v2-0001-Add-UNIQUE-null-treatment-option.patch

From ffd6c8e0ce24f3c56bd44e71588d4165e20e9157 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Wed, 29 Dec 2021 10:49:57 +0100
Subject: [PATCH v2] Add UNIQUE null treatment option

The SQL standard has been ambiguous about whether null values in
unique constraints should be considered equal or not.  Different
implementations have different behaviors.  In the SQL:202x draft, this
has been formalized by making this implementation-defined and adding
an option on unique constraint definitions UNIQUE [ NULLS [NOT]
DISTINCT ] to choose a behavior explicitly.

This patch adds this option to PostgreSQL.  The default behavior
remains UNIQUE NULLS DISTINCT.  Making this happen in the btree code
is pretty easy; most of the patch is just to carry the flag around to
all the places that need it.

The CREATE UNIQUE INDEX syntax extension is not from the standard,
it's my own invention.

XXX I named all the internal flags, catalog columns, etc. in the
negative ("nulls not distinct") so that the default PostgreSQL
behavior is the default if the flag is false.  But perhaps the double
negatives make some code harder to read.

Discussion: https://www.postgresql.org/message-id/flat/84e5ee1b-387e-9a54-c326-9082674bde78@enterprisedb.com
---
 doc/src/sgml/catalogs.sgml                 | 13 +++++
 doc/src/sgml/ddl.sgml                      | 29 ++++++++--
 doc/src/sgml/information_schema.sgml       | 12 +++++
 doc/src/sgml/ref/alter_table.sgml          |  4 +-
 doc/src/sgml/ref/create_index.sgml         | 13 +++++
 doc/src/sgml/ref/create_table.sgml         | 11 ++--
 src/backend/access/nbtree/nbtinsert.c      | 10 ++--
 src/backend/access/nbtree/nbtsort.c        | 15 +++++-
 src/backend/catalog/index.c                |  7 +++
 src/backend/catalog/information_schema.sql |  9 +++-
 src/backend/catalog/sql_features.txt       |  1 +
 src/backend/catalog/toasting.c             |  1 +
 src/backend/commands/indexcmds.c           |  3 +-
 src/backend/nodes/copyfuncs.c              |  2 +
 src/backend/nodes/equalfuncs.c             |  2 +
 src/backend/nodes/makefuncs.c              |  3 +-
 src/backend/nodes/outfuncs.c               |  2 +
 src/backend/parser/gram.y                  | 47 ++++++++++-------
 src/backend/parser/parse_utilcmd.c         |  3 ++
 src/backend/utils/adt/ruleutils.c          | 23 +++++---
 src/backend/utils/cache/relcache.c         |  1 +
 src/backend/utils/sort/tuplesort.c         |  8 ++-
 src/bin/pg_dump/pg_dump.c                  | 19 +++++--
 src/bin/pg_dump/pg_dump.h                  |  1 +
 src/bin/psql/describe.c                    | 19 +++++--
 src/include/catalog/pg_index.h             |  1 +
 src/include/nodes/execnodes.h              |  1 +
 src/include/nodes/makefuncs.h              |  2 +-
 src/include/nodes/parsenodes.h             |  2 +
 src/include/utils/tuplesort.h              |  1 +
 src/test/regress/expected/constraints.out  | 23 ++++++++
 src/test/regress/expected/create_index.out | 61 ++++++++++++++++++++++
 src/test/regress/sql/constraints.sql       | 14 +++++
 src/test/regress/sql/create_index.sql      | 37 +++++++++++++
 34 files changed, 342 insertions(+), 58 deletions(-)

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 03e2537b07..f0d49e4841 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -4275,6 +4275,19 @@ <title><structname>pg_index</structname> Columns</title>
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>indnullsnotdistinct</structfield> <type>bool</type>
+      </para>
+      <para>
+       This value is only used for unique indexes.  If false, this unique
+       index will consider null values distinct (so the index can contain
+       multiple null values in a column, the default PostgreSQL behavior).  If
+       it is true, it will consider null values to be equal (so the index can
+       only contain one null value in a column).
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>indisprimary</structfield> <type>bool</type>
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 64d9030652..f622285ba0 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -759,14 +759,33 @@ <title>Unique Constraints</title>
     In general, a unique constraint is violated if there is more than
     one row in the table where the values of all of the
     columns included in the constraint are equal.
-    However, two null values are never considered equal in this
+    By default, two null values are not considered equal in this
     comparison.  That means even in the presence of a
     unique constraint it is possible to store duplicate
     rows that contain a null value in at least one of the constrained
-    columns.  This behavior conforms to the SQL standard, but we have
-    heard that other SQL databases might not follow this rule.  So be
-    careful when developing applications that are intended to be
-    portable.
+    columns.  This behavior can be changed by adding the clause <literal>NULLS
+    NOT DISTINCT</literal>, like
+<programlisting>
+CREATE TABLE products (
+    product_no integer UNIQUE <emphasis>NULLS NOT DISTINCT</emphasis>,
+    name text,
+    price numeric
+);
+</programlisting>
+    or
+<programlisting>
+CREATE TABLE products (
+    product_no integer,
+    name text,
+    price numeric,
+    UNIQUE <emphasis>NULLS NOT DISTINCT</emphasis> (product_no)
+);
+</programlisting>
+    The default behavior can be specified explicitly using <literal>NULLS
+    DISTINCT</literal>.  The default null treatment in unique constraints is
+    implementation-defined according to the SQL standard, and other
+    implementations have a different behavior.  So be careful when developing
+    applications that are intended to be portable.
    </para>
   </sect2>
 
diff --git a/doc/src/sgml/information_schema.sgml b/doc/src/sgml/information_schema.sgml
index c5e68c175f..350c75bc31 100644
--- a/doc/src/sgml/information_schema.sgml
+++ b/doc/src/sgml/information_schema.sgml
@@ -6899,6 +6899,18 @@ <title><structname>table_constraints</structname> Columns</title>
        <literal>YES</literal>)
       </para></entry>
      </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>nulls_distinct</structfield> <type>yes_or_no</type>
+      </para>
+      <para>
+       If the constraint is a unique constraint, then <literal>YES</literal>
+       if the constraint treats nulls as distinct or <literal>NO</literal> if
+       it treats nulls as not distinct, otherwise null for other types of
+       constraints.
+      </para></entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index a76e2e7322..b20f6c651c 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -103,7 +103,7 @@
   DEFAULT <replaceable>default_expr</replaceable> |
   GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED |
   GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
-  UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
+  UNIQUE [ NULLS [ NOT ] DISTINCT ] <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
   REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
     [ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
@@ -113,7 +113,7 @@
 
