;
@@ -215,7 +215,9 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 
 		if (collproviderstr)
 		{
-			if (pg_strcasecmp(collproviderstr, "icu") == 0)
+			if (pg_strcasecmp(collproviderstr, "builtin") == 0)
+				collprovider = COLLPROVIDER_BUILTIN;
+			else if (pg_strcasecmp(collproviderstr, "icu") == 0)
 				collprovider = COLLPROVIDER_ICU;
 			else if (pg_strcasecmp(collproviderstr, "libc") == 0)
 				collprovider = COLLPROVIDER_LIBC;
@@ -245,7 +247,17 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 		if (lcctypeEl)
 			collctype = defGetString(lcctypeEl);
 
-		if (collprovider == COLLPROVIDER_LIBC)
+		if (collprovider == COLLPROVIDER_BUILTIN)
+		{
+			if (!colllocale)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("parameter \"locale\" must be specified")));
+
+			colllocale = builtin_validate_locale(GetDatabaseEncoding(),
+												 colllocale);
+		}
+		else if (collprovider == COLLPROVIDER_LIBC)
 		{
 			if (!collcollate)
 				ereport(ERROR,
@@ -305,7 +317,17 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
 					 errmsg("ICU rules cannot be specified unless locale provider is ICU")));
 
-		if (collprovider == COLLPROVIDER_ICU)
+		if (collprovider == COLLPROVIDER_BUILTIN)
+		{
+			/*
+			 * Behavior may be different in different encodings, so set
+			 * collencoding to the current database encoding. No validation is
+			 * required, because the "builtin" provider is compatible with any
+			 * encoding.
+			 */
+			collencoding = GetDatabaseEncoding();
+		}
+		else if (collprovider == COLLPROVIDER_ICU)
 		{
 #ifdef USE_ICU
 			/*
@@ -334,7 +356,18 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 	}
 
 	if (!collversion)
-		collversion = get_collation_actual_version(collprovider, collprovider == COLLPROVIDER_ICU ? colllocale : collcollate);
+	{
+		const char *locale;
+
+		if (collprovider == COLLPROVIDER_ICU)
+			locale = colllocale;
+		else if (collprovider == COLLPROVIDER_LIBC)
+			locale = collcollate;
+		else
+			locale = NULL;		/* COLLPROVIDER_BUILTIN */
+
+		collversion = get_collation_actual_version(collprovider, locale);
+	}
 
 	newoid = CollationCreate(collName,
 							 collNamespace,
@@ -409,6 +442,7 @@ AlterCollation(AlterCollationStmt *stmt)
 	Form_pg_collation collForm;
 	Datum		datum;
 	bool		isnull;
+	char	   *locale;
 	char	   *oldversion;
 	char	   *newversion;
 	ObjectAddress address;
@@ -435,8 +469,20 @@ AlterCollation(AlterCollationStmt *stmt)
 	datum = SysCacheGetAttr(COLLOID, tup, Anum_pg_collation_collversion, &isnull);
 	oldversion = isnull ? NULL : TextDatumGetCString(datum);
 
-	datum = SysCacheGetAttrNotNull(COLLOID, tup, collForm->collprovider == COLLPROVIDER_ICU ? Anum_pg_collation_colllocale : Anum_pg_collation_collcollate);
-	newversion = get_collation_actual_version(collForm->collprovider, TextDatumGetCString(datum));
+	if (collForm->collprovider == COLLPROVIDER_ICU)
+	{
+		datum = SysCacheGetAttrNotNull(COLLOID, tup, Anum_pg_collation_colllocale);
+		locale = TextDatumGetCString(datum);
+	}
+	else if (collForm->collprovider == COLLPROVIDER_LIBC)
+	{
+		datum = SysCacheGetAttrNotNull(COLLOID, tup, Anum_pg_collation_collcollate);
+		locale = TextDatumGetCString(datum);
+	}
+	else
+		locale = NULL;			/* COLLPROVIDER_BUILTIN */
+
+	newversion = get_collation_actual_version(collForm->collprovider, locale);
 
 	/* cannot change from NULL to non-NULL or vice versa */
 	if ((!oldversion && newversion) || (oldversion && !newversion))
@@ -500,11 +546,18 @@ pg_collation_actual_version(PG_FUNCTION_ARGS)
 
 		provider = ((Form_pg_database) GETSTRUCT(dbtup))->datlocprovider;
 
-		datum = SysCacheGetAttrNotNull(DATABASEOID, dbtup,
-									   provider == COLLPROVIDER_ICU ?
-									   Anum_pg_database_datlocale : Anum_pg_database_datcollate);
-
-		locale = TextDatumGetCString(datum);
+		if (provider == COLLPROVIDER_ICU)
+		{
+			datum = SysCacheGetAttrNotNull(DATABASEOID, dbtup, Anum_pg_database_datlocale);
+			locale = TextDatumGetCString(datum);
+		}
+		else if (provider == COLLPROVIDER_LIBC)
+		{
+			datum = SysCacheGetAttrNotNull(DATABASEOID, dbtup, Anum_pg_database_datcollate);
+			locale = TextDatumGetCString(datum);
+		}
+		else
+			locale = NULL;		/* COLLPROVIDER_BUILTIN */
 
 		ReleaseSysCache(dbtup);
 	}
@@ -521,11 +574,19 @@ pg_collation_actual_version(PG_FUNCTION_ARGS)
 
 		provider = ((Form_pg_collation) GETSTRUCT(colltp))->collprovider;
 		Assert(provider != COLLPROVIDER_DEFAULT);
-		datum = SysCacheGetAttrNotNull(COLLOID, colltp,
-									   provider == COLLPROVIDER_ICU ?
-									   Anum_pg_collation_colllocale : Anum_pg_collation_collcollate);
 
-		locale = TextDatumGetCString(datum);
+		if (provider == COLLPROVIDER_ICU)
+		{
+			datum = SysCacheGetAttrNotNull(COLLOID, colltp, Anum_pg_collation_colllocale);
+			locale = TextDatumGetCString(datum);
+		}
+		else if (provider == COLLPROVIDER_LIBC)
+		{
+			datum = SysCacheGetAttrNotNull(COLLOID, colltp, Anum_pg_collation_collcollate);
+			locale = TextDatumGetCString(datum);
+		}
+		else
+			locale = NULL;		/* COLLPROVIDER_BUILTIN */
 
 		ReleaseSysCache(colltp);
 	}
diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index d1de46e759..d7a21adc5c 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -698,6 +698,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	DefElem    *dtemplate = NULL;
 	DefElem    *dencoding = NULL;
 	DefElem    *dlocale = NULL;
+	DefElem    *dbuiltinlocale = NULL;
 	DefElem    *dcollate = NULL;
 	DefElem    *dctype = NULL;
 	DefElem    *diculocale = NULL;
@@ -713,7 +714,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	const char *dbtemplate = NULL;
 	char	   *dbcollate = NULL;
 	char	   *dbctype = NULL;
-	char	   *dblocale = NULL;
+	const char *dblocale = NULL;
 	char	   *dbicurules = NULL;
 	char		dblocprovider = '\0';
 	char	   *canonname;
@@ -762,6 +763,12 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 				errorConflictingDefElem(defel, pstate);
 			dlocale = defel;
 		}
+		else if (strcmp(defel->defname, "builtin_locale") == 0)
+		{
+			if (dbuiltinlocale)
+				errorConflictingDefElem(defel, pstate);
+			dbuiltinlocale = defel;
+		}
 		else if (strcmp(defel->defname, "lc_collate") == 0)
 		{
 			if (dcollate)
@@ -897,7 +904,10 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	{
 		dbcollate = defGetString(dlocale);
 		dbctype = defGetString(dlocale);
+		dblocale = defGetString(dlocale);
 	}
+	if (dbuiltinlocale && dbuiltinlocale->arg)
+		dblocale = defGetString(dbuiltinlocale);
 	if (dcollate && dcollate->arg)
 		dbcollate = defGetString(dcollate);
 	if (dctype && dctype->arg)
@@ -910,7 +920,9 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	{
 		char	   *locproviderstr = defGetString(dlocprovider);
 
-		if (pg_strcasecmp(locproviderstr, "icu") == 0)
+		if (pg_strcasecmp(locproviderstr, "builtin") == 0)
+			dblocprovider = COLLPROVIDER_BUILTIN;
+		else if (pg_strcasecmp(locproviderstr, "icu") == 0)
 			dblocprovider = COLLPROVIDER_ICU;
 		else if (pg_strcasecmp(locproviderstr, "libc") == 0)
 			dblocprovider = COLLPROVIDER_LIBC;
@@ -1027,14 +1039,9 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 		dbctype = src_ctype;
 	if (dblocprovider == '\0')
 		dblocprovider = src_locprovider;
-	if (dblocale == NULL && dblocprovider == COLLPROVIDER_ICU)
-	{
-		if (dlocale && dlocale->arg)
-			dblocale = defGetString(dlocale);
-		else
-			dblocale = src_locale;
-	}
-	if (dbicurules == NULL && dblocprovider == COLLPROVIDER_ICU)
+	if (dblocale == NULL)
+		dblocale = src_locale;
+	if (dbicurules == NULL)
 		dbicurules = src_icurules;
 
 	/* Some encodings are client only */
@@ -1059,6 +1066,27 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 
 	check_encoding_locale_matches(encoding, dbcollate, dbctype);
 
+	if (dblocprovider == COLLPROVIDER_BUILTIN)
+	{
+		/*
+		 * This would happen if template0 uses the libc provider but the new
+		 * database uses builtin.
+		 */
+		if (!dblocale)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("LOCALE must be specified for the builtin provider")));
+
+		dblocale = builtin_validate_locale(encoding, dblocale);
+	}
+	else
+	{
+		if (dbuiltinlocale && dbuiltinlocale->arg)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+					 errmsg("BUILTIN_LOCALE cannot be specified unless locale provider is builtin")));
+	}
+
 	if (dblocprovider == COLLPROVIDER_ICU)
 	{
 		if (!(is_encoding_supported_by_icu(encoding)))
@@ -1100,7 +1128,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	}
 	else
 	{
-		if (dblocale)
+		if (diculocale && diculocale->arg)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
 					 errmsg("ICU locale cannot be specified unless locale provider is ICU")));
@@ -1111,6 +1139,10 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 					 errmsg("ICU rules cannot be specified unless locale provider is ICU")));
 	}
 
+	/* for libc, locale comes from datcollate and datctype */
+	if (dblocprovider == COLLPROVIDER_LIBC)
+		dblocale = NULL;
+
 	/*
 	 * Check that the new encoding and locale settings match the source
 	 * database.  We insist on this because we simply copy the source data ---
@@ -1196,8 +1228,16 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	if (src_collversion && !dcollversion)
 	{
 		char	   *actual_versionstr;
+		const char *locale;
 
