v20-0004-Create-custom-compression-methods.patch

From 2b12cef37af56f27c9c3be254c067137abb15583 Mon Sep 17 00:00:00 2001
From: Dilip Kumar <dilipkumar@localhost.localdomain>
Date: Fri, 29 Jan 2021 15:44:09 +0530
Subject: [PATCH v20 4/6] Create custom compression methods

Provide syntax to create custom compression methods.

Dilip Kumar based on the patches from Ildus Kurbangaliev.
Design input from Tomas Vondra and Robert Haas
Reviewed by Robert Haas, Tomas Vondra, Alexander Korotkov and Justin Pryzby
---
 doc/src/sgml/ref/alter_table.sgml              |  7 +--
 doc/src/sgml/ref/create_access_method.sgml     | 13 +++---
 doc/src/sgml/ref/create_table.sgml             |  3 +-
 src/backend/access/common/detoast.c            | 59 +++++++++++++++++++++++---
 src/backend/access/common/toast_internals.c    | 19 +++++++--
 src/backend/access/compression/compress_lz4.c  | 21 ++++-----
 src/backend/access/compression/compress_pglz.c | 20 ++++-----
 src/backend/access/compression/compressamapi.c | 27 ++++++++++++
 src/backend/access/index/amapi.c               | 50 ++++++++++++++++------
 src/backend/commands/amcmds.c                  |  5 +++
 src/backend/executor/nodeModifyTable.c         |  2 +-
 src/backend/parser/gram.y                      |  1 +
 src/backend/utils/adt/varlena.c                |  7 +--
 src/bin/pg_dump/pg_dump.c                      |  3 ++
 src/bin/psql/tab-complete.c                    |  6 +++
 src/include/access/amapi.h                     |  1 +
 src/include/access/compressamapi.h             | 16 +++++--
 src/include/access/detoast.h                   |  8 ++++
 src/include/access/toast_internals.h           | 15 +++++++
 src/test/regress/expected/compression.out      | 40 ++++++++++++++++-
 src/test/regress/expected/compression_1.out    | 41 +++++++++++++++++-
 src/test/regress/sql/compression.sql           | 10 +++++
 22 files changed, 312 insertions(+), 62 deletions(-)

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 081fe07..46f34bf 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -390,9 +390,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     </term>
     <listitem>
      <para>
-      This clause adds compression to a column. The Compression method can be
-      set to any available compression method.  The built-in methods are
-      <literal>pglz</literal> and <literal>lz4</literal>.
+      This clause adds compression to a column. The Compression method could be
+      created with <xref linkend="sql-create-access-method"/> or
+      it can be set to any available compression method.  The built-in methods
+      are <literal>pglz</literal> and <literal>lz4</literal>.
       The <literal>PRESERVE</literal> list contains a list of compression
       methods used in the column and determines which of them may be kept.
       Without <literal>PRESERVE</literal> or if any of the pre-existing
diff --git a/doc/src/sgml/ref/create_access_method.sgml b/doc/src/sgml/ref/create_access_method.sgml
index dae43db..79f1290 100644
--- a/doc/src/sgml/ref/create_access_method.sgml
+++ b/doc/src/sgml/ref/create_access_method.sgml
@@ -61,7 +61,7 @@ CREATE ACCESS METHOD <replaceable class="parameter">name</replaceable>
     <listitem>
      <para>
       This clause specifies the type of access method to define.
-      Only <literal>TABLE</literal> and <literal>INDEX</literal>
+      <literal>TABLE</literal>, <literal>INDEX</literal> and <literal>COMPRESSION</literal>
       are supported at present.
      </para>
     </listitem>
@@ -77,12 +77,15 @@ CREATE ACCESS METHOD <replaceable class="parameter">name</replaceable>
       declared to take a single argument of type <type>internal</type>,
       and its return type depends on the type of access method;
       for <literal>TABLE</literal> access methods, it must
-      be <type>table_am_handler</type> and for <literal>INDEX</literal>
-      access methods, it must be <type>index_am_handler</type>.
+      be <type>table_am_handler</type>, for <literal>INDEX</literal>
+      access methods, it must be <type>index_am_handler</type> and
+      for <literal>COMPRESSION</literal> access methods, it must be
+      <type>compression_am_handler</type>.
       The C-level API that the handler function must implement varies
       depending on the type of access method. The table access method API
-      is described in <xref linkend="tableam"/> and the index access method
-      API is described in <xref linkend="indexam"/>.
+      is described in <xref linkend="tableam"/>, the index access method
+      API is described in <xref linkend="indexam"/> an the compression access
+      method API is described in <xref linkend="compression-am"/>.
      </para>
     </listitem>
    </varlistentry>
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index d514c7f..10b468f 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -998,7 +998,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     <listitem>
      <para>
       This clause adds the compression method to a column.  The Compression
-      method can be set from available compression methods.  The built-in
+      method could be created with <xref linkend="sql-create-access-method"/> or
+      it can be set from the available compression methods.  The built-in
       methods are <literal>pglz</literal> and <literal>lz4</literal>.
       If no compression method is specified, then compressible types will have
       the default compression method <literal>pglz</literal>.
diff --git a/src/backend/access/common/detoast.c b/src/backend/access/common/detoast.c
index c904640..e032c30 100644
--- a/src/backend/access/common/detoast.c
+++ b/src/backend/access/common/detoast.c
@@ -458,12 +458,43 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 }
 