 [ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
 { CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
-  UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
+  UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
   EXCLUDE [ USING <replaceable class="parameter">index_method</replaceable> ] ( <replaceable class="parameter">exclude_element</replaceable> WITH <replaceable class="parameter">operator</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> [ WHERE ( <replaceable class="parameter">predicate</replaceable> ) ] |
   FOREIGN KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ]
diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
index 89a4d746f6..91eaaabc90 100644
--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -24,6 +24,7 @@
 CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class="parameter">name</replaceable> ] ON [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ USING <replaceable class="parameter">method</replaceable> ]
     ( { <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ <replaceable class="parameter">opclass</replaceable> [ ( <replaceable class="parameter">opclass_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] ) ] ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
     [ INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
+    [ NULLS [ NOT ] DISTINCT ]
     [ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
     [ TABLESPACE <replaceable class="parameter">tablespace_name</replaceable> ]
     [ WHERE <replaceable class="parameter">predicate</replaceable> ]
@@ -334,6 +335,18 @@ <title>Parameters</title>
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><literal>NULLS DISTINCT</literal></term>
+      <term><literal>NULLS NOT DISTINCT</literal></term>
+      <listitem>
+       <para>
+        Specifies whether for a unique index, null values should be considered
+        distinct (not equal).  The default is that they are distinct, so that
+        a unique index could contain multiple null values in a column.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><replaceable class="parameter">storage_parameter</replaceable></term>
       <listitem>
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index b97bb9ded1..7e4ef312c0 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -67,7 +67,7 @@
   DEFAULT <replaceable>default_expr</replaceable> |
   GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED |
   GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
-  UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
+  UNIQUE [ NULLS [ NOT ] DISTINCT ] <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
   REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
     [ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
@@ -77,7 +77,7 @@
 
 [ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
 { CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
-  UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
+  UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
   EXCLUDE [ USING <replaceable class="parameter">index_method</replaceable> ] ( <replaceable class="parameter">exclude_element</replaceable> WITH <replaceable class="parameter">operator</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> [ WHERE ( <replaceable class="parameter">predicate</replaceable> ) ] |
   FOREIGN KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ]
@@ -917,8 +917,8 @@ <title>Parameters</title>
    </varlistentry>
 
    <varlistentry>
-    <term><literal>UNIQUE</literal> (column constraint)</term>
-    <term><literal>UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>
+    <term><literal>UNIQUE [ NULLS [ NOT ] DISTINCT ]</literal> (column constraint)</term>
+    <term><literal>UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>
     <optional> <literal>INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...])</literal> </optional> (table constraint)</term>
 
     <listitem>
@@ -934,7 +934,8 @@ <title>Parameters</title>
 
      <para>
       For the purpose of a unique constraint, null values are not
-      considered equal.
+      considered equal, unless <literal>NULLS NOT DISTINCT</literal> is
+      specified.
      </para>
 
      <para>
diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
index 37ee0b4d6e..44640bc35d 100644
--- a/src/backend/access/nbtree/nbtinsert.c
+++ b/src/backend/access/nbtree/nbtinsert.c
@@ -112,7 +112,7 @@ _bt_doinsert(Relation rel, IndexTuple itup,
 
 	if (checkingunique)
 	{
-		if (!itup_key->anynullkeys)
+		if (!itup_key->anynullkeys || rel->rd_index->indnullsnotdistinct)
 		{
 			/* No (heapkeyspace) scantid until uniqueness established */
 			itup_key->scantid = NULL;
@@ -398,9 +398,9 @@ _bt_search_insert(Relation rel, BTInsertState insertstate)
  * _bt_findinsertloc() to reuse most of the binary search work we do
  * here.
  *
- * Do not call here when there are NULL values in scan key.  NULL should be
- * considered unequal to NULL when checking for duplicates, but we are not
- * prepared to handle that correctly.
+ * This code treats NULLs as equal, unlike the default semantics for unique
+ * indexes.  So do not call here when there are NULL values in scan key and
+ * the index uses the default NULLS DISTINCT mode.
  */
 static TransactionId
 _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,
@@ -444,7 +444,7 @@ _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,
 	 * Scan over all equal tuples, looking for live conflicts.
 	 */
 	Assert(!insertstate->bounds_valid || insertstate->low == offset);
-	Assert(!itup_key->anynullkeys);
+	Assert(!itup_key->anynullkeys || rel->rd_index->indnullsnotdistinct);
 	Assert(itup_key->scantid == NULL);
 	for (;;)
 	{
diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c
index 1e02be9746..66dc201000 100644
--- a/src/backend/access/nbtree/nbtsort.c
+++ b/src/backend/access/nbtree/nbtsort.c
@@ -89,6 +89,7 @@ typedef struct BTSpool
 	Relation	heap;
 	Relation	index;
 	bool		isunique;
+	bool		nulls_not_distinct;
 } BTSpool;
 
 /*
@@ -106,6 +107,7 @@ typedef struct BTShared
 	Oid			heaprelid;
 	Oid			indexrelid;
 	bool		isunique;
+	bool		nulls_not_distinct;
 	bool		isconcurrent;
 	int			scantuplesortstates;
 
@@ -206,6 +208,7 @@ typedef struct BTLeader
 typedef struct BTBuildState
 {
 	bool		isunique;
+	bool		nulls_not_distinct;
 	bool		havedead;
 	Relation	heap;
 	BTSpool    *spool;
@@ -307,6 +310,7 @@ btbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 #endif							/* BTREE_BUILD_STATS */
 
 	buildstate.isunique = indexInfo->ii_Unique;
+	buildstate.nulls_not_distinct = indexInfo->ii_NullsNotDistinct;
 	buildstate.havedead = false;
 	buildstate.heap = heap;
 	buildstate.spool = NULL;
@@ -380,6 +384,7 @@ _bt_spools_heapscan(Relation heap, Relation index, BTBuildState *buildstate,
 	btspool->heap = heap;
 	btspool->index = index;
 	btspool->isunique = indexInfo->ii_Unique;
+	btspool->nulls_not_distinct = indexInfo->ii_NullsNotDistinct;
 
 	/* Save as primary spool */
 	buildstate->spool = btspool;
@@ -429,6 +434,7 @@ _bt_spools_heapscan(Relation heap, Relation index, BTBuildState *buildstate,
 	 */
 	buildstate->spool->sortstate =
 		tuplesort_begin_index_btree(heap, index, buildstate->isunique,
+									buildstate->nulls_not_distinct,
 									maintenance_work_mem, coordinate,
 									false);
 
@@ -468,7 +474,7 @@ _bt_spools_heapscan(Relation heap, Relation index, BTBuildState *buildstate,
 		 * full, so we give it only work_mem
 		 */
 		buildstate->spool2->sortstate =
-			tuplesort_begin_index_btree(heap, index, false, work_mem,
+			tuplesort_begin_index_btree(heap, index, false, false, work_mem,
 										coordinate2, false);
 	}
 
@@ -1554,6 +1560,7 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	btshared->heaprelid = RelationGetRelid(btspool->heap);
 	btshared->indexrelid = RelationGetRelid(btspool->index);
 	btshared->isunique = btspool->isunique;
+	btshared->nulls_not_distinct = btspool->nulls_not_distinct;
 	btshared->isconcurrent = isconcurrent;
 	btshared->scantuplesortstates = scantuplesortstates;
 	ConditionVariableInit(&btshared->workersdonecv);
@@ -1747,6 +1754,7 @@ _bt_leader_participate_as_worker(BTBuildState *buildstate)
 	leaderworker->heap = buildstate->spool->heap;
 	leaderworker->index = buildstate->spool->index;
 	leaderworker->isunique = buildstate->spool->isunique;
+	leaderworker->nulls_not_distinct = buildstate->spool->nulls_not_distinct;
 