-		actual_versionstr = get_collation_actual_version(dblocprovider, dblocprovider == COLLPROVIDER_ICU ? dblocale : dbcollate);
+		if (dblocprovider == COLLPROVIDER_ICU)
+			locale = dblocale;
+		else if (dblocprovider == COLLPROVIDER_LIBC)
+			locale = dbcollate;
+		else
+			locale = NULL;		/* COLLPROVIDER_BUILTIN */
+
+		actual_versionstr = get_collation_actual_version(dblocprovider, locale);
 		if (!actual_versionstr)
 			ereport(ERROR,
 					(errmsg("template database \"%s\" has a collation version, but no actual collation version could be determined",
@@ -1225,7 +1265,18 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	 * collation version, which is normally only the case for template0.
 	 */
 	if (dbcollversion == NULL)
-		dbcollversion = get_collation_actual_version(dblocprovider, dblocprovider == COLLPROVIDER_ICU ? dblocale : dbcollate);
+	{
+		const char *locale;
+
+		if (dblocprovider == COLLPROVIDER_ICU)
+			locale = dblocale;
+		else if (dblocprovider == COLLPROVIDER_LIBC)
+			locale = dbcollate;
+		else
+			locale = NULL;		/* COLLPROVIDER_BUILTIN */
+
+		dbcollversion = get_collation_actual_version(dblocprovider, locale);
+	}
 
 	/* Resolve default tablespace for new database */
 	if (dtablespacename && dtablespacename->arg)
@@ -1364,8 +1415,8 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	 * block on the unique index, and fail after we commit).
 	 */
 
-	Assert((dblocprovider == COLLPROVIDER_ICU && dblocale) ||
-		   (dblocprovider != COLLPROVIDER_ICU && !dblocale));
+	Assert((dblocprovider != COLLPROVIDER_LIBC && dblocale) ||
+		   (dblocprovider == COLLPROVIDER_LIBC && !dblocale));
 
 	/* Form tuple */
 	new_record[Anum_pg_database_oid - 1] = ObjectIdGetDatum(dboid);
@@ -2446,6 +2497,7 @@ AlterDatabaseRefreshColl(AlterDatabaseRefreshCollStmt *stmt)
 	ObjectAddress address;
 	Datum		datum;
 	bool		isnull;
+	char	   *locale;
 	char	   *oldversion;
 	char	   *newversion;
 
@@ -2472,10 +2524,24 @@ AlterDatabaseRefreshColl(AlterDatabaseRefreshCollStmt *stmt)
 	datum = heap_getattr(tuple, Anum_pg_database_datcollversion, RelationGetDescr(rel), &isnull);
 	oldversion = isnull ? NULL : TextDatumGetCString(datum);
 
-	datum = heap_getattr(tuple, datForm->datlocprovider == COLLPROVIDER_ICU ? Anum_pg_database_datlocale : Anum_pg_database_datcollate, RelationGetDescr(rel), &isnull);
-	if (isnull)
-		elog(ERROR, "unexpected null in pg_database");
-	newversion = get_collation_actual_version(datForm->datlocprovider, TextDatumGetCString(datum));
+	if (datForm->datlocprovider == COLLPROVIDER_ICU)
+	{
+		datum = heap_getattr(tuple, Anum_pg_database_datlocale, RelationGetDescr(rel), &isnull);
+		if (isnull)
+			elog(ERROR, "unexpected null in pg_database");
+		locale = TextDatumGetCString(datum);
+	}
+	else if (datForm->datlocprovider == COLLPROVIDER_LIBC)
+	{
+		datum = heap_getattr(tuple, Anum_pg_database_datcollate, RelationGetDescr(rel), &isnull);
+		if (isnull)
+			elog(ERROR, "unexpected null in pg_database");
+		locale = TextDatumGetCString(datum);
+	}
+	else
+		locale = NULL;			/* COLLPROVIDER_BUILTIN */
+
+	newversion = get_collation_actual_version(datForm->datlocprovider, locale);
 
 	/* cannot change from NULL to non-NULL or vice versa */
 	if ((!oldversion && newversion) || (oldversion && !newversion))
@@ -2660,6 +2726,7 @@ pg_database_collation_actual_version(PG_FUNCTION_ARGS)
 	HeapTuple	tp;
 	char		datlocprovider;
 	Datum		datum;
+	char	   *locale;
 	char	   *version;
 
 	tp = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(dbid));
@@ -2670,8 +2737,20 @@ pg_database_collation_actual_version(PG_FUNCTION_ARGS)
 
 	datlocprovider = ((Form_pg_database) GETSTRUCT(tp))->datlocprovider;
 
-	datum = SysCacheGetAttrNotNull(DATABASEOID, tp, datlocprovider == COLLPROVIDER_ICU ? Anum_pg_database_datlocale : Anum_pg_database_datcollate);
-	version = get_collation_actual_version(datlocprovider, TextDatumGetCString(datum));
+	if (datlocprovider == COLLPROVIDER_ICU)
+	{
+		datum = SysCacheGetAttrNotNull(DATABASEOID, tp, Anum_pg_database_datlocale);
+		locale = TextDatumGetCString(datum);
+	}
+	else if (datlocprovider == COLLPROVIDER_LIBC)
+	{
+		datum = SysCacheGetAttrNotNull(DATABASEOID, tp, Anum_pg_database_datcollate);
+		locale = TextDatumGetCString(datum);
+	}
+	else
+		locale = NULL;			/* COLLPROVIDER_BUILTIN */
+
+	version = get_collation_actual_version(datlocprovider, locale);
 
 	ReleaseSysCache(tp);
 
diff --git a/src/backend/regex/regc_pg_locale.c b/src/backend/regex/regc_pg_locale.c
index 6a26388bfa..d612731319 100644
--- a/src/backend/regex/regc_pg_locale.c
+++ b/src/backend/regex/regc_pg_locale.c
@@ -16,6 +16,8 @@
  */
 
 #include "catalog/pg_collation.h"
+#include "common/unicode_case.h"
+#include "common/unicode_category.h"
 #include "utils/pg_locale.h"
 
 /*
@@ -64,6 +66,7 @@
 typedef enum
 {
 	PG_REGEX_LOCALE_C,			/* C locale (encoding independent) */
+	PG_REGEX_BUILTIN,			/* built-in Unicode semantics */
 	PG_REGEX_LOCALE_WIDE,		/* Use <wctype.h> functions */
 	PG_REGEX_LOCALE_1BYTE,		/* Use <ctype.h> functions */
 	PG_REGEX_LOCALE_WIDE_L,		/* Use locale_t <wctype.h> functions */
@@ -75,6 +78,8 @@ static PG_Locale_Strategy pg_regex_strategy;
 static pg_locale_t pg_regex_locale;
 static Oid	pg_regex_collation;
 
+static bool regex_builtin_cclass_posix = false;
+
 /*
  * Hard-wired character properties for C locale
  */
@@ -266,7 +271,15 @@ pg_set_regex_collation(Oid collation)
 		if (GetDatabaseEncoding() == PG_UTF8)
 		{
 			if (pg_regex_locale)
-				pg_regex_strategy = PG_REGEX_LOCALE_WIDE_L;
+			{
+				if (pg_regex_locale->provider == COLLPROVIDER_BUILTIN)
+				{
+					pg_regex_strategy = PG_REGEX_BUILTIN;
+					regex_builtin_cclass_posix = pg_regex_locale->info.builtin.properties_posix;
+				}
+				else
+					pg_regex_strategy = PG_REGEX_LOCALE_WIDE_L;
+			}
 			else
 				pg_regex_strategy = PG_REGEX_LOCALE_WIDE;
 		}
@@ -290,6 +303,8 @@ pg_wc_isdigit(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISDIGIT));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isdigit(c, regex_builtin_cclass_posix);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswdigit((wint_t) c);
@@ -322,6 +337,8 @@ pg_wc_isalpha(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISALPHA));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isalpha(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalpha((wint_t) c);
@@ -354,6 +371,8 @@ pg_wc_isalnum(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISALNUM));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isalnum(c, regex_builtin_cclass_posix);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalnum((wint_t) c);
@@ -395,6 +414,8 @@ pg_wc_isupper(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISUPPER));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isupper(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswupper((wint_t) c);
@@ -427,6 +448,8 @@ pg_wc_islower(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISLOWER));
+		case PG_REGEX_BUILTIN:
+			return pg_u_islower(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswlower((wint_t) c);
@@ -459,6 +482,8 @@ pg_wc_isgraph(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISGRAPH));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isgraph(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswgraph((wint_t) c);
@@ -491,6 +516,8 @@ pg_wc_isprint(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISPRINT));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isprint(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswprint((wint_t) c);
@@ -523,6 +550,8 @@ pg_wc_ispunct(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISPUNCT));
+		case PG_REGEX_BUILTIN:
+			return pg_u_ispunct(c, regex_builtin_cclass_posix);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswpunct((wint_t) c);
@@ -555,6 +584,8 @@ pg_wc_isspace(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISSPACE));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isspace(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswspace((wint_t) c);
@@ -588,6 +619,8 @@ pg_wc_toupper(pg_wchar c)
 			if (c <= (pg_wchar) 127)
 				return pg_ascii_toupper((unsigned char) c);
 			return c;
+		case PG_REGEX_BUILTIN:
+			return unicode_uppercase_simple(c);
 		case PG_REGEX_LOCALE_WIDE:
 			/* force C behavior for ASCII characters, per comments above */
 			if (c <= (pg_wchar) 127)
@@ -628,6 +661,8 @@ pg_wc_tolower(pg_wchar c)
 			if (c <= (pg_wchar) 127)
 				return pg_ascii_tolower((unsigned char) c);
 			return c;
+		case PG_REGEX_BUILTIN:
+			return unicode_lowercase_simple(c);
 		case PG_REGEX_LOCALE_WIDE:
 			/* force C behavior for ASCII characters, per comments above */
 			if (c <= (pg_wchar) 127)
@@ -792,6 +827,9 @@ pg_ctype_get_cache(pg_wc_probefunc probefunc, int cclasscode)
 			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
 #endif
 			break;
+		case PG_REGEX_BUILTIN:
+			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
+			break;
 		case PG_REGEX_LOCALE_WIDE:
 		case PG_REGEX_LOCALE_WIDE_L:
 			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
@@ -809,6 +847,7 @@ pg_ctype_get_cache(pg_wc_probefunc probefunc, int cclasscode)
 			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
 			break;
 		default:
+			Assert(false);
 			max_chr = 0;		/* can't get here, but keep compiler quiet */
 			break;
 	}
diff --git a/src/backend/utils/adt/formatting.c b/src/backend/utils/adt/formatting.c
index 829aaa8d0e..0d3ce8a33e 100644
--- a/src/backend/utils/adt/formatting.c
+++ b/src/backend/utils/adt/formatting.c
@@ -77,6 +77,8 @@
 