 /* ----------
+ * toast_get_compression_oid -
+ *
+ * Return the Oid of the compresion method stored in the compressed data.
+ * For built-in methods, we only store the built-in compression method id in
+ * first 2-bits of the rawsize and that is directly mapped to the compression
+ * method Oid.  And, for the custom compression method we store the Oid of the
+ * compression method in the custom compression header.
+ */
+Oid
+toast_get_compression_oid(struct varlena *attr)
+{
+	CompressionId cmid;
+
+	/*
+	 * If it is custom compression id then get the Oid from the custom
+	 * compression header otherwise, directly translate the built-in
+	 * compression id to compression method Oid.
+	 */
+	cmid = TOAST_COMPRESS_METHOD(attr);
+	if (IsCustomCompression(cmid))
+	{
+		toast_compress_header_custom	*hdr;
+
+		hdr = (toast_compress_header_custom *) attr;
+		return hdr->cmoid;
+	}
+	else
+		return CompressionIdToOid(cmid);
+}
+
+/* ----------
  * toast_get_compression_handler - get the compression handler routines
  *
  * helper function for toast_decompress_datum and toast_decompress_datum_slice
  */
 static inline const CompressionAmRoutine *
-toast_get_compression_handler(struct varlena *attr)
+toast_get_compression_handler(struct varlena *attr, int32 *header_size)
 {
 	const CompressionAmRoutine *cmroutine;
 	CompressionId cmid;
@@ -477,10 +508,21 @@ toast_get_compression_handler(struct varlena *attr)
 	{
 		case PGLZ_COMPRESSION_ID:
 			cmroutine = &pglz_compress_methods;
+			*header_size = TOAST_COMPRESS_HDRSZ;
 			break;
 		case LZ4_COMPRESSION_ID:
 			cmroutine = &lz4_compress_methods;
+			*header_size = TOAST_COMPRESS_HDRSZ;
 			break;
+		case CUSTOM_COMPRESSION_ID:
+		{
+			toast_compress_header_custom	*hdr;
+
+			hdr = (toast_compress_header_custom *) attr;
+			cmroutine = GetCompressionAmRoutineByAmId(hdr->cmoid);
+			*header_size = TOAST_CUSTOM_COMPRESS_HDRSZ;
+			break;
+		}
 		default:
 			elog(ERROR, "Invalid compression method id %d", cmid);
 	}
@@ -496,9 +538,11 @@ toast_get_compression_handler(struct varlena *attr)
 static struct varlena *
 toast_decompress_datum(struct varlena *attr)
 {
-	const CompressionAmRoutine *cmroutine = toast_get_compression_handler(attr);
+	int32	header_size;
+	const CompressionAmRoutine *cmroutine =
+				toast_get_compression_handler(attr, &header_size);
 
-	return cmroutine->datum_decompress(attr);
+	return cmroutine->datum_decompress(attr, header_size);
 }
 
 
@@ -512,16 +556,19 @@ toast_decompress_datum(struct varlena *attr)
 static struct varlena *
 toast_decompress_datum_slice(struct varlena *attr, int32 slicelength)
 {
-	const CompressionAmRoutine *cmroutine = toast_get_compression_handler(attr);
+	int32	header_size;
+	const CompressionAmRoutine *cmroutine =
+				toast_get_compression_handler(attr, &header_size);
 
 	/*
 	 * If the handler supports the slice decompression then decompress the
 	 * slice otherwise decompress complete data.
 	 */
 	if (cmroutine->datum_decompress_slice)
-		return cmroutine->datum_decompress_slice(attr, slicelength);
+		return cmroutine->datum_decompress_slice(attr, header_size,
+												 slicelength);
 	else
-		return cmroutine->datum_decompress(attr);
+		return cmroutine->datum_decompress(attr, header_size);
 }
 
 /* ----------
diff --git a/src/backend/access/common/toast_internals.c b/src/backend/access/common/toast_internals.c
index b04c5a5..a353906 100644
--- a/src/backend/access/common/toast_internals.c
+++ b/src/backend/access/common/toast_internals.c
@@ -48,6 +48,7 @@ toast_compress_datum(Datum value, Oid cmoid)
 {
 	struct varlena *tmp = NULL;
 	int32		valsize;
+	bool		isCustomCompression = false;
 	const CompressionAmRoutine *cmroutine = NULL;
 