 	/* Initialize second spool, if required */
 	if (!btleader->btshared->isunique)
@@ -1846,6 +1854,7 @@ _bt_parallel_build_main(dsm_segment *seg, shm_toc *toc)
 	btspool->heap = heapRel;
 	btspool->index = indexRel;
 	btspool->isunique = btshared->isunique;
+	btspool->nulls_not_distinct = btshared->nulls_not_distinct;
 
 	/* Look up shared state private to tuplesort.c */
 	sharedsort = shm_toc_lookup(toc, PARALLEL_KEY_TUPLESORT, false);
@@ -1928,6 +1937,7 @@ _bt_parallel_scan_and_sort(BTSpool *btspool, BTSpool *btspool2,
 	btspool->sortstate = tuplesort_begin_index_btree(btspool->heap,
 													 btspool->index,
 													 btspool->isunique,
+													 btspool->nulls_not_distinct,
 													 sortmem, coordinate,
 													 false);
 
@@ -1950,13 +1960,14 @@ _bt_parallel_scan_and_sort(BTSpool *btspool, BTSpool *btspool2,
 		coordinate2->nParticipants = -1;
 		coordinate2->sharedsort = sharedsort2;
 		btspool2->sortstate =
-			tuplesort_begin_index_btree(btspool->heap, btspool->index, false,
+			tuplesort_begin_index_btree(btspool->heap, btspool->index, false, false,
 										Min(sortmem, work_mem), coordinate2,
 										false);
 	}
 
 	/* Fill in buildstate for _bt_build_callback() */
 	buildstate.isunique = btshared->isunique;
+	buildstate.nulls_not_distinct = btshared->nulls_not_distinct;
 	buildstate.havedead = false;
 	buildstate.heap = btspool->heap;
 	buildstate.spool = btspool;
diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index 1757cd3446..222404c5b8 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -613,6 +613,7 @@ UpdateIndexRelation(Oid indexoid,
 	values[Anum_pg_index_indnatts - 1] = Int16GetDatum(indexInfo->ii_NumIndexAttrs);
 	values[Anum_pg_index_indnkeyatts - 1] = Int16GetDatum(indexInfo->ii_NumIndexKeyAttrs);
 	values[Anum_pg_index_indisunique - 1] = BoolGetDatum(indexInfo->ii_Unique);
+	values[Anum_pg_index_indnullsnotdistinct - 1] = BoolGetDatum(indexInfo->ii_NullsNotDistinct);
 	values[Anum_pg_index_indisprimary - 1] = BoolGetDatum(primary);
 	values[Anum_pg_index_indisexclusion - 1] = BoolGetDatum(isexclusion);
 	values[Anum_pg_index_indimmediate - 1] = BoolGetDatum(immediate);
@@ -1349,6 +1350,7 @@ index_concurrently_create_copy(Relation heapRelation, Oid oldIndexId,
 							indexExprs,
 							indexPreds,
 							oldInfo->ii_Unique,
+							oldInfo->ii_NullsNotDistinct,
 							false,	/* not ready for inserts */
 							true);
 
@@ -2410,6 +2412,7 @@ BuildIndexInfo(Relation index)
 					   RelationGetIndexExpressions(index),
 					   RelationGetIndexPredicate(index),
 					   indexStruct->indisunique,
+					   indexStruct->indnullsnotdistinct,
 					   indexStruct->indisready,
 					   false);
 
@@ -2469,6 +2472,7 @@ BuildDummyIndexInfo(Relation index)
 					   RelationGetDummyIndexExpressions(index),
 					   NIL,
 					   indexStruct->indisunique,
+					   indexStruct->indnullsnotdistinct,
 					   indexStruct->indisready,
 					   false);
 
@@ -2502,6 +2506,9 @@ CompareIndexInfo(IndexInfo *info1, IndexInfo *info2,
 	if (info1->ii_Unique != info2->ii_Unique)
 		return false;
 
+	if (info1->ii_NullsNotDistinct != info2->ii_NullsNotDistinct)
+		return false;
+
 	/* indexes are only equivalent if they have the same access method */
 	if (info1->ii_Am != info2->ii_Am)
 		return false;
diff --git a/src/backend/catalog/information_schema.sql b/src/backend/catalog/information_schema.sql
index 11d9dd60c2..caafb00c44 100644
--- a/src/backend/catalog/information_schema.sql
+++ b/src/backend/catalog/information_schema.sql
@@ -1838,7 +1838,11 @@ CREATE VIEW table_constraints AS
              AS is_deferrable,
            CAST(CASE WHEN c.condeferred THEN 'YES' ELSE 'NO' END AS yes_or_no)
              AS initially_deferred,
-           CAST('YES' AS yes_or_no) AS enforced
+           CAST('YES' AS yes_or_no) AS enforced,
+           CAST(CASE WHEN c.contype = 'u'
+                     THEN CASE WHEN (SELECT NOT indnullsnotdistinct FROM pg_index WHERE indexrelid = conindid) THEN 'YES' ELSE 'NO' END
+                     END
+                AS yes_or_no) AS nulls_distinct
 
     FROM pg_namespace nc,
          pg_namespace nr,
@@ -1868,7 +1872,8 @@ CREATE VIEW table_constraints AS
            CAST('CHECK' AS character_data) AS constraint_type,
            CAST('NO' AS yes_or_no) AS is_deferrable,
            CAST('NO' AS yes_or_no) AS initially_deferred,
-           CAST('YES' AS yes_or_no) AS enforced
+           CAST('YES' AS yes_or_no) AS enforced,
+           CAST(NULL AS yes_or_no) AS nulls_distinct
 
     FROM pg_namespace nr,
          pg_class r,
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index b8a78f4d41..097d9c4784 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -228,6 +228,7 @@ F263	Comma-separated predicates in simple CASE expression			NO
 F271	Compound character literals			YES	
 F281	LIKE enhancements			YES	
 F291	UNIQUE predicate			NO	
+F292	UNIQUE null treatment			YES	SQL:202x draft
 F301	CORRESPONDING in query expressions			NO	
 F302	INTERSECT table operator			YES	
 F302	INTERSECT table operator	01	INTERSECT DISTINCT table operator	YES	
diff --git a/src/backend/catalog/toasting.c b/src/backend/catalog/toasting.c
index 0db90c2011..9614f64b92 100644
--- a/src/backend/catalog/toasting.c
+++ b/src/backend/catalog/toasting.c
@@ -301,6 +301,7 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
 	indexInfo->ii_ExclusionStrats = NULL;
 	indexInfo->ii_OpclassOptions = NULL;
 	indexInfo->ii_Unique = true;
+	indexInfo->ii_NullsNotDistinct = false;
 	indexInfo->ii_ReadyForInserts = true;
 	indexInfo->ii_Concurrent = false;
 	indexInfo->ii_BrokenHotChain = false;
diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 8d3104821e..875880a21f 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -226,7 +226,7 @@ CheckIndexCompatible(Oid oldId,
 	 * ii_NumIndexKeyAttrs with same value.
 	 */
 	indexInfo = makeIndexInfo(numberOfAttributes, numberOfAttributes,
-							  accessMethodId, NIL, NIL, false, false, false);
+							  accessMethodId, NIL, NIL, false, false, false, false);
 	typeObjectId = (Oid *) palloc(numberOfAttributes * sizeof(Oid));
 	collationObjectId = (Oid *) palloc(numberOfAttributes * sizeof(Oid));
 	classObjectId = (Oid *) palloc(numberOfAttributes * sizeof(Oid));
@@ -867,6 +867,7 @@ DefineIndex(Oid relationId,
 							  NIL,	/* expressions, NIL for now */
 							  make_ands_implicit((Expr *) stmt->whereClause),
 							  stmt->unique,
+							  stmt->nulls_not_distinct,
 							  !concurrent,
 							  concurrent);
 