 #include "catalog/pg_collation.h"
 #include "catalog/pg_type.h"
+#include "common/unicode_case.h"
+#include "common/unicode_category.h"
 #include "mb/pg_wchar.h"
 #include "nodes/miscnodes.h"
 #include "parser/scansup.h"
@@ -1680,6 +1682,35 @@ str_tolower(const char *buff, size_t nbytes, Oid collid)
 		}
 		else
 #endif
+		if (mylocale && mylocale->provider == COLLPROVIDER_BUILTIN)
+		{
+			const char *src = buff;
+			size_t		srclen = nbytes;
+			size_t		dstsize = srclen + 1;
+			char	   *dst = palloc(dstsize);
+			size_t		needed;
+
+			Assert(GetDatabaseEncoding() == PG_UTF8);
+
+			/* first try buffer of equal size */
+			dstsize = srclen + 1;
+			result = palloc(dstsize);
+
+			needed = unicode_strlower(dst, dstsize, src, srclen,
+									  mylocale->info.builtin.casemap_full);
+			if (needed + 1 > dstsize)
+			{
+				/* grow buffer if needed and retry */
+				dstsize = needed + 1;
+				dst = repalloc(dst, dstsize);
+				needed = unicode_strlower(dst, dstsize, src, srclen,
+										  mylocale->info.builtin.casemap_full);
+				Assert(needed + 1 == dstsize);
+			}
+
+			result = dst;
+		}
+		else
 		{
 			if (pg_database_encoding_max_length() > 1)
 			{
@@ -1798,6 +1829,35 @@ str_toupper(const char *buff, size_t nbytes, Oid collid)
 		}
 		else
 #endif
+		if (mylocale && mylocale->provider == COLLPROVIDER_BUILTIN)
+		{
+			const char *src = buff;
+			size_t		srclen = nbytes;
+			size_t		dstsize = srclen + 1;
+			char	   *dst = palloc(dstsize);
+			size_t		needed;
+
+			Assert(GetDatabaseEncoding() == PG_UTF8);
+
+			/* first try buffer of equal size */
+			dstsize = srclen + 1;
+			result = palloc(dstsize);
+
+			needed = unicode_strupper(dst, dstsize, src, srclen,
+									  mylocale->info.builtin.casemap_full);
+			if (needed + 1 > dstsize)
+			{
+				/* grow buffer if needed and retry */
+				dstsize = needed + 1;
+				dst = repalloc(dst, dstsize);
+				needed = unicode_strupper(dst, dstsize, src, srclen,
+										  mylocale->info.builtin.casemap_full);
+				Assert(needed + 1 == dstsize);
+			}
+
+			result = dst;
+		}
+		else
 		{
 			if (pg_database_encoding_max_length() > 1)
 			{
@@ -1861,6 +1921,48 @@ str_toupper(const char *buff, size_t nbytes, Oid collid)
 	return result;
 }
 
+struct WordBoundaryState
+{
+	const char *str;
+	size_t		len;
+	size_t		offset;
+	bool		init;
+	bool		prev_alnum;
+	bool		posix;
+};
+
+/*
+ * Simple word boundary iterator that draws boundaries each time the result of
+ * pg_u_isalnum() changes.
+ */
+static size_t
+initcap_wbnext(void *state)
+{
+	struct WordBoundaryState *wbstate = (struct WordBoundaryState *) state;
+
+	while (wbstate->offset < wbstate->len &&
+		   wbstate->str[wbstate->offset] != '\0')
+	{
+		pg_wchar	u = utf8_to_unicode((unsigned char *) wbstate->str +
+										wbstate->offset);
+		bool		curr_alnum = pg_u_isalnum(u, wbstate->posix);
+
+		if (!wbstate->init || curr_alnum != wbstate->prev_alnum)
+		{
+			size_t		prev_offset = wbstate->offset;
+
+			wbstate->init = true;
+			wbstate->offset += unicode_utf8len(u);
+			wbstate->prev_alnum = curr_alnum;
+			return prev_offset;
+		}
+
+		wbstate->offset += unicode_utf8len(u);
+	}
+
+	return wbstate->len;
+}
+
 /*
  * collation-aware, wide-character-aware initcap function
  *
@@ -1917,6 +2019,53 @@ str_initcap(const char *buff, size_t nbytes, Oid collid)
 		}
 		else
 #endif
+		if (mylocale && mylocale->provider == COLLPROVIDER_BUILTIN)
+		{
+			const char *src = buff;
+			size_t		srclen = nbytes;
+			size_t		dstsize = srclen + 1;
+			char	   *dst = palloc(dstsize);
+			size_t		needed;
+			struct WordBoundaryState wbstate = {
+				.str = src,
+				.len = srclen,
+				.offset = 0,
+				.init = false,
+				.prev_alnum = false,
+				.posix = mylocale->info.builtin.properties_posix,
+			};
+
+			Assert(GetDatabaseEncoding() == PG_UTF8);
+
+			/* first try buffer of equal size */
+			dstsize = srclen + 1;
+			result = palloc(dstsize);
+
+			needed = unicode_strtitle(dst, dstsize, src, srclen,
+									  mylocale->info.builtin.titlecase,
+									  mylocale->info.builtin.adjust_to_cased,
+									  mylocale->info.builtin.casemap_full,
+									  initcap_wbnext, &wbstate);
+			if (needed + 1 > dstsize)
+			{
+				/* reset iterator */
+				wbstate.offset = 0;
+				wbstate.prev_alnum = false;
+
+				/* grow buffer if needed and retry */
+				dstsize = needed + 1;
+				dst = repalloc(dst, dstsize);
+				needed = unicode_strtitle(dst, dstsize, src, srclen,
+										  mylocale->info.builtin.titlecase,
+										  mylocale->info.builtin.adjust_to_cased,
+										  mylocale->info.builtin.casemap_full,
+										  initcap_wbnext, &wbstate);
+				Assert(needed + 1 == dstsize);
+			}
+
+			result = dst;
+		}
+		else
 		{
 			if (pg_database_encoding_max_length() > 1)
 			{
diff --git a/src/backend/utils/adt/pg_locale.c b/src/backend/utils/adt/pg_locale.c
index 45fe847320..0c129cd3ca 100644
--- a/src/backend/utils/adt/pg_locale.c
+++ b/src/backend/utils/adt/pg_locale.c
@@ -1269,7 +1269,19 @@ lookup_collation_cache(Oid collation, bool set_flags)
 			elog(ERROR, "cache lookup failed for collation %u", collation);
 		collform = (Form_pg_collation) GETSTRUCT(tp);
 
-		if (collform->collprovider == COLLPROVIDER_LIBC)
+		if (collform->collprovider == COLLPROVIDER_BUILTIN)
+		{
+			Datum		datum;
+			const char *colllocale;
+
+			datum = SysCacheGetAttrNotNull(COLLOID, tp, Anum_pg_collation_colllocale);
+			colllocale = TextDatumGetCString(datum);
+
+			cache_entry->collate_is_c = true;
+			cache_entry->ctype_is_c = ((strcmp(colllocale, "C") == 0) ||
+									   (strcmp(colllocale, "POSIX") == 0));
+		}
+		else if (collform->collprovider == COLLPROVIDER_LIBC)
 		{
 			Datum		datum;
 			const char *collcollate;
@@ -1320,16 +1332,30 @@ lc_collate_is_c(Oid collation)
 	if (collation == DEFAULT_COLLATION_OID)
 	{
 		static int	result = -1;
-		char	   *localeptr;
-
-		if (default_locale.provider == COLLPROVIDER_ICU)
-			return false;
+		const char *localeptr;
 
 		if (result >= 0)
 			return (bool) result;
-		localeptr = setlocale(LC_COLLATE, NULL);
-		if (!localeptr)
-			elog(ERROR, "invalid LC_COLLATE setting");
+
+		if (default_locale.provider == COLLPROVIDER_BUILTIN)
+		{
+			result = true;
+			return (bool) result;
+		}
+		else if (default_locale.provider == COLLPROVIDER_ICU)
+		{
+			result = false;
+			return (bool) result;
+		}
+		else if (default_locale.provider == COLLPROVIDER_LIBC)
+		{
+			localeptr = setlocale(LC_CTYPE, NULL);
+			if (!localeptr)
+				elog(ERROR, "invalid LC_CTYPE setting");
+		}
+		else
+			elog(ERROR, "unexpected collation provider '%c'",
+				 default_locale.provider);
 
 		if (strcmp(localeptr, "C") == 0)
 			result = true;
@@ -1373,16 +1399,29 @@ lc_ctype_is_c(Oid collation)
 	if (collation == DEFAULT_COLLATION_OID)
 	{
 		static int	result = -1;
-		char	   *localeptr;
-
-		if (default_locale.provider == COLLPROVIDER_ICU)
-			return false;
+		const char *localeptr;
 
 		if (result >= 0)
 			return (bool) result;
-		localeptr = setlocale(LC_CTYPE, NULL);
-		if (!localeptr)
-			elog(ERROR, "invalid LC_CTYPE setting");
+
+		if (default_locale.provider == COLLPROVIDER_BUILTIN)
+		{
+			localeptr = default_locale.info.builtin.locale;
+		}
+		else if (default_locale.provider == COLLPROVIDER_ICU)
+		{
+			result = false;
+			return (bool) result;
+		}
+		else if (default_locale.provider == COLLPROVIDER_LIBC)
+		{
+			localeptr = setlocale(LC_CTYPE, NULL);
+			if (!localeptr)
+				elog(ERROR, "invalid LC_CTYPE setting");
+		}
+		else
+			elog(ERROR, "unexpected collation provider '%c'",
+				 default_locale.provider);
 
 		if (strcmp(localeptr, "C") == 0)
 			result = true;
@@ -1390,6 +1429,7 @@ lc_ctype_is_c(Oid collation)
 			result = true;
 		else
 			result = false;
+
 		return (bool) result;
 	}
 
@@ -1520,10 +1560,10 @@ pg_newlocale_from_collation(Oid collid)
 
 	if (collid == DEFAULT_COLLATION_OID)
 	{
-		if (default_locale.provider == COLLPROVIDER_ICU)
-			return &default_locale;
-		else
+		if (default_locale.provider == COLLPROVIDER_LIBC)
 			return (pg_locale_t) 0;
+		else
+			return &default_locale;
 	}
 
 	cache_entry = lookup_collation_cache(collid, false);
@@ -1548,7 +1588,50 @@ pg_newlocale_from_collation(Oid collid)
 		result.provider = collform->collprovider;
 		result.deterministic = collform->collisdeterministic;
 
-		if (collform->collprovider == COLLPROVIDER_LIBC)
+		if (collform->collprovider == COLLPROVIDER_BUILTIN)
+		{
+			bool		casemap_full;
+			bool		adjust_to_cased;
+			bool		titlecase;
+			bool		properties_posix;
+
+			const char *locstr;
+
+			datum = SysCacheGetAttrNotNull(COLLOID, tp, Anum_pg_collation_colllocale);
+			locstr = TextDatumGetCString(datum);
+
+			if (strcmp(locstr, "PG_UNICODE_FAST") == 0)
+			{
+				casemap_full = true;
+				adjust_to_cased = true;
+				titlecase = true;
+				properties_posix = false;
+			}
+			else if (strcmp(locstr, "C.UTF-8") == 0)
+			{
+				casemap_full = false;
+				adjust_to_cased = false;
+				titlecase = false;
+				properties_posix = true;
+			}
+			else if (strcmp(locstr, "C") == 0)
+			{
+				casemap_full = false;
+				adjust_to_cased = false;
+				titlecase = false;
+				properties_posix = true;
+			}
+			else
+				elog(ERROR, "unexpected builtin locale: %s", locstr);
+
+			result.info.builtin.locale = MemoryContextStrdup(TopMemoryContext,
+															 locstr);
+			result.info.builtin.casemap_full = casemap_full;
+			result.info.builtin.adjust_to_cased = adjust_to_cased;
+			result.info.builtin.titlecase = titlecase;
+			result.info.builtin.properties_posix = properties_posix;
+		}
+		else if (collform->collprovider == COLLPROVIDER_LIBC)
 		{
 			const char *collcollate;
 			const char *collctype pg_attribute_unused();
@@ -1627,6 +1710,7 @@ pg_newlocale_from_collation(Oid collid)
 
 			collversionstr = TextDatumGetCString(datum);
 