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index df0b747883..fab86b605b 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -3068,6 +3068,7 @@ _copyConstraint(const Constraint *from)
 	COPY_NODE_FIELD(raw_expr);
 	COPY_STRING_FIELD(cooked_expr);
 	COPY_SCALAR_FIELD(generated_when);
+	COPY_SCALAR_FIELD(nulls_not_distinct);
 	COPY_NODE_FIELD(keys);
 	COPY_NODE_FIELD(including);
 	COPY_NODE_FIELD(exclusions);
@@ -3660,6 +3661,7 @@ _copyIndexStmt(const IndexStmt *from)
 	COPY_SCALAR_FIELD(oldCreateSubid);
 	COPY_SCALAR_FIELD(oldFirstRelfilenodeSubid);
 	COPY_SCALAR_FIELD(unique);
+	COPY_SCALAR_FIELD(nulls_not_distinct);
 	COPY_SCALAR_FIELD(primary);
 	COPY_SCALAR_FIELD(isconstraint);
 	COPY_SCALAR_FIELD(deferrable);
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index cb7ddd463c..0544f220f5 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -1400,6 +1400,7 @@ _equalIndexStmt(const IndexStmt *a, const IndexStmt *b)
 	COMPARE_SCALAR_FIELD(oldCreateSubid);
 	COMPARE_SCALAR_FIELD(oldFirstRelfilenodeSubid);
 	COMPARE_SCALAR_FIELD(unique);
+	COMPARE_SCALAR_FIELD(nulls_not_distinct);
 	COMPARE_SCALAR_FIELD(primary);
 	COMPARE_SCALAR_FIELD(isconstraint);
 	COMPARE_SCALAR_FIELD(deferrable);
@@ -2710,6 +2711,7 @@ _equalConstraint(const Constraint *a, const Constraint *b)
 	COMPARE_NODE_FIELD(raw_expr);
 	COMPARE_STRING_FIELD(cooked_expr);
 	COMPARE_SCALAR_FIELD(generated_when);
+	COMPARE_SCALAR_FIELD(nulls_not_distinct);
 	COMPARE_NODE_FIELD(keys);
 	COMPARE_NODE_FIELD(including);
 	COMPARE_NODE_FIELD(exclusions);
diff --git a/src/backend/nodes/makefuncs.c b/src/backend/nodes/makefuncs.c
index 7d1a01d1ed..4967414a7d 100644
--- a/src/backend/nodes/makefuncs.c
+++ b/src/backend/nodes/makefuncs.c
@@ -741,7 +741,7 @@ make_ands_implicit(Expr *clause)
  */
 IndexInfo *
 makeIndexInfo(int numattrs, int numkeyattrs, Oid amoid, List *expressions,
-			  List *predicates, bool unique, bool isready, bool concurrent)
+			  List *predicates, bool unique, bool nulls_not_distinct, bool isready, bool concurrent)
 {
 	IndexInfo  *n = makeNode(IndexInfo);
 
@@ -750,6 +750,7 @@ makeIndexInfo(int numattrs, int numkeyattrs, Oid amoid, List *expressions,
 	Assert(n->ii_NumIndexKeyAttrs != 0);
 	Assert(n->ii_NumIndexKeyAttrs <= n->ii_NumIndexAttrs);
 	n->ii_Unique = unique;
+	n->ii_NullsNotDistinct = nulls_not_distinct;
 	n->ii_ReadyForInserts = isready;
 	n->ii_Concurrent = concurrent;
 
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 91a89b6d51..8a9f216963 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -2774,6 +2774,7 @@ _outIndexStmt(StringInfo str, const IndexStmt *node)
 	WRITE_UINT_FIELD(oldCreateSubid);
 	WRITE_UINT_FIELD(oldFirstRelfilenodeSubid);
 	WRITE_BOOL_FIELD(unique);
+	WRITE_BOOL_FIELD(nulls_not_distinct);
 	WRITE_BOOL_FIELD(primary);
 	WRITE_BOOL_FIELD(isconstraint);
 	WRITE_BOOL_FIELD(deferrable);
@@ -3706,6 +3707,7 @@ _outConstraint(StringInfo str, const Constraint *node)
 
 		case CONSTR_UNIQUE:
 			appendStringInfoString(str, "UNIQUE");
+			WRITE_BOOL_FIELD(nulls_not_distinct);
 			WRITE_NODE_FIELD(keys);
 			WRITE_NODE_FIELD(including);
 			WRITE_NODE_FIELD(options);
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 3d4dd43e47..0490e5b9a9 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -625,6 +625,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <ival>	opt_window_exclusion_clause
 %type <str>		opt_existing_window_name
 %type <boolean> opt_if_not_exists
+%type <boolean> opt_unique_null_treatment
 %type <ival>	generated_when override_kind
 %type <partspec>	PartitionSpec OptPartitionSpec
 %type <partelem>	part_elem
@@ -3623,15 +3624,16 @@ ColConstraintElem:
 					n->location = @1;
 					$$ = (Node *)n;
 				}
-			| UNIQUE opt_definition OptConsTableSpace
+			| UNIQUE opt_unique_null_treatment opt_definition OptConsTableSpace
 				{
 					Constraint *n = makeNode(Constraint);
 					n->contype = CONSTR_UNIQUE;
 					n->location = @1;
+					n->nulls_not_distinct = !$2;
 					n->keys = NULL;
-					n->options = $2;
+					n->options = $3;
 					n->indexname = NULL;
-					n->indexspace = $3;
+					n->indexspace = $4;
 					$$ = (Node *)n;
 				}
 			| PRIMARY KEY opt_definition OptConsTableSpace
@@ -3716,6 +3718,12 @@ ColConstraintElem:
 				}
 		;
 
+opt_unique_null_treatment:
+			NULLS_P DISTINCT		{ $$ = true; }
+			| NULLS_P NOT DISTINCT	{ $$ = false; }
+			| /*EMPTY*/				{ $$ = true; }
+		;
+
 generated_when:
 			ALWAYS			{ $$ = ATTRIBUTE_IDENTITY_ALWAYS; }
 			| BY DEFAULT	{ $$ = ATTRIBUTE_IDENTITY_BY_DEFAULT; }
@@ -3828,18 +3836,19 @@ ConstraintElem:
 					n->initially_valid = !n->skip_validation;
 					$$ = (Node *)n;
 				}
-			| UNIQUE '(' columnList ')' opt_c_include opt_definition OptConsTableSpace
+			| UNIQUE opt_unique_null_treatment '(' columnList ')' opt_c_include opt_definition OptConsTableSpace
 				ConstraintAttributeSpec
 				{
 					Constraint *n = makeNode(Constraint);
 					n->contype = CONSTR_UNIQUE;
 					n->location = @1;
-					n->keys = $3;
-					n->including = $5;
-					n->options = $6;
+					n->nulls_not_distinct = !$2;
+					n->keys = $4;
+					n->including = $6;
+					n->options = $7;
 					n->indexname = NULL;
-					n->indexspace = $7;
-					processCASbits($8, @8, "UNIQUE",
+					n->indexspace = $8;
+					processCASbits($9, @9, "UNIQUE",
 								   &n->deferrable, &n->initdeferred, NULL,
 								   NULL, yyscanner);
 					$$ = (Node *)n;
@@ -7413,7 +7422,7 @@ defacl_privilege_target:
 