+			Assert(collform->collprovider != COLLPROVIDER_BUILTIN);
 			datum = SysCacheGetAttrNotNull(COLLOID, tp, collform->collprovider == COLLPROVIDER_ICU ? Anum_pg_collation_colllocale : Anum_pg_collation_collcollate);
 
 			actual_versionstr = get_collation_actual_version(collform->collprovider,
@@ -1678,6 +1762,14 @@ get_collation_actual_version(char collprovider, const char *collcollate)
 {
 	char	   *collversion = NULL;
 
+	/*
+	 * The only two supported locales (C and C.UTF-8) are both based on memcmp
+	 * and do not change. (The ctype behavior can change, but the versioning
+	 * does not track that.)
+	 */
+	if (collprovider == COLLPROVIDER_BUILTIN)
+		return NULL;
+
 #ifdef USE_ICU
 	if (collprovider == COLLPROVIDER_ICU)
 	{
@@ -2444,6 +2536,43 @@ pg_strnxfrm_prefix(char *dest, size_t destsize, const char *src,
 	return result;
 }
 
+const char *
+builtin_validate_locale(int encoding, const char *locale)
+{
+	const char *canonical_name = NULL;
+	int			required_encoding = -1;
+
+	if (strcmp(locale, "C") == 0 || strcmp(locale, "POSIX") == 0)
+	{
+		canonical_name = "C";
+	}
+	else if (strcmp(locale, "PG_UNICODE_FAST") == 0)
+	{
+		required_encoding = PG_UTF8;
+		canonical_name = "PG_UNICODE_FAST";
+	}
+	else if (strcmp(locale, "C.UTF-8") == 0 || strcmp(locale, "C.UTF8") == 0)
+	{
+		required_encoding = PG_UTF8;
+		canonical_name = "C.UTF-8";
+	}
+
+	if (!canonical_name)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("invalid locale name \"%s\" for builtin provider",
+						locale)));
+
+	if (required_encoding >= 0 && encoding != required_encoding)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("encoding \"%s\" does not match locale \"%s\"",
+						pg_encoding_to_char(encoding), locale)));
+
+	return canonical_name;
+}
+
+
 #ifdef USE_ICU
 
 /*
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 154912ecb4..95c00f559d 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -425,7 +425,48 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
 		strcmp(ctype, "POSIX") == 0)
 		database_ctype_is_c = true;
 
-	if (dbform->datlocprovider == COLLPROVIDER_ICU)
+	if (dbform->datlocprovider == COLLPROVIDER_BUILTIN)
+	{
+		bool		casemap_full;
+		bool		adjust_to_cased;
+		bool		titlecase;
+		bool		properties_posix;
+
+		datum = SysCacheGetAttrNotNull(DATABASEOID, tup, Anum_pg_database_datlocale);
+		datlocale = TextDatumGetCString(datum);
+
+		if (strcmp(datlocale, "PG_UNICODE_FAST") == 0)
+		{
+			casemap_full = true;
+			adjust_to_cased = true;
+			titlecase = true;
+			properties_posix = false;
+		}
+		else if (strcmp(datlocale, "C.UTF-8") == 0)
+		{
+			casemap_full = false;
+			adjust_to_cased = false;
+			titlecase = false;
+			properties_posix = true;
+		}
+		else if (strcmp(datlocale, "C") == 0)
+		{
+			casemap_full = false;
+			adjust_to_cased = false;
+			titlecase = false;
+			properties_posix = true;
+		}
+		else
+			elog(ERROR, "unexpected builtin locale: %s", datlocale);
+
+		default_locale.info.builtin.locale = MemoryContextStrdup(
+																 TopMemoryContext, datlocale);
+		default_locale.info.builtin.casemap_full = casemap_full;
+		default_locale.info.builtin.adjust_to_cased = adjust_to_cased;
+		default_locale.info.builtin.titlecase = titlecase;
+		default_locale.info.builtin.properties_posix = properties_posix;
+	}
+	else if (dbform->datlocprovider == COLLPROVIDER_ICU)
 	{
 		char	   *icurules;
 
@@ -463,10 +504,16 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
 	{
 		char	   *actual_versionstr;
 		char	   *collversionstr;
+		char	   *locale;
 
 		collversionstr = TextDatumGetCString(datum);
 
-		actual_versionstr = get_collation_actual_version(dbform->datlocprovider, dbform->datlocprovider == COLLPROVIDER_ICU ? datlocale : collate);
+		if (dbform->datlocprovider == COLLPROVIDER_LIBC)
+			locale = collate;
+		else
+			locale = datlocale;
+
+		actual_versionstr = get_collation_actual_version(dbform->datlocprovider, locale);
 		if (!actual_versionstr)
 			/* should not happen */
 			elog(WARNING,
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 90f793632a..7419c38722 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -146,6 +146,7 @@ static char *lc_time = NULL;
 static char *lc_messages = NULL;
 static char locale_provider = COLLPROVIDER_LIBC;
 static char *datlocale = NULL;
+static bool icu_locale_specified = false;
 static char *icu_rules = NULL;
 static const char *default_text_search_config = NULL;
 static char *username = NULL;
@@ -2390,14 +2391,13 @@ setlocales(void)
 	lc_messages = canonname;
 #endif
 
+	if (locale_provider != COLLPROVIDER_LIBC && datlocale == NULL)
+		pg_fatal("locale must be specified unless provider is libc");
+
 	if (locale_provider == COLLPROVIDER_ICU)
 	{
 		char	   *langtag;
 
-		/* acquire default locale from the environment, if not specified */
-		if (datlocale == NULL)
-			pg_fatal("ICU locale must be specified");
-
 		/* canonicalize to a language tag */
 		langtag = icu_language_tag(datlocale);
 		printf(_("Using language tag \"%s\" for ICU locale \"%s\".\n"),
@@ -2442,7 +2442,8 @@ usage(const char *progname)
 			 "                            set default locale in the respective category for\n"
 			 "                            new databases (default taken from environment)\n"));
 	printf(_("      --no-locale           equivalent to --locale=C\n"));
-	printf(_("      --locale-provider={libc|icu}\n"
+	printf(_("      --builtin-locale=LOCALE   set builtin locale name for new databases\n"));
+	printf(_("      --locale-provider={builtin|libc|icu}\n"
 			 "                            set default locale provider for new databases\n"));
 	printf(_("      --pwfile=FILE         read password for the new superuser from file\n"));
 	printf(_("  -T, --text-search-config=CFG\n"
@@ -2593,20 +2594,28 @@ setup_locale_encoding(void)
 {
 	setlocales();
 
-	if (locale_provider == COLLPROVIDER_LIBC &&
-		strcmp(lc_ctype, lc_collate) == 0 &&
-		strcmp(lc_ctype, lc_time) == 0 &&
-		strcmp(lc_ctype, lc_numeric) == 0 &&
-		strcmp(lc_ctype, lc_monetary) == 0 &&
-		strcmp(lc_ctype, lc_messages) == 0 &&
-		(!datlocale || strcmp(lc_ctype, datlocale) == 0))
+	if (locale_provider == COLLPROVIDER_BUILTIN &&
+		strcmp(lc_ctype, "C") == 0 &&
+		strcmp(lc_collate, "C") == 0 &&
+		strcmp(lc_time, "C") == 0 &&
+		strcmp(lc_numeric, "C") == 0 &&
+		strcmp(lc_monetary, "C") == 0 &&
+		strcmp(lc_messages, "C") == 0)
+		printf(_("The database cluster will be initialized with no locale.\n"));
+	else if (locale_provider == COLLPROVIDER_LIBC &&
+			 strcmp(lc_ctype, lc_collate) == 0 &&
+			 strcmp(lc_ctype, lc_time) == 0 &&
+			 strcmp(lc_ctype, lc_numeric) == 0 &&
+			 strcmp(lc_ctype, lc_monetary) == 0 &&
+			 strcmp(lc_ctype, lc_messages) == 0 &&
+			 (!datlocale || strcmp(lc_ctype, datlocale) == 0))
 		printf(_("The database cluster will be initialized with locale \"%s\".\n"), lc_ctype);
 	else
 	{
 		printf(_("The database cluster will be initialized with this locale configuration:\n"));
-		printf(_("  provider:    %s\n"), collprovider_name(locale_provider));
-		if (datlocale)
-			printf(_("  ICU locale:  %s\n"), datlocale);
+		printf(_("  default collation provider:  %s\n"), collprovider_name(locale_provider));
+		if (locale_provider != COLLPROVIDER_LIBC)
+			printf(_("  default collation locale:    %s\n"), datlocale);
 		printf(_("  LC_COLLATE:  %s\n"
 				 "  LC_CTYPE:    %s\n"
 				 "  LC_MESSAGES: %s\n"
@@ -3099,9 +3108,10 @@ main(int argc, char *argv[])
 		{"allow-group-access", no_argument, NULL, 'g'},
 		{"discard-caches", no_argument, NULL, 14},
 		{"locale-provider", required_argument, NULL, 15},
-		{"icu-locale", required_argument, NULL, 16},
-		{"icu-rules", required_argument, NULL, 17},
-		{"sync-method", required_argument, NULL, 18},
+		{"builtin-locale", required_argument, NULL, 16},
+		{"icu-locale", required_argument, NULL, 17},
+		{"icu-rules", required_argument, NULL, 18},
+		{"sync-method", required_argument, NULL, 19},
 		{NULL, 0, NULL, 0}
 	};
 
@@ -3269,7 +3279,9 @@ main(int argc, char *argv[])
 										 "-c debug_discard_caches=1");
 				break;
 			case 15:
-				if (strcmp(optarg, "icu") == 0)
+				if (strcmp(optarg, "builtin") == 0)
+					locale_provider = COLLPROVIDER_BUILTIN;
+				else if (strcmp(optarg, "icu") == 0)
 					locale_provider = COLLPROVIDER_ICU;
 				else if (strcmp(optarg, "libc") == 0)
 					locale_provider = COLLPROVIDER_LIBC;
@@ -3280,9 +3292,13 @@ main(int argc, char *argv[])
 				datlocale = pg_strdup(optarg);
 				break;
 			case 17:
-				icu_rules = pg_strdup(optarg);
+				datlocale = pg_strdup(optarg);
+				icu_locale_specified = true;
 				break;
 			case 18:
+				icu_rules = pg_strdup(optarg);
+				break;
+			case 19:
 				if (!parse_sync_method(optarg, &sync_method))
 					exit(1);
 				break;
@@ -3312,7 +3328,7 @@ main(int argc, char *argv[])
 		exit(1);
 	}
 