 IndexStmt:	CREATE opt_unique INDEX opt_concurrently opt_index_name
 			ON relation_expr access_method_clause '(' index_params ')'
-			opt_include opt_reloptions OptTableSpace where_clause
+			opt_include opt_unique_null_treatment opt_reloptions OptTableSpace where_clause
 				{
 					IndexStmt *n = makeNode(IndexStmt);
 					n->unique = $2;
@@ -7423,9 +7432,10 @@ IndexStmt:	CREATE opt_unique INDEX opt_concurrently opt_index_name
 					n->accessMethod = $8;
 					n->indexParams = $10;
 					n->indexIncludingParams = $12;
-					n->options = $13;
-					n->tableSpace = $14;
-					n->whereClause = $15;
+					n->nulls_not_distinct = !$13;
+					n->options = $14;
+					n->tableSpace = $15;
+					n->whereClause = $16;
 					n->excludeOpNames = NIL;
 					n->idxcomment = NULL;
 					n->indexOid = InvalidOid;
@@ -7443,7 +7453,7 @@ IndexStmt:	CREATE opt_unique INDEX opt_concurrently opt_index_name
 				}
 			| CREATE opt_unique INDEX opt_concurrently IF_P NOT EXISTS name
 			ON relation_expr access_method_clause '(' index_params ')'
-			opt_include opt_reloptions OptTableSpace where_clause
+			opt_include opt_unique_null_treatment opt_reloptions OptTableSpace where_clause
 				{
 					IndexStmt *n = makeNode(IndexStmt);
 					n->unique = $2;
@@ -7453,9 +7463,10 @@ IndexStmt:	CREATE opt_unique INDEX opt_concurrently opt_index_name
 					n->accessMethod = $11;
 					n->indexParams = $13;
 					n->indexIncludingParams = $15;
-					n->options = $16;
-					n->tableSpace = $17;
-					n->whereClause = $18;
+					n->nulls_not_distinct = !$16;
+					n->options = $17;
+					n->tableSpace = $18;
+					n->whereClause = $19;
 					n->excludeOpNames = NIL;
 					n->idxcomment = NULL;
 					n->indexOid = InvalidOid;
@@ -13804,7 +13815,7 @@ a_expr:		c_expr									{ $$ = $1; }
 					else
 						$$ = (Node *) makeA_Expr(AEXPR_OP_ALL, $2, $1, $5, @2);
 				}
-			| UNIQUE select_with_parens
+			| UNIQUE opt_unique_null_treatment select_with_parens
 				{
 					/* Not sure how to get rid of the parentheses
 					 * but there are lots of shift/reduce errors without them.
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index 2d857a301b..9964450466 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -1581,6 +1581,7 @@ generateClonedIndexStmt(RangeVar *heapRel, Relation source_idx,
 	index->oldCreateSubid = InvalidSubTransactionId;
 	index->oldFirstRelfilenodeSubid = InvalidSubTransactionId;
 	index->unique = idxrec->indisunique;
+	index->nulls_not_distinct = idxrec->indnullsnotdistinct;
 	index->primary = idxrec->indisprimary;
 	index->transformed = true;	/* don't need transformIndexStmt */
 	index->concurrent = false;
@@ -2112,6 +2113,7 @@ transformIndexConstraints(CreateStmtContext *cxt)
 				equal(index->whereClause, priorindex->whereClause) &&
 				equal(index->excludeOpNames, priorindex->excludeOpNames) &&
 				strcmp(index->accessMethod, priorindex->accessMethod) == 0 &&
+				index->nulls_not_distinct == priorindex->nulls_not_distinct &&
 				index->deferrable == priorindex->deferrable &&
 				index->initdeferred == priorindex->initdeferred)
 			{
@@ -2178,6 +2180,7 @@ transformIndexConstraint(Constraint *constraint, CreateStmtContext *cxt)
 		 * DefineIndex will check for it.
 		 */
 	}
+	index->nulls_not_distinct = constraint->nulls_not_distinct;
 	index->isconstraint = true;
 	index->deferrable = constraint->deferrable;
 	index->initdeferred = constraint->initdeferred;
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 8da525c715..606279f3b5 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -1442,6 +1442,9 @@ pg_get_indexdef_worker(Oid indexrelid, int colno,
 	{
 		appendStringInfoChar(&buf, ')');
 
+		if (idxrec->indnullsnotdistinct)
+			appendStringInfo(&buf, " NULLS NOT DISTINCT");
+
 		/*
 		 * If it has options, append "WITH (options)"
 		 */
@@ -2310,9 +2313,20 @@ pg_get_constraintdef_worker(Oid constraintId, bool fullCommand,
 
 				/* Start off the constraint definition */
 				if (conForm->contype == CONSTRAINT_PRIMARY)
-					appendStringInfoString(&buf, "PRIMARY KEY (");
+					appendStringInfoString(&buf, "PRIMARY KEY ");
 				else
-					appendStringInfoString(&buf, "UNIQUE (");
+					appendStringInfoString(&buf, "UNIQUE ");
+
+				indexId = conForm->conindid;
+
+				indtup = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(indexId));
+				if (!HeapTupleIsValid(indtup))
+					elog(ERROR, "cache lookup failed for index %u", indexId);
+				if (conForm->contype == CONSTRAINT_UNIQUE &&
+					((Form_pg_index) GETSTRUCT(indtup))->indnullsnotdistinct)
+					appendStringInfoString(&buf, "NULLS NOT DISTINCT ");
+
+				appendStringInfoString(&buf, "(");
 
 				/* Fetch and build target column list */
 				val = SysCacheGetAttr(CONSTROID, tup,
@@ -2325,12 +2339,7 @@ pg_get_constraintdef_worker(Oid constraintId, bool fullCommand,
 
 				appendStringInfoChar(&buf, ')');
 
-				indexId = conForm->conindid;
-
 				/* Build including column list (from pg_index.indkeys) */
-				indtup = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(indexId));
-				if (!HeapTupleIsValid(indtup))
-					elog(ERROR, "cache lookup failed for index %u", indexId);
 				val = SysCacheGetAttr(INDEXRELID, indtup,
 									  Anum_pg_index_indnatts, &isnull);
 				if (isnull)
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 105d8d4601..a389ce0445 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -2274,6 +2274,7 @@ RelationReloadIndexInfo(Relation relation)
 		 * the array fields are allowed to change, though.
 		 */
 		relation->rd_index->indisunique = index->indisunique;
+		relation->rd_index->indnullsnotdistinct = index->indnullsnotdistinct;
 		relation->rd_index->indisprimary = index->indisprimary;
 		relation->rd_index->indisexclusion = index->indisexclusion;
 		relation->rd_index->indimmediate = index->indimmediate;
diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
index 90e26745df..d9a8b86fb9 100644
--- a/src/backend/utils/sort/tuplesort.c
+++ b/src/backend/utils/sort/tuplesort.c
@@ -459,6 +459,7 @@ struct Tuplesortstate
 
 	/* These are specific to the index_btree subcase: */
 	bool		enforceUnique;	/* complain if we find duplicate tuples */
+	bool		uniqueNullsNotDistinct;	/* unique constraint null treatment */
 
 	/* These are specific to the index_hash subcase: */
 	uint32		high_mask;		/* masks for sortable part of hash code */
@@ -1065,6 +1066,7 @@ Tuplesortstate *
 tuplesort_begin_index_btree(Relation heapRel,
 							Relation indexRel,
 							bool enforceUnique,
+							bool uniqueNullsNotDistinct,
 							int workMem,
 							SortCoordinate coordinate,
 							bool randomAccess)
@@ -1103,6 +1105,7 @@ tuplesort_begin_index_btree(Relation heapRel,
 	state->heapRel = heapRel;
 	state->indexRel = indexRel;
 	state->enforceUnique = enforceUnique;
+	state->uniqueNullsNotDistinct = uniqueNullsNotDistinct;
 
 	indexScanKey = _bt_mkscankey(indexRel, NULL);
 
@@ -4200,14 +4203,15 @@ comparetup_index_btree(const SortTuple *a, const SortTuple *b,
 
 	/*
 	 * If btree has asked us to enforce uniqueness, complain if two equal
-	 * tuples are detected (unless there was at least one NULL field).
+	 * tuples are detected (unless there was at least one NULL field and NULLS
+	 * NOT DISTINCT was not set).
 	 *
 	 * It is sufficient to make the test here, because if two tuples are equal
 	 * they *must* get compared at some stage of the sort --- otherwise the
 	 * sort algorithm wouldn't have checked whether one must appear before the
 	 * other.
 	 */
-	if (state->enforceUnique && !equal_hasnull)
+	if (state->enforceUnique && !(!state->uniqueNullsNotDistinct && equal_hasnull))
 	{
 		Datum		values[INDEX_MAX_KEYS];
 		bool		isnull[INDEX_MAX_KEYS];
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index b52f3ccda2..82a4521ce7 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -6467,6 +6467,7 @@ getIndexes(Archive *fout, TableInfo tblinfo[], int numTables)
 				i_indkey,
 				i_indisclustered,
 				i_indisreplident,
+				i_indnullsnotdistinct,
 				i_contype,
 				i_conname,
 				i_condeferrable,
@@ -6543,14 +6544,21 @@ getIndexes(Archive *fout, TableInfo tblinfo[], int numTables)
 						  "(SELECT pg_catalog.array_agg(attstattarget ORDER BY attnum) "
 						  "  FROM pg_catalog.pg_attribute "
 						  "  WHERE attrelid = i.indexrelid AND "
-						  "    attstattarget >= 0) AS indstatvals ");
+						  "    attstattarget >= 0) AS indstatvals, ");
 	else
 		appendPQExpBuffer(query,
 						  "0 AS parentidx, "
 						  "i.indnatts AS indnkeyatts, "
 						  "i.indnatts AS indnatts, "
 						  "'' AS indstatcols, "
-						  "'' AS indstatvals ");
+						  "'' AS indstatvals, ");
+
+	if (fout->remoteVersion >= 150000)
+		appendPQExpBuffer(query,
+						  "i.indnullsnotdistinct ");
+	else
+		appendPQExpBuffer(query,
+						  "false AS indnullsnotdistinct ");
 
 	/*
 	 * The point of the messy-looking outer join is to find a constraint that
@@ -6613,6 +6621,7 @@ getIndexes(Archive *fout, TableInfo tblinfo[], int numTables)
 	i_indkey = PQfnumber(res, "indkey");
 	i_indisclustered = PQfnumber(res, "indisclustered");
 	i_indisreplident = PQfnumber(res, "indisreplident");
+	i_indnullsnotdistinct = PQfnumber(res, "indnullsnotdistinct");
 	i_contype = PQfnumber(res, "contype");
 	i_conname = PQfnumber(res, "conname");
 	i_condeferrable = PQfnumber(res, "condeferrable");
@@ -6689,6 +6698,7 @@ getIndexes(Archive *fout, TableInfo tblinfo[], int numTables)
 						  indxinfo[j].indkeys, indxinfo[j].indnattrs);
 			indxinfo[j].indisclustered = (PQgetvalue(res, j, i_indisclustered)[0] == 't');
 			indxinfo[j].indisreplident = (PQgetvalue(res, j, i_indisreplident)[0] == 't');
+			indxinfo[j].indnullsnotdistinct = (PQgetvalue(res, j, i_indnullsnotdistinct)[0] == 't');
 			indxinfo[j].parentidx = atooid(PQgetvalue(res, j, i_parentidx));
 			indxinfo[j].partattaches = (SimplePtrList)
 			{
@@ -16050,8 +16060,11 @@ dumpConstraint(Archive *fout, const ConstraintInfo *coninfo)
 		}
 		else
 		{
-			appendPQExpBuffer(q, "%s (",
+			appendPQExpBuffer(q, "%s",
 							  coninfo->contype == 'p' ? "PRIMARY KEY" : "UNIQUE");
+			if (indxinfo->indnullsnotdistinct)
+				appendPQExpBuffer(q, " NULLS NOT DISTINCT");
+			appendPQExpBuffer(q, " (");
 			for (k = 0; k < indxinfo->indnkeyattrs; k++)
 			{
 				int			indkey = (int) indxinfo->indkeys[k];
diff --git a/src/bin/pg_dump/pg_dump.h b/src/bin/pg_dump/pg_dump.h
index f011ace8a8..06a2979f04 100644
--- a/src/bin/pg_dump/pg_dump.h
+++ b/src/bin/pg_dump/pg_dump.h
@@ -398,6 +398,7 @@ typedef struct _indxInfo
 								 * contains both key and nonkey attributes */
 	bool		indisclustered;
 	bool		indisreplident;
+	bool		indnullsnotdistinct;
 	Oid			parentidx;		/* if a partition, parent index OID */
 	SimplePtrList partattaches; /* if partitioned, partition attach objects */
 
diff --git a/src/bin/psql/describe.c b/src/bin/psql/describe.c
index c28788e84f..a59200a792 100644
--- a/src/bin/psql/describe.c
+++ b/src/bin/psql/describe.c
@@ -2181,6 +2181,11 @@ describeOneTableDetails(const char *schemaname,
 		else
 			appendPQExpBufferStr(&buf, "false AS indisreplident,\n");
 