-	if (datlocale && locale_provider != COLLPROVIDER_ICU)
+	if (icu_locale_specified && locale_provider != COLLPROVIDER_ICU)
 		pg_fatal("%s cannot be specified unless locale provider \"%s\" is chosen",
 				 "--icu-locale", "icu");
 
diff --git a/src/bin/initdb/t/001_initdb.pl b/src/bin/initdb/t/001_initdb.pl
index 03376cc0f7..242f4581a5 100644
--- a/src/bin/initdb/t/001_initdb.pl
+++ b/src/bin/initdb/t/001_initdb.pl
@@ -117,7 +117,7 @@ if ($ENV{with_icu} eq 'yes')
 {
 	command_fails_like(
 		[ 'initdb', '--no-sync', '--locale-provider=icu', "$tempdir/data2" ],
-		qr/initdb: error: ICU locale must be specified/,
+		qr/initdb: error: locale must be specified unless provider is libc/,
 		'locale provider ICU requires --icu-locale');
 
 	command_ok(
@@ -138,7 +138,7 @@ if ($ENV{with_icu} eq 'yes')
 			'--lc-monetary=C', '--lc-time=C',
 			"$tempdir/data4"
 		],
-		qr/^\s+ICU locale:\s+und\n/ms,
+		qr/^\s+default collation locale:\s+und\n/ms,
 		'options --locale-provider=icu --locale=und --lc-*=C');
 
 	command_fails_like(
@@ -184,6 +184,59 @@ else
 		'locale provider ICU fails since no ICU support');
 }
 
+command_fails(
+	[ 'initdb', '--no-sync', '--locale-provider=builtin', "$tempdir/data6" ],
+	'locale provider builtin fails without --locale');
+
+command_ok(
+	[
+		'initdb', '--no-sync',
+		'--locale-provider=builtin', '--locale=C',
+		"$tempdir/data7"
+	],
+	'locale provider builtin with --locale');
+
+command_ok(
+	[
+		'initdb', '--no-sync',
+		'--locale-provider=builtin', '-E UTF-8',
+		'--builtin-locale=C.UTF-8', "$tempdir/data8"
+	],
+	'locale provider builtin with -E UTF-8 --builtin-locale=C.UTF-8');
+
+command_fails(
+	[
+		'initdb', '--no-sync',
+		'--locale-provider=builtin', '-E SQL_ASCII',
+		'--builtin-locale=C.UTF-8', "$tempdir/data9"
+	],
+	'locale provider builtin with --builtin-locale=C.UTF-8 fails for SQL_ASCII'
+);
+
+command_ok(
+	[
+		'initdb', '--no-sync',
+		'--locale-provider=builtin', '--lc-ctype=C',
+		'--locale=C', "$tempdir/data10"
+	],
+	'locale provider builtin with --lc-ctype');
+
+command_fails(
+	[
+		'initdb', '--no-sync',
+		'--locale-provider=builtin', '--icu-locale=en',
+		"$tempdir/dataX"
+	],
+	'fails for locale provider builtin with ICU locale');
+
+command_fails(
+	[
+		'initdb', '--no-sync',
+		'--locale-provider=builtin', '--icu-rules=""',
+		"$tempdir/dataX"
+	],
+	'fails for locale provider builtin with ICU rules');
+
 command_fails(
 	[ 'initdb', '--no-sync', '--locale-provider=xyz', "$tempdir/dataX" ],
 	'fails for invalid locale provider');
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index a67b4b8225..450c322205 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -3114,7 +3114,9 @@ dumpDatabase(Archive *fout)
 	}
 
 	appendPQExpBufferStr(creaQry, " LOCALE_PROVIDER = ");
-	if (datlocprovider[0] == 'c')
+	if (datlocprovider[0] == 'b')
+		appendPQExpBufferStr(creaQry, "builtin");
+	else if (datlocprovider[0] == 'c')
 		appendPQExpBufferStr(creaQry, "libc");
 	else if (datlocprovider[0] == 'i')
 		appendPQExpBufferStr(creaQry, "icu");
@@ -3122,27 +3124,33 @@ dumpDatabase(Archive *fout)
 		pg_fatal("unrecognized locale provider: %s",
 				 datlocprovider);
 
-	if (strlen(collate) > 0 && strcmp(collate, ctype) == 0)
+	if (!locale && datlocprovider[0] != 'c')
+		pg_log_warning("database '%s' with provider '%s' missing datlocale",
+					   datname, datlocprovider);
+
+	if (locale && datlocprovider[0] == 'c')
+		pg_log_warning("database '%s' with provider 'c' has non-NULL locale '%s'",
+					   datname, locale);
+
+	/* if collate and ctype are equal, and locale is NULL, use LOCALE */
+	if (!locale && strlen(collate) > 0 && strcmp(collate, ctype) == 0)
+		locale = collate;
+
+	/* output LC_COLLATE and LC_CTYPE if different from LOCALE */
+	if (strlen(collate) > 0 && (!locale || strcmp(collate, locale) != 0))
 	{
-		appendPQExpBufferStr(creaQry, " LOCALE = ");
+		appendPQExpBufferStr(creaQry, " LC_COLLATE = ");
 		appendStringLiteralAH(creaQry, collate, fout);
 	}
-	else
+	if (strlen(ctype) > 0 && (!locale || strcmp(ctype, locale) != 0))
 	{
-		if (strlen(collate) > 0)
-		{
-			appendPQExpBufferStr(creaQry, " LC_COLLATE = ");
-			appendStringLiteralAH(creaQry, collate, fout);
-		}
-		if (strlen(ctype) > 0)
-		{
-			appendPQExpBufferStr(creaQry, " LC_CTYPE = ");
-			appendStringLiteralAH(creaQry, ctype, fout);
-		}
+		appendPQExpBufferStr(creaQry, " LC_CTYPE = ");
+		appendStringLiteralAH(creaQry, ctype, fout);
 	}
+
 	if (locale)
 	{
-		appendPQExpBufferStr(creaQry, " ICU_LOCALE = ");
+		appendPQExpBufferStr(creaQry, " LOCALE = ");
 		appendStringLiteralAH(creaQry, locale, fout);
 	}
 
@@ -13870,7 +13878,9 @@ dumpCollation(Archive *fout, const CollInfo *collinfo)
 					  fmtQualifiedDumpable(collinfo));
 
 	appendPQExpBufferStr(q, "provider = ");
-	if (collprovider[0] == 'c')
+	if (collprovider[0] == 'b')
+		appendPQExpBufferStr(q, "builtin");
+	else if (collprovider[0] == 'c')
 		appendPQExpBufferStr(q, "libc");
 	else if (collprovider[0] == 'i')
 		appendPQExpBufferStr(q, "icu");
@@ -13891,6 +13901,13 @@ dumpCollation(Archive *fout, const CollInfo *collinfo)
 
 		/* no locale -- the default collation cannot be reloaded anyway */
 	}
+	else if (collprovider[0] == 'b')
+	{
+		if (collcollate || collctype || colllocale || collicurules)
+			pg_log_warning("invalid collation \"%s\"", qcollname);
+
+		appendPQExpBufferStr(q, ", locale = 'C'");
+	}
 	else if (collprovider[0] == 'i')
 	{
 		if (fout->remoteVersion >= 150000)
diff --git a/src/bin/pg_upgrade/t/002_pg_upgrade.pl b/src/bin/pg_upgrade/t/002_pg_upgrade.pl
index 41d06d272b..94bf086ba8 100644
--- a/src/bin/pg_upgrade/t/002_pg_upgrade.pl
+++ b/src/bin/pg_upgrade/t/002_pg_upgrade.pl
@@ -110,13 +110,16 @@ my $oldversion = int($oldnode->pg_version =~ s/([0-9]*).*/$1/rg);
 # can test that pg_upgrade copies the locale settings of template0
 # from the old to the new cluster.
 
-my $original_encoding = "6";    # UTF-8
-my $original_provider = "c";
-my $original_locale = "C";
-my $original_datlocale = "";
-my $provider_field = "'c' AS datlocprovider";
-my $datlocale_field = "NULL AS datlocale";
-if ($oldversion >= 15 && $ENV{with_icu} eq 'yes')
+my %encoding_number = ('UTF-8' => 6, 'SQL_ASCII' => 0);
+my $provider_field;
+my $datlocale_field;
+my $original_encoding;
+my $original_provider;
+my $original_datcollate = "C";
+my $original_datctype = "C";
+my $original_datlocale;
+
+if ($oldversion >= 15)
 {
 	$provider_field = "datlocprovider";
 	if ($oldversion >= 17)
@@ -127,18 +130,52 @@ if ($oldversion >= 15 && $ENV{with_icu} eq 'yes')
 	{
 		$datlocale_field = "daticulocale AS datlocale";
 	}
+}
+else
+{
+	$provider_field = "'c' AS datlocprovider";
+	$datlocale_field = "NULL AS datlocale";
+}
+
+if ($oldversion >= 17)
+{
+	$original_encoding = "UTF-8";
+	$original_provider = "b";
+	$original_datlocale = "C.UTF-8";
+}
+elsif ($oldversion >= 15 && $ENV{with_icu} eq 'yes')
+{
+	$original_encoding = "UTF-8";
 	$original_provider = "i";
 	$original_datlocale = "fr-CA";
 }
+else
+{
+	my $original_encoding = "SQL_ASCII";
+	my $original_provider = "c";
+	my $original_datlocale = "";
+}
 
 my @initdb_params = @custom_opts;
 
-push @initdb_params, ('--encoding', 'UTF-8');
-push @initdb_params, ('--locale', $original_locale);
-if ($original_provider eq "i")
+push @initdb_params, ('--encoding', $original_encoding);
+push @initdb_params, ('--lc-collate', $original_datcollate);
+push @initdb_params, ('--lc-ctype', $original_datctype);
+
+# add --locale-provider, if supported
+my %provider_name = ('b' => 'builtin', 'i' => 'icu', 'c' => 'libc');
+if ($oldnode->pg_version >= 15)
 {
-	push @initdb_params, ('--locale-provider', 'icu');
-	push @initdb_params, ('--icu-locale', 'fr-CA');
+	push @initdb_params,
+	  ('--locale-provider', $provider_name{$original_provider});
+	if ($original_provider eq 'b')
+	{
+		push @initdb_params, ('--builtin-locale', $original_datlocale);
+	}
+	elsif ($original_provider eq 'i')
+	{
+		push @initdb_params, ('--icu-locale', $original_datlocale);
+	}
 }
 
 $node_params{extra} = \@initdb_params;
@@ -151,7 +188,7 @@ $result = $oldnode->safe_psql(
 	"SELECT encoding, $provider_field, datcollate, datctype, $datlocale_field
                  FROM pg_database WHERE datname='template0'");
 is( $result,
-	"$original_encoding|$original_provider|$original_locale|$original_locale|$original_datlocale",
+	"$encoding_number{$original_encoding}|$original_provider|$original_datcollate|$original_datctype|$original_datlocale",
 	"check locales in original cluster");
 
 # The default location of the source code is the root of this directory.
@@ -327,7 +364,8 @@ if (defined($ENV{oldinstall}))
 }
 