+		if (pset.sversion >= 150000)
+			appendPQExpBufferStr(&buf, "i.indnullsnotdistinct,\n");
+		else
+			appendPQExpBufferStr(&buf, "false AS indnullsnotdistinct,\n");
+
 		appendPQExpBuffer(&buf, "  a.amname, c2.relname, "
 						  "pg_catalog.pg_get_expr(i.indpred, i.indrelid, true)\n"
 						  "FROM pg_catalog.pg_index i, pg_catalog.pg_class c, pg_catalog.pg_class c2, pg_catalog.pg_am a\n"
@@ -2205,14 +2210,20 @@ describeOneTableDetails(const char *schemaname,
 			char	   *deferrable = PQgetvalue(result, 0, 4);
 			char	   *deferred = PQgetvalue(result, 0, 5);
 			char	   *indisreplident = PQgetvalue(result, 0, 6);
-			char	   *indamname = PQgetvalue(result, 0, 7);
-			char	   *indtable = PQgetvalue(result, 0, 8);
-			char	   *indpred = PQgetvalue(result, 0, 9);
+			char	   *indnullsnotdistinct = PQgetvalue(result, 0, 7);
+			char	   *indamname = PQgetvalue(result, 0, 8);
+			char	   *indtable = PQgetvalue(result, 0, 9);
+			char	   *indpred = PQgetvalue(result, 0, 10);
 
 			if (strcmp(indisprimary, "t") == 0)
 				printfPQExpBuffer(&tmpbuf, _("primary key, "));
 			else if (strcmp(indisunique, "t") == 0)
-				printfPQExpBuffer(&tmpbuf, _("unique, "));
+			{
+				printfPQExpBuffer(&tmpbuf, _("unique"));
+				if (strcmp(indnullsnotdistinct, "t") == 0)
+					appendPQExpBufferStr(&tmpbuf, _(" nulls not distinct"));
+				appendPQExpBuffer(&tmpbuf, _(", "));
+			}
 			else
 				resetPQExpBuffer(&tmpbuf);
 			appendPQExpBuffer(&tmpbuf, "%s, ", indamname);
diff --git a/src/include/catalog/pg_index.h b/src/include/catalog/pg_index.h
index a5a6075d4d..8c8300284e 100644
--- a/src/include/catalog/pg_index.h
+++ b/src/include/catalog/pg_index.h
@@ -34,6 +34,7 @@ CATALOG(pg_index,2610,IndexRelationId) BKI_SCHEMA_MACRO
 	int16		indnatts;		/* total number of columns in index */
 	int16		indnkeyatts;	/* number of key columns in index */
 	bool		indisunique;	/* is this a unique index? */
+	bool		indnullsnotdistinct;	/* null treatment in unique index */
 	bool		indisprimary;	/* is this index for primary key? */
 	bool		indisexclusion; /* is this index for exclusion constraint? */
 	bool		indimmediate;	/* is uniqueness enforced immediately? */
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index ddc3529332..2b628d0ef4 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -171,6 +171,7 @@ typedef struct IndexInfo
 	uint16	   *ii_UniqueStrats;	/* array with one entry per column */
 	Datum	   *ii_OpclassOptions;	/* array with one entry per column */
 	bool		ii_Unique;
+	bool		ii_NullsNotDistinct;
 	bool		ii_ReadyForInserts;
 	bool		ii_Concurrent;
 	bool		ii_BrokenHotChain;
diff --git a/src/include/nodes/makefuncs.h b/src/include/nodes/makefuncs.h
index eea87f847d..9edacd921b 100644
--- a/src/include/nodes/makefuncs.h
+++ b/src/include/nodes/makefuncs.h
@@ -96,7 +96,7 @@ extern List *make_ands_implicit(Expr *clause);
 
 extern IndexInfo *makeIndexInfo(int numattrs, int numkeyattrs, Oid amoid,
 								List *expressions, List *predicates,
-								bool unique, bool isready, bool concurrent);
+								bool unique, bool nulls_not_distinct, bool isready, bool concurrent);
 
 extern DefElem *makeDefElem(char *name, Node *arg, int location);
 extern DefElem *makeDefElemExtended(char *nameSpace, char *name, Node *arg,
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 4c5a8a39bf..24ec61ce87 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -2276,6 +2276,7 @@ typedef struct Constraint
 	char		generated_when; /* ALWAYS or BY DEFAULT */
 
 	/* Fields used for unique constraints (UNIQUE and PRIMARY KEY): */
+	bool		nulls_not_distinct;	/* null treatment for UNIQUE constraints */
 	List	   *keys;			/* String nodes naming referenced key
 								 * column(s) */
 	List	   *including;		/* String nodes naming referenced nonkey
@@ -2904,6 +2905,7 @@ typedef struct IndexStmt
 	SubTransactionId oldFirstRelfilenodeSubid;	/* rd_firstRelfilenodeSubid of
 												 * oldNode */
 	bool		unique;			/* is index unique? */
+	bool		nulls_not_distinct;	/* null treatment for UNIQUE constraints */
 	bool		primary;		/* is index a primary key? */
 	bool		isconstraint;	/* is it for a pkey/unique constraint? */
 	bool		deferrable;		/* is the constraint DEFERRABLE? */
diff --git a/src/include/utils/tuplesort.h b/src/include/utils/tuplesort.h
index f94949370b..b30ca09598 100644
--- a/src/include/utils/tuplesort.h
+++ b/src/include/utils/tuplesort.h
@@ -208,6 +208,7 @@ extern Tuplesortstate *tuplesort_begin_cluster(TupleDesc tupDesc,
 extern Tuplesortstate *tuplesort_begin_index_btree(Relation heapRel,
 												   Relation indexRel,
 												   bool enforceUnique,
+												   bool uniqueNullsNotDistinct,
 												   int workMem, SortCoordinate coordinate,
 												   bool randomAccess);
 extern Tuplesortstate *tuplesort_begin_index_hash(Relation heapRel,
diff --git a/src/test/regress/expected/constraints.out b/src/test/regress/expected/constraints.out
index e32cf8bb57..36ccbb5f15 100644
--- a/src/test/regress/expected/constraints.out
+++ b/src/test/regress/expected/constraints.out
@@ -445,6 +445,29 @@ SELECT * FROM UNIQUE_TBL;
  6 | six-upsert-insert
 (7 rows)
 