 # Create an invalid database, will be deleted below
-$oldnode->safe_psql('postgres', qq(
+$oldnode->safe_psql(
+	'postgres', qq(
   CREATE DATABASE regression_invalid;
   UPDATE pg_database SET datconnlimit = -2 WHERE datname = 'regression_invalid';
 ));
@@ -370,7 +408,7 @@ command_checks_all(
 		$mode, '--check',
 	],
 	1,
-	[qr/invalid/], # pg_upgrade prints errors on stdout :(
+	[qr/invalid/],    # pg_upgrade prints errors on stdout :(
 	[qr//],
 	'invalid database causes failure');
 rmtree($newnode->data_dir . "/pg_upgrade_output.d");
@@ -434,7 +472,7 @@ $result = $newnode->safe_psql(
 	"SELECT encoding, $provider_field, datcollate, datctype, $datlocale_field
                  FROM pg_database WHERE datname='template0'");
 is( $result,
-	"$original_encoding|$original_provider|$original_locale|$original_locale|$original_datlocale",
+	"$encoding_number{$original_encoding}|$original_provider|$original_datcollate|$original_datctype|$original_datlocale",
 	"check that locales in new cluster match original cluster");
 
 # Second dump from the upgraded instance.
diff --git a/src/bin/psql/describe.c b/src/bin/psql/describe.c
index b943569050..c649477505 100644
--- a/src/bin/psql/describe.c
+++ b/src/bin/psql/describe.c
@@ -926,7 +926,7 @@ listAllDbs(const char *pattern, bool verbose)
 					  gettext_noop("Encoding"));
 	if (pset.sversion >= 150000)
 		appendPQExpBuffer(&buf,
-						  "  CASE d.datlocprovider WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
+						  "  CASE d.datlocprovider WHEN 'b' THEN 'builtin' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
 						  gettext_noop("Locale Provider"));
 	else
 		appendPQExpBuffer(&buf,
@@ -4974,7 +4974,7 @@ listCollations(const char *pattern, bool verbose, bool showSystem)
 
 	if (pset.sversion >= 100000)
 		appendPQExpBuffer(&buf,
-						  "  CASE c.collprovider WHEN 'd' THEN 'default' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
+						  "  CASE c.collprovider WHEN 'd' THEN 'default' WHEN 'b' THEN 'builtin' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
 						  gettext_noop("Provider"));
 	else
 		appendPQExpBuffer(&buf,
diff --git a/src/bin/scripts/createdb.c b/src/bin/scripts/createdb.c
index 14970a6a5f..4af4b98181 100644
--- a/src/bin/scripts/createdb.c
+++ b/src/bin/scripts/createdb.c
@@ -40,8 +40,9 @@ main(int argc, char *argv[])
 		{"locale", required_argument, NULL, 'l'},
 		{"maintenance-db", required_argument, NULL, 3},
 		{"locale-provider", required_argument, NULL, 4},
-		{"icu-locale", required_argument, NULL, 5},
-		{"icu-rules", required_argument, NULL, 6},
+		{"builtin-locale", required_argument, NULL, 5},
+		{"icu-locale", required_argument, NULL, 6},
+		{"icu-rules", required_argument, NULL, 7},
 		{NULL, 0, NULL, 0}
 	};
 
@@ -67,6 +68,7 @@ main(int argc, char *argv[])
 	char	   *lc_ctype = NULL;
 	char	   *locale = NULL;
 	char	   *locale_provider = NULL;
+	char	   *builtin_locale = NULL;
 	char	   *icu_locale = NULL;
 	char	   *icu_rules = NULL;
 
@@ -134,9 +136,12 @@ main(int argc, char *argv[])
 				locale_provider = pg_strdup(optarg);
 				break;
 			case 5:
-				icu_locale = pg_strdup(optarg);
+				builtin_locale = pg_strdup(optarg);
 				break;
 			case 6:
+				icu_locale = pg_strdup(optarg);
+				break;
+			case 7:
 				icu_rules = pg_strdup(optarg);
 				break;
 			default:
@@ -216,6 +221,11 @@ main(int argc, char *argv[])
 		appendPQExpBufferStr(&sql, " LOCALE ");
 		appendStringLiteralConn(&sql, locale, conn);
 	}
+	if (builtin_locale)
+	{
+		appendPQExpBufferStr(&sql, " BUILTIN_LOCALE ");
+		appendStringLiteralConn(&sql, builtin_locale, conn);
+	}
 	if (lc_collate)
 	{
 		appendPQExpBufferStr(&sql, " LC_COLLATE ");
@@ -296,7 +306,7 @@ help(const char *progname)
 	printf(_("      --lc-ctype=LOCALE        LC_CTYPE setting for the database\n"));
 	printf(_("      --icu-locale=LOCALE      ICU locale setting for the database\n"));
 	printf(_("      --icu-rules=RULES        ICU rules setting for the database\n"));
-	printf(_("      --locale-provider={libc|icu}\n"
+	printf(_("      --locale-provider={builtin|libc|icu}\n"
 			 "                               locale provider for the database's default collation\n"));
 	printf(_("  -O, --owner=OWNER            database user to own the new database\n"));
 	printf(_("  -S, --strategy=STRATEGY      database creation strategy wal_log or file_copy\n"));
diff --git a/src/bin/scripts/t/020_createdb.pl b/src/bin/scripts/t/020_createdb.pl
index 37e47b0078..3ba623f9d1 100644
--- a/src/bin/scripts/t/020_createdb.pl
+++ b/src/bin/scripts/t/020_createdb.pl
@@ -105,6 +105,84 @@ else
 		'create database with ICU fails since no ICU support');
 }
 
+$node->command_fails(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'tbuiltin1'
+	],
+	'create database with provider "builtin" fails without --locale');
+
+$node->command_ok(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'--locale=C', 'tbuiltin2'
+	],
+	'create database with provider "builtin" and locale "C"');
+
+$node->command_ok(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'--locale=C', '--lc-collate=C',
+		'tbuiltin3'
+	],
+	'create database with provider "builtin" and LC_COLLATE=C');
+
+$node->command_ok(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'--locale=C', '--lc-ctype=C',
+		'tbuiltin4'
+	],
+	'create database with provider "builtin" and LC_CTYPE=C');
+
+$node->command_ok(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'-E UTF-8', '--builtin-locale=C.UTF8',
+		'tbuiltin5'
+	],
+	'create database with provider "builtin" and --builtin-locale C.UTF-8');
+
+$node->command_fails(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'-E LATIN1', '--builtin-locale=C.UTF-8',
+		'tbuiltin6'
+	],
+	'create database with provider "builtin" and --builtin-locale C.UTF-8');
+
+$node->command_fails(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'--locale=C', '--icu-locale=en',
+		'tbuiltin7'
+	],
+	'create database with provider "builtin" and ICU_LOCALE="en"');
+
+$node->command_fails(
+	[
+		'createdb', '-T',
+		'template0', '--locale-provider=builtin',
+		'--locale=C', '--icu-rules=""',
+		'tbuiltin8'
+	],
+	'create database with provider "builtin" and ICU_RULES=""');
+
+$node->command_fails(
+	[
+		'createdb', '-T',
+		'template1', '--locale-provider=builtin',
+		'--locale=C', 'tbuiltin9'
+	],
+	'create database with provider "builtin" not matching template');
+
 $node->command_fails([ 'createdb', 'foobar1' ],
 	'fails if database already exists');
 
diff --git a/src/include/catalog/pg_collation.dat b/src/include/catalog/pg_collation.dat
index 7396ff10c4..938432e8a4 100644
--- a/src/include/catalog/pg_collation.dat
+++ b/src/include/catalog/pg_collation.dat
@@ -23,9 +23,9 @@
   descr => 'standard POSIX collation',
   collname => 'POSIX', collprovider => 'c', collencoding => '-1',
   collcollate => 'POSIX', collctype => 'POSIX' },
-{ oid => '962', descr => 'sorts by Unicode code point',
-  collname => 'ucs_basic', collprovider => 'c', collencoding => '6',
-  collcollate => 'C', collctype => 'C' },
+{ oid => '962', descr => 'sorts by Unicode code point, C character semantics',
+  collname => 'ucs_basic', collprovider => 'b', collencoding => '6',
+  colllocale => 'C' },
 { oid => '963',
   descr => 'sorts using the Unicode Collation Algorithm with default settings',
   collname => 'unicode', collprovider => 'i', collencoding => '-1',
diff --git a/src/include/catalog/pg_collation.h b/src/include/catalog/pg_collation.h
index a3e196fb53..5ce289d74b 100644
--- a/src/include/catalog/pg_collation.h
+++ b/src/include/catalog/pg_collation.h
@@ -68,6 +68,7 @@ MAKE_SYSCACHE(COLLOID, pg_collation_oid_index, 8);
 #ifdef EXPOSE_TO_CLIENT_CODE
 
 #define COLLPROVIDER_DEFAULT	'd'
+#define COLLPROVIDER_BUILTIN	'b'
 #define COLLPROVIDER_ICU		'i'
 #define COLLPROVIDER_LIBC		'c'
 
@@ -76,6 +77,8 @@ collprovider_name(char c)
 {
 	switch (c)
 	{
+		case COLLPROVIDER_BUILTIN:
+			return "builtin";
 		case COLLPROVIDER_ICU:
 			return "icu";
 		case COLLPROVIDER_LIBC:
diff --git a/src/include/utils/pg_locale.h b/src/include/utils/pg_locale.h
index 28c925b5af..2348f56b6f 100644
--- a/src/include/utils/pg_locale.h
+++ b/src/include/utils/pg_locale.h
@@ -76,6 +76,14 @@ struct pg_locale_struct
 	bool		deterministic;
 	union
 	{
+		struct
+		{
+			const char *locale;
+			bool		casemap_full;
+			bool		adjust_to_cased;
+			bool		titlecase;
+			bool		properties_posix;
+		}			builtin;
 		locale_t	lt;
 #ifdef USE_ICU
 		struct
@@ -112,7 +120,7 @@ extern size_t pg_strxfrm_prefix(char *dest, const char *src, size_t destsize,
 								pg_locale_t locale);
 extern size_t pg_strnxfrm_prefix(char *dest, size_t destsize, const char *src,
 								 size_t srclen, pg_locale_t locale);
-
+extern const char *builtin_validate_locale(int encoding, const char *loc_str);
 extern void icu_validate_locale(const char *loc_str);
 extern char *icu_language_tag(const char *loc_str, int elevel);
 
diff --git a/src/test/icu/t/010_database.pl b/src/test/icu/t/010_database.pl
index 8a1fc12ec6..5f8ef16803 100644
--- a/src/test/icu/t/010_database.pl
+++ b/src/test/icu/t/010_database.pl
@@ -27,9 +27,8 @@ CREATE TABLE icu (def text, en text COLLATE "en-x-icu", upfirst text COLLATE upp
 INSERT INTO icu VALUES ('a', 'a', 'a'), ('b', 'b', 'b'), ('A', 'A', 'A'), ('B', 'B', 'B');
 });
 
-is( $node1->safe_psql('dbicu', q{SELECT icu_unicode_version() IS NOT NULL}),
-	qq(t),
-	'ICU unicode version defined');
+is($node1->safe_psql('dbicu', q{SELECT icu_unicode_version() IS NOT NULL}),
+	qq(t), 'ICU unicode version defined');
 
 is( $node1->safe_psql('dbicu', q{SELECT def FROM icu ORDER BY def}),
 	qq(A
@@ -63,14 +62,13 @@ is( $node1->psql(
 	0,
 	"C locale works for ICU");
 
-# Test that LOCALE works for ICU locales if LC_COLLATE and LC_CTYPE
-# are specified
-is( $node1->psql(
-		'postgres',
-		q{CREATE DATABASE dbicu2 LOCALE_PROVIDER icu LOCALE '@colStrength=primary'
-      LC_COLLATE='C' LC_CTYPE='C' TEMPLATE template0 ENCODING UTF8}
-	),
-	0,
-	"LOCALE works for ICU locales if LC_COLLATE and LC_CTYPE are specified");
+my ($ret, $stdout, $stderr) = $node1->psql('postgres',
+	q{CREATE DATABASE dbicu LOCALE_PROVIDER builtin LOCALE 'C' TEMPLATE dbicu}
+);
+isnt($ret, 0, "locale provider must match template: exit code not 0");
+like(
+	$stderr,
+	qr/ERROR:  new locale provider \(builtin\) does not match locale provider of the template database \(icu\)/,
+	"locale provider must match template: error message");
 
 done_testing();
diff --git a/src/test/regress/expected/collate.out b/src/test/regress/expected/collate.out
index 0649564485..ece4a8e99d 100644
--- a/src/test/regress/expected/collate.out
+++ b/src/test/regress/expected/collate.out
@@ -650,6 +650,26 @@ EXPLAIN (COSTS OFF)
 (3 rows)
 
 -- CREATE/DROP COLLATION
+CREATE COLLATION builtin_c ( PROVIDER = builtin, LOCALE = "C" );
+CREATE COLLATION builtin_posix ( PROVIDER = builtin, LOCALE = "POSIX" );
+SELECT b FROM collate_test1 ORDER BY b COLLATE builtin_c;
+  b  
+-----
+ ABD
+ Abc
+ abc
+ bbc
+(4 rows)
+
+CREATE COLLATION builtin2 ( PROVIDER = builtin ); -- fails
+ERROR:  parameter "locale" must be specified
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "en_US" ); -- fails
+ERROR:  invalid locale name "en_US" for builtin provider
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LC_CTYPE = "C", LC_COLLATE = "C" ); -- fails
+ERROR:  parameter "locale" must be specified
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "POSIX", LC_CTYPE = "POSIX" ); -- fails
+ERROR:  conflicting or redundant options
+DETAIL:  LOCALE cannot be specified together with LC_COLLATE or LC_CTYPE.
 CREATE COLLATION mycoll1 FROM "C";
 CREATE COLLATION mycoll2 ( LC_COLLATE = "POSIX", LC_CTYPE = "POSIX" );
 CREATE COLLATION mycoll3 FROM "default";  -- intentionally unsupported
@@ -754,7 +774,7 @@ DETAIL:  FROM cannot be specified together with any other options.
 -- must get rid of them.
 --
 DROP SCHEMA collate_tests CASCADE;
-NOTICE:  drop cascades to 19 other objects
+NOTICE:  drop cascades to 21 other objects
 DETAIL:  drop cascades to table collate_test1
 drop cascades to table collate_test_like
 drop cascades to table collate_test2
@@ -771,6 +791,8 @@ drop cascades to function dup(anyelement)
 drop cascades to table collate_test20
 drop cascades to table collate_test21
 drop cascades to table collate_test22
+drop cascades to collation builtin_c
+drop cascades to collation builtin_posix
 drop cascades to collation mycoll2
 drop cascades to table collate_test23
 drop cascades to view collate_on_int
diff --git a/src/test/regress/expected/collate.utf8.out b/src/test/regress/expected/collate.utf8.out
new file mode 100644
index 0000000000..7535df9024
--- /dev/null
+++ b/src/test/regress/expected/collate.utf8.out
@@ -0,0 +1,288 @@
+/*
+ * This test is for collations and character operations when using the
+ * builtin provider with the C.UTF-8 locale.
+ */
+/* skip test if not UTF8 server encoding */
+SELECT getdatabaseencoding() <> 'UTF8' AS skip_test \gset
+\if :skip_test
+\quit
+\endif
+SET client_encoding TO UTF8;
+--
+-- Test builtin PG_UNICODE_FAST locale.
+--
+CREATE COLLATION BUILTIN_UNICODE_FAST
+  ( provider = builtin, locale = 'PG_UNICODE_FAST' );
+CREATE TABLE builtin_test1 (
+  t TEXT COLLATE BUILTIN_UNICODE_FAST
+);
+INSERT INTO builtin_test1 VALUES
+  ('abc DEF 123abc'),
+  ('ábc sßs ßss DÉF'),
+  ('ǄxxǄ ǆxxǅ ǅxxǆ'),
+  ('ȺȺȺ'),
+  ('ⱥⱥⱥ'),
+  ('ⱥȺ');
+SELECT
+    t, lower(t), initcap(t), upper(t),
+    length(convert_to(t, 'UTF8')) AS t_bytes,
+    length(convert_to(lower(t), 'UTF8')) AS lower_t_bytes,
+    length(convert_to(initcap(t), 'UTF8')) AS initcap_t_bytes,
+    length(convert_to(upper(t), 'UTF8')) AS upper_t_bytes
+  FROM builtin_test1;
+        t        |      lower      |     initcap      |       upper       | t_bytes | lower_t_bytes | initcap_t_bytes | upper_t_bytes 
+-----------------+-----------------+------------------+-------------------+---------+---------------+-----------------+---------------
+ abc DEF 123abc  | abc def 123abc  | Abc Def 123Abc   | ABC DEF 123ABC    |      14 |            14 |              14 |            14
+ ábc sßs ßss DÉF | ábc sßs ßss déf | Ábc Sßs Ssss Déf | ÁBC SSSS SSSS DÉF |      19 |            19 |              19 |            19
+ ǄxxǄ ǆxxǅ ǅxxǆ  | ǆxxǆ ǆxxǆ ǆxxǆ  | ǅxxǆ ǅxxǆ ǅxxǆ   | ǄXXǄ ǄXXǄ ǄXXǄ    |      20 |            20 |              20 |            20
+ ȺȺȺ             | ⱥⱥⱥ             | Ⱥⱥⱥ              | ȺȺȺ               |       6 |             9 |               8 |             6
+ ⱥⱥⱥ             | ⱥⱥⱥ             | Ⱥⱥⱥ              | ȺȺȺ               |       9 |             9 |               8 |             6
+ ⱥȺ              | ⱥⱥ              | Ⱥⱥ               | ȺȺ                |       5 |             6 |               5 |             4
+(6 rows)
+
+DROP TABLE builtin_test1;
+-- test Final_Sigma
+SELECT lower('ΑΣ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 03A3
+ lower 
+-------
+ ας
+(1 row)
+
+SELECT lower('ΑΣ0' COLLATE BUILTIN_UNICODE_FAST); -- 0391 03A3 0030
+ lower 
+-------
+ ας0
+(1 row)
+
+SELECT lower('ἈΣ̓' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0343 03A3 0343
+ lower 
+-------
+ ἀς̓
+(1 row)
+
+SELECT lower('ᾼΣͅ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0345 03A3 0345
+ lower 
+-------
+ ᾳςͅ
+(1 row)
+
+-- test !Final_Sigma
+SELECT lower('Σ' COLLATE BUILTIN_UNICODE_FAST); -- 03A3
+ lower 
+-------
+ σ
+(1 row)
+
+SELECT lower('0Σ' COLLATE BUILTIN_UNICODE_FAST); -- 0030 03A3
+ lower 
+-------
+ 0σ
+(1 row)
+
+SELECT lower('ΑΣΑ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 03A3 0391
+ lower 
+-------
+ ασα
+(1 row)
+
+SELECT lower('ἈΣ̓Α' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0343 03A3 0343 0391
+ lower 
+-------
+ ἀσ̓α
+(1 row)
+
+SELECT lower('ᾼΣͅΑ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0345 03A3 0345 0391
+ lower 
+-------
+ ᾳσͅα
+(1 row)
+
+-- properties
+SELECT 'xyz' ~ '[[:alnum:]]' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'xyz' !~ '[[:upper:]]' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT '@' !~ '[[:alnum:]]' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT '=' !~ '[[:punct:]]' COLLATE BUILTIN_UNICODE_FAST; -- symbols are not punctuation
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'a8a' ~ '[[:digit:]]' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT '൧' ~ '\d' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+-- case mapping
+SELECT 'xYz' ~* 'XyZ' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'xAb' ~* '[W-Y]' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'xAb' !~* '[c-d]' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'Δ' ~* '[α-λ]' COLLATE BUILTIN_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'δ' ~* '[Γ-Λ]' COLLATE BUILTIN_UNICODE_FAST; -- same as above with cases reversed
+ ?column? 
+----------
+ t
+(1 row)
+
+DROP COLLATION BUILTIN_UNICODE_FAST;
+--
+-- Test builtin C.UTF-8 locale.
+--
+CREATE COLLATION BUILTIN_C_UTF8 ( provider = builtin, locale = 'C.UTF-8' );
+CREATE TABLE builtin_test2 (
+  t TEXT COLLATE BUILTIN_C_UTF8
+);
+INSERT INTO builtin_test2 VALUES
+  ('abc DEF 123abc'),
+  ('ábc sßs ßss DÉF'),
+  ('ǄxxǄ ǆxxǅ ǅxxǆ'),
+  ('ȺȺȺ'),
+  ('ⱥⱥⱥ'),
+  ('ⱥȺ');
+SELECT
+    t, lower(t), initcap(t), upper(t),
+    length(convert_to(t, 'UTF8')) AS t_bytes,
+    length(convert_to(lower(t), 'UTF8')) AS lower_t_bytes,
+    length(convert_to(initcap(t), 'UTF8')) AS initcap_t_bytes,
+    length(convert_to(upper(t), 'UTF8')) AS upper_t_bytes
+  FROM builtin_test2;
+        t        |      lower      |     initcap     |      upper      | t_bytes | lower_t_bytes | initcap_t_bytes | upper_t_bytes 
+-----------------+-----------------+-----------------+-----------------+---------+---------------+-----------------+---------------
+ abc DEF 123abc  | abc def 123abc  | Abc Def 123abc  | ABC DEF 123ABC  |      14 |            14 |              14 |            14
+ ábc sßs ßss DÉF | ábc sßs ßss déf | Ábc Sßs ßss Déf | ÁBC SßS ßSS DÉF |      19 |            19 |              19 |            19
+ ǄxxǄ ǆxxǅ ǅxxǆ  | ǆxxǆ ǆxxǆ ǆxxǆ  | Ǆxxǆ Ǆxxǆ Ǆxxǆ  | ǄXXǄ ǄXXǄ ǄXXǄ  |      20 |            20 |              20 |            20
+ ȺȺȺ             | ⱥⱥⱥ             | Ⱥⱥⱥ             | ȺȺȺ             |       6 |             9 |               8 |             6
+ ⱥⱥⱥ             | ⱥⱥⱥ             | Ⱥⱥⱥ             | ȺȺȺ             |       9 |             9 |               8 |             6
+ ⱥȺ              | ⱥⱥ              | Ⱥⱥ              | ȺȺ              |       5 |             6 |               5 |             4
+(6 rows)
+
+DROP TABLE builtin_test2;
+-- negative test: Final_Sigma not used for builtin locale C.UTF-8
+SELECT lower('ΑΣ' COLLATE BUILTIN_C_UTF8);
+ lower 
+-------
+ ασ
+(1 row)
+
+SELECT lower('ΑͺΣͺ' COLLATE BUILTIN_C_UTF8);
+ lower 
+-------
+ αͺσͺ
+(1 row)
+
+SELECT lower('Α΄Σ΄' COLLATE BUILTIN_C_UTF8);
+ lower 
+-------
+ α΄σ΄
+(1 row)
+
+-- properties
+SELECT 'xyz' ~ '[[:alnum:]]' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'xyz' !~ '[[:upper:]]' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT '@' !~ '[[:alnum:]]' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT '=' ~ '[[:punct:]]' COLLATE BUILTIN_C_UTF8; -- symbols are punctuation in posix
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'a8a' ~ '[[:digit:]]' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT '൧' !~ '\d' COLLATE BUILTIN_C_UTF8; -- only 0-9 considered digits in posix
+ ?column? 
+----------
+ t
+(1 row)
+
+-- case mapping
+SELECT 'xYz' ~* 'XyZ' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'xAb' ~* '[W-Y]' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'xAb' !~* '[c-d]' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'Δ' ~* '[α-λ]' COLLATE BUILTIN_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT 'δ' ~* '[Γ-Λ]' COLLATE BUILTIN_C_UTF8; -- same as above with cases reversed
+ ?column? 
+----------
+ t
+(1 row)
+
+DROP COLLATION BUILTIN_C_UTF8;
diff --git a/src/test/regress/expected/collate.utf8_1.out b/src/test/regress/expected/collate.utf8_1.out
new file mode 100644
index 0000000000..e73fdf50c3
--- /dev/null
+++ b/src/test/regress/expected/collate.utf8_1.out
@@ -0,0 +1,8 @@
+/*
+ * This test is for collations and character operations when using the
+ * builtin provider with the C.UTF-8 locale.
+ */
+/* skip test if not UTF8 server encoding */
+SELECT getdatabaseencoding() <> 'UTF8' AS skip_test \gset
+\if :skip_test
+\quit
diff --git a/src/test/regress/parallel_schedule b/src/test/regress/parallel_schedule
index 1d8a414eea..e48cb4b7a3 100644
--- a/src/test/regress/parallel_schedule
+++ b/src/test/regress/parallel_schedule
@@ -78,9 +78,9 @@ test: brin_bloom brin_multi
 # psql depends on create_am
 # amutils depends on geometry, create_index_spgist, hash_index, brin
 # ----------
-test: create_table_like alter_generic alter_operator misc async dbsize merge misc_functions sysviews tsrf tid tidscan tidrangescan collate.icu.utf8 incremental_sort create_role without_overlaps
+test: create_table_like alter_generic alter_operator misc async dbsize merge misc_functions sysviews tsrf tid tidscan tidrangescan collate.utf8 collate.icu.utf8 incremental_sort create_role without_overlaps
 