+DROP TABLE UNIQUE_TBL;
+CREATE TABLE UNIQUE_TBL (i int UNIQUE NULLS NOT DISTINCT, t text);
+INSERT INTO UNIQUE_TBL VALUES (1, 'one');
+INSERT INTO UNIQUE_TBL VALUES (2, 'two');
+INSERT INTO UNIQUE_TBL VALUES (1, 'three');
+ERROR:  duplicate key value violates unique constraint "unique_tbl_i_key"
+DETAIL:  Key (i)=(1) already exists.
+INSERT INTO UNIQUE_TBL VALUES (4, 'four');
+INSERT INTO UNIQUE_TBL VALUES (5, 'one');
+INSERT INTO UNIQUE_TBL (t) VALUES ('six');
+INSERT INTO UNIQUE_TBL (t) VALUES ('seven');
+ERROR:  duplicate key value violates unique constraint "unique_tbl_i_key"
+DETAIL:  Key (i)=(null) already exists.
+SELECT * FROM UNIQUE_TBL;
+ i |  t   
+---+------
+ 1 | one
+ 2 | two
+ 4 | four
+ 5 | one
+   | six
+(5 rows)
+
 DROP TABLE UNIQUE_TBL;
 CREATE TABLE UNIQUE_TBL (i int, t text,
 	UNIQUE(i,t));
diff --git a/src/test/regress/expected/create_index.out b/src/test/regress/expected/create_index.out
index daf75dd5c4..53c8e830ce 100644
--- a/src/test/regress/expected/create_index.out
+++ b/src/test/regress/expected/create_index.out
@@ -1272,6 +1272,67 @@ SELECT count(*) FROM tenk1 WHERE stringu1 = 'TVAAAA';
 DROP INDEX hash_tuplesort_idx;
 RESET maintenance_work_mem;
 --
+-- Test unique null behavior
+--
+CREATE TABLE unique_tbl (i int, t text);
+CREATE UNIQUE INDEX unique_idx1 ON unique_tbl (i) NULLS DISTINCT;
+CREATE UNIQUE INDEX unique_idx2 ON unique_tbl (i) NULLS NOT DISTINCT;
+INSERT INTO unique_tbl VALUES (1, 'one');
+INSERT INTO unique_tbl VALUES (2, 'two');
+INSERT INTO unique_tbl VALUES (3, 'three');
+INSERT INTO unique_tbl VALUES (4, 'four');
+INSERT INTO unique_tbl VALUES (5, 'one');
+INSERT INTO unique_tbl (t) VALUES ('six');
+INSERT INTO unique_tbl (t) VALUES ('seven');  -- error from unique_idx2
+ERROR:  duplicate key value violates unique constraint "unique_idx2"
+DETAIL:  Key (i)=(null) already exists.
+DROP INDEX unique_idx1, unique_idx2;
+INSERT INTO unique_tbl (t) VALUES ('seven');
+-- build indexes on filled table
+CREATE UNIQUE INDEX unique_idx3 ON unique_tbl (i) NULLS DISTINCT;  -- ok
+CREATE UNIQUE INDEX unique_idx4 ON unique_tbl (i) NULLS NOT DISTINCT;  -- error
+ERROR:  could not create unique index "unique_idx4"
+DETAIL:  Key (i)=(null) is duplicated.
+DELETE FROM unique_tbl WHERE t = 'seven';
+CREATE UNIQUE INDEX unique_idx4 ON unique_tbl (i) NULLS NOT DISTINCT;  -- ok now
+\d unique_tbl
+             Table "public.unique_tbl"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ i      | integer |           |          | 
+ t      | text    |           |          | 
+Indexes:
+    "unique_idx3" UNIQUE, btree (i)
+    "unique_idx4" UNIQUE, btree (i) NULLS NOT DISTINCT
+
+\d unique_idx3
+      Index "public.unique_idx3"
+ Column |  Type   | Key? | Definition 
+--------+---------+------+------------
+ i      | integer | yes  | i
+unique, btree, for table "public.unique_tbl"
+
+\d unique_idx4
+      Index "public.unique_idx4"
+ Column |  Type   | Key? | Definition 
+--------+---------+------+------------
+ i      | integer | yes  | i
+unique nulls not distinct, btree, for table "public.unique_tbl"
+
+SELECT pg_get_indexdef('unique_idx3'::regclass);
+                           pg_get_indexdef                            
+----------------------------------------------------------------------
+ CREATE UNIQUE INDEX unique_idx3 ON public.unique_tbl USING btree (i)
+(1 row)
+
+SELECT pg_get_indexdef('unique_idx4'::regclass);
+                                     pg_get_indexdef                                     
+-----------------------------------------------------------------------------------------
+ CREATE UNIQUE INDEX unique_idx4 ON public.unique_tbl USING btree (i) NULLS NOT DISTINCT
+(1 row)
+
+DROP TABLE unique_tbl;
+--
 -- Test functional index
 --
 CREATE TABLE func_index_heap (f1 text, f2 text);
diff --git a/src/test/regress/sql/constraints.sql b/src/test/regress/sql/constraints.sql
index 458f805778..34de0c969a 100644
--- a/src/test/regress/sql/constraints.sql
+++ b/src/test/regress/sql/constraints.sql
@@ -306,6 +306,20 @@ CREATE TABLE UNIQUE_TBL (i int UNIQUE, t text);
 
 DROP TABLE UNIQUE_TBL;
 
+CREATE TABLE UNIQUE_TBL (i int UNIQUE NULLS NOT DISTINCT, t text);
+
+INSERT INTO UNIQUE_TBL VALUES (1, 'one');
+INSERT INTO UNIQUE_TBL VALUES (2, 'two');
+INSERT INTO UNIQUE_TBL VALUES (1, 'three');
+INSERT INTO UNIQUE_TBL VALUES (4, 'four');
+INSERT INTO UNIQUE_TBL VALUES (5, 'one');
+INSERT INTO UNIQUE_TBL (t) VALUES ('six');
+INSERT INTO UNIQUE_TBL (t) VALUES ('seven');
+
+SELECT * FROM UNIQUE_TBL;
+
+DROP TABLE UNIQUE_TBL;
+
 CREATE TABLE UNIQUE_TBL (i int, t text,
 	UNIQUE(i,t));
 
diff --git a/src/test/regress/sql/create_index.sql b/src/test/regress/sql/create_index.sql
index 8b353be16e..9003950a1f 100644
--- a/src/test/regress/sql/create_index.sql
+++ b/src/test/regress/sql/create_index.sql
@@ -387,6 +387,43 @@ CREATE INDEX hash_tuplesort_idx ON tenk1 USING hash (stringu1 name_ops) WITH (fi
 RESET maintenance_work_mem;
 
 
+--
+-- Test unique null behavior
+--
+CREATE TABLE unique_tbl (i int, t text);
+
+CREATE UNIQUE INDEX unique_idx1 ON unique_tbl (i) NULLS DISTINCT;
+CREATE UNIQUE INDEX unique_idx2 ON unique_tbl (i) NULLS NOT DISTINCT;
+
+INSERT INTO unique_tbl VALUES (1, 'one');
+INSERT INTO unique_tbl VALUES (2, 'two');
+INSERT INTO unique_tbl VALUES (3, 'three');
+INSERT INTO unique_tbl VALUES (4, 'four');
+INSERT INTO unique_tbl VALUES (5, 'one');
+INSERT INTO unique_tbl (t) VALUES ('six');
+INSERT INTO unique_tbl (t) VALUES ('seven');  -- error from unique_idx2
+
+DROP INDEX unique_idx1, unique_idx2;
+
+INSERT INTO unique_tbl (t) VALUES ('seven');
+
+-- build indexes on filled table
+CREATE UNIQUE INDEX unique_idx3 ON unique_tbl (i) NULLS DISTINCT;  -- ok
+CREATE UNIQUE INDEX unique_idx4 ON unique_tbl (i) NULLS NOT DISTINCT;  -- error
+
+DELETE FROM unique_tbl WHERE t = 'seven';
+
+CREATE UNIQUE INDEX unique_idx4 ON unique_tbl (i) NULLS NOT DISTINCT;  -- ok now
+
+\d unique_tbl
+\d unique_idx3
+\d unique_idx4
+SELECT pg_get_indexdef('unique_idx3'::regclass);
+SELECT pg_get_indexdef('unique_idx4'::regclass);
+
+DROP TABLE unique_tbl;
+
+
 --
 -- Test functional index
 --

base-commit: cab5b9ab2c066ba904f13de2681872dcda31e207
-- 
2.34.1