-# collate.*.utf8 tests cannot be run in parallel with each other
+# collate.linux.utf8 and collate.icu.utf8 tests cannot be run in parallel with each other
 test: rules psql psql_crosstab amutils stats_ext collate.linux.utf8 collate.windows.win1252
 
 # ----------
diff --git a/src/test/regress/sql/collate.sql b/src/test/regress/sql/collate.sql
index c3d40fc195..01d5c69fe4 100644
--- a/src/test/regress/sql/collate.sql
+++ b/src/test/regress/sql/collate.sql
@@ -244,6 +244,16 @@ EXPLAIN (COSTS OFF)
 
 -- CREATE/DROP COLLATION
 
+CREATE COLLATION builtin_c ( PROVIDER = builtin, LOCALE = "C" );
+CREATE COLLATION builtin_posix ( PROVIDER = builtin, LOCALE = "POSIX" );
+
+SELECT b FROM collate_test1 ORDER BY b COLLATE builtin_c;
+
+CREATE COLLATION builtin2 ( PROVIDER = builtin ); -- fails
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "en_US" ); -- fails
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LC_CTYPE = "C", LC_COLLATE = "C" ); -- fails
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "POSIX", LC_CTYPE = "POSIX" ); -- fails
+
 CREATE COLLATION mycoll1 FROM "C";
 CREATE COLLATION mycoll2 ( LC_COLLATE = "POSIX", LC_CTYPE = "POSIX" );
 CREATE COLLATION mycoll3 FROM "default";  -- intentionally unsupported
diff --git a/src/test/regress/sql/collate.utf8.sql b/src/test/regress/sql/collate.utf8.sql
new file mode 100644
index 0000000000..ba92dcb7d2
--- /dev/null
+++ b/src/test/regress/sql/collate.utf8.sql
@@ -0,0 +1,123 @@
+/*
+ * This test is for collations and character operations when using the
+ * builtin provider with the C.UTF-8 locale.
+ */
+
+/* skip test if not UTF8 server encoding */
+SELECT getdatabaseencoding() <> 'UTF8' AS skip_test \gset
+\if :skip_test
+\quit
+\endif
+
+SET client_encoding TO UTF8;
+
+--
+-- Test builtin PG_UNICODE_FAST locale.
+--
+
+CREATE COLLATION BUILTIN_UNICODE_FAST
+  ( provider = builtin, locale = 'PG_UNICODE_FAST' );
+
+CREATE TABLE builtin_test1 (
+  t TEXT COLLATE BUILTIN_UNICODE_FAST
+);
+INSERT INTO builtin_test1 VALUES
+  ('abc DEF 123abc'),
+  ('ábc sßs ßss DÉF'),
+  ('ǄxxǄ ǆxxǅ ǅxxǆ'),
+  ('ȺȺȺ'),
+  ('ⱥⱥⱥ'),
+  ('ⱥȺ');
+
+SELECT
+    t, lower(t), initcap(t), upper(t),
+    length(convert_to(t, 'UTF8')) AS t_bytes,
+    length(convert_to(lower(t), 'UTF8')) AS lower_t_bytes,
+    length(convert_to(initcap(t), 'UTF8')) AS initcap_t_bytes,
+    length(convert_to(upper(t), 'UTF8')) AS upper_t_bytes
+  FROM builtin_test1;
+
+DROP TABLE builtin_test1;
+
+-- test Final_Sigma
+SELECT lower('ΑΣ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 03A3
+SELECT lower('ΑΣ0' COLLATE BUILTIN_UNICODE_FAST); -- 0391 03A3 0030
+SELECT lower('ἈΣ̓' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0343 03A3 0343
+SELECT lower('ᾼΣͅ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0345 03A3 0345
+
+-- test !Final_Sigma
+SELECT lower('Σ' COLLATE BUILTIN_UNICODE_FAST); -- 03A3
+SELECT lower('0Σ' COLLATE BUILTIN_UNICODE_FAST); -- 0030 03A3
+SELECT lower('ΑΣΑ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 03A3 0391
+SELECT lower('ἈΣ̓Α' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0343 03A3 0343 0391
+SELECT lower('ᾼΣͅΑ' COLLATE BUILTIN_UNICODE_FAST); -- 0391 0345 03A3 0345 0391
+
+-- properties
+
+SELECT 'xyz' ~ '[[:alnum:]]' COLLATE BUILTIN_UNICODE_FAST;
+SELECT 'xyz' !~ '[[:upper:]]' COLLATE BUILTIN_UNICODE_FAST;
+SELECT '@' !~ '[[:alnum:]]' COLLATE BUILTIN_UNICODE_FAST;
+SELECT '=' !~ '[[:punct:]]' COLLATE BUILTIN_UNICODE_FAST; -- symbols are not punctuation
+SELECT 'a8a' ~ '[[:digit:]]' COLLATE BUILTIN_UNICODE_FAST;
+SELECT '൧' ~ '\d' COLLATE BUILTIN_UNICODE_FAST;
+
+-- case mapping
+
+SELECT 'xYz' ~* 'XyZ' COLLATE BUILTIN_UNICODE_FAST;
+SELECT 'xAb' ~* '[W-Y]' COLLATE BUILTIN_UNICODE_FAST;
+SELECT 'xAb' !~* '[c-d]' COLLATE BUILTIN_UNICODE_FAST;
+SELECT 'Δ' ~* '[α-λ]' COLLATE BUILTIN_UNICODE_FAST;
+SELECT 'δ' ~* '[Γ-Λ]' COLLATE BUILTIN_UNICODE_FAST; -- same as above with cases reversed
+
+DROP COLLATION BUILTIN_UNICODE_FAST;
+
+--
+-- Test builtin C.UTF-8 locale.
+--
+
+CREATE COLLATION BUILTIN_C_UTF8 ( provider = builtin, locale = 'C.UTF-8' );
+
+CREATE TABLE builtin_test2 (
+  t TEXT COLLATE BUILTIN_C_UTF8
+);
+INSERT INTO builtin_test2 VALUES
+  ('abc DEF 123abc'),
+  ('ábc sßs ßss DÉF'),
+  ('ǄxxǄ ǆxxǅ ǅxxǆ'),
+  ('ȺȺȺ'),
+  ('ⱥⱥⱥ'),
+  ('ⱥȺ');
+
+SELECT
+    t, lower(t), initcap(t), upper(t),
+    length(convert_to(t, 'UTF8')) AS t_bytes,
+    length(convert_to(lower(t), 'UTF8')) AS lower_t_bytes,
+    length(convert_to(initcap(t), 'UTF8')) AS initcap_t_bytes,
+    length(convert_to(upper(t), 'UTF8')) AS upper_t_bytes
+  FROM builtin_test2;
+
+DROP TABLE builtin_test2;
+
+-- negative test: Final_Sigma not used for builtin locale C.UTF-8
+SELECT lower('ΑΣ' COLLATE BUILTIN_C_UTF8);
+SELECT lower('ΑͺΣͺ' COLLATE BUILTIN_C_UTF8);
+SELECT lower('Α΄Σ΄' COLLATE BUILTIN_C_UTF8);
+
+-- properties
+
+SELECT 'xyz' ~ '[[:alnum:]]' COLLATE BUILTIN_C_UTF8;
+SELECT 'xyz' !~ '[[:upper:]]' COLLATE BUILTIN_C_UTF8;
+SELECT '@' !~ '[[:alnum:]]' COLLATE BUILTIN_C_UTF8;
+SELECT '=' ~ '[[:punct:]]' COLLATE BUILTIN_C_UTF8; -- symbols are punctuation in posix
+SELECT 'a8a' ~ '[[:digit:]]' COLLATE BUILTIN_C_UTF8;
+SELECT '൧' !~ '\d' COLLATE BUILTIN_C_UTF8; -- only 0-9 considered digits in posix
+
+-- case mapping
+
+SELECT 'xYz' ~* 'XyZ' COLLATE BUILTIN_C_UTF8;
+SELECT 'xAb' ~* '[W-Y]' COLLATE BUILTIN_C_UTF8;
+SELECT 'xAb' !~* '[c-d]' COLLATE BUILTIN_C_UTF8;
+SELECT 'Δ' ~* '[α-λ]' COLLATE BUILTIN_C_UTF8;
+SELECT 'δ' ~* '[Γ-Λ]' COLLATE BUILTIN_C_UTF8; -- same as above with cases reversed
+
+DROP COLLATION BUILTIN_C_UTF8;
-- 
2.34.1