 	Assert(!VARATT_IS_EXTERNAL(DatumGetPointer(value)));
@@ -65,11 +66,16 @@ toast_compress_datum(Datum value, Oid cmoid)
 			cmroutine = &lz4_compress_methods;
 			break;
 		default:
-			elog(ERROR, "Invalid compression method oid %u", cmoid);
+			isCustomCompression = true;
+			cmroutine = GetCompressionAmRoutineByAmId(cmoid);
+			break;
 	}
 
 	/* Call the actual compression function */
-	tmp = cmroutine->datum_compress((const struct varlena *) value);
+	tmp = cmroutine->datum_compress((const struct varlena *)value,
+									isCustomCompression ?
+									TOAST_CUSTOM_COMPRESS_HDRSZ :
+									TOAST_COMPRESS_HDRSZ);
 	if (!tmp)
 		return PointerGetDatum(NULL);
 
@@ -88,7 +94,14 @@ toast_compress_datum(Datum value, Oid cmoid)
 	if (VARSIZE(tmp) < valsize - 2)
 	{
 		/* successful compression */
-		TOAST_COMPRESS_SET_SIZE_AND_METHOD(tmp, valsize, CompressionOidToId(cmoid));
+		TOAST_COMPRESS_SET_SIZE_AND_METHOD(tmp, valsize, isCustomCompression ?
+										   CUSTOM_COMPRESSION_ID :
+										   CompressionOidToId(cmoid));
+
+		/* For custom compression, set the oid of the compression method */
+		if (isCustomCompression)
+			TOAST_COMPRESS_SET_CMOID(tmp, cmoid);
+
 		return PointerGetDatum(tmp);
 	}
 	else
diff --git a/src/backend/access/compression/compress_lz4.c b/src/backend/access/compression/compress_lz4.c
index e8b035d..34b0dc1 100644
--- a/src/backend/access/compression/compress_lz4.c
+++ b/src/backend/access/compression/compress_lz4.c
@@ -30,7 +30,7 @@
  * compressed varlena, or NULL if compression fails.
  */
 static struct varlena *
-lz4_cmcompress(const struct varlena *value)
+lz4_cmcompress(const struct varlena *value, int32 header_size)
 {
 #ifndef HAVE_LIBLZ4
 	ereport(ERROR,
@@ -45,10 +45,10 @@ lz4_cmcompress(const struct varlena *value)
 	valsize = VARSIZE_ANY_EXHDR(value);
 
 	max_size = LZ4_compressBound(VARSIZE_ANY_EXHDR(value));
-	tmp = (struct varlena *) palloc(max_size + TOAST_COMPRESS_HDRSZ);
+	tmp = (struct varlena *) palloc(max_size + header_size);
 
 	len = LZ4_compress_default(VARDATA_ANY(value),
-							   (char *) tmp + TOAST_COMPRESS_HDRSZ,
+							   (char *) tmp + header_size,
 							   valsize, max_size);
 	if (len <= 0)
 	{
@@ -56,7 +56,7 @@ lz4_cmcompress(const struct varlena *value)
 		elog(ERROR, "lz4: could not compress data");
 	}
 
-	SET_VARSIZE_COMPRESSED(tmp, len + TOAST_COMPRESS_HDRSZ);
+	SET_VARSIZE_COMPRESSED(tmp, len + header_size);
 
 	return tmp;
 #endif
@@ -68,7 +68,7 @@ lz4_cmcompress(const struct varlena *value)
  * Returns the decompressed varlena.
  */
 static struct varlena *
-lz4_cmdecompress(const struct varlena *value)
+lz4_cmdecompress(const struct varlena *value, int32 header_size)
 {
 #ifndef HAVE_LIBLZ4
 	ereport(ERROR,
@@ -81,9 +81,9 @@ lz4_cmdecompress(const struct varlena *value)
 	result = (struct varlena *) palloc(TOAST_COMPRESS_RAWSIZE(value) + VARHDRSZ);
 	SET_VARSIZE(result, TOAST_COMPRESS_RAWSIZE(value) + VARHDRSZ);
 
-	rawsize = LZ4_decompress_safe(TOAST_COMPRESS_RAWDATA(value),
+	rawsize = LZ4_decompress_safe((char *) value + header_size,
 								  VARDATA(result),
-								  VARSIZE(value) - TOAST_COMPRESS_HDRSZ,
+								  VARSIZE(value) - header_size,
 								  TOAST_COMPRESS_RAWSIZE(value));
 	if (rawsize < 0)
 		elog(ERROR, "lz4: compressed data is corrupted");
@@ -100,7 +100,8 @@ lz4_cmdecompress(const struct varlena *value)
  * Decompresses part of the data. Returns the decompressed varlena.
  */
 static struct varlena *
-lz4_cmdecompress_slice(const struct varlena *value, int32 slicelength)
+lz4_cmdecompress_slice(const struct varlena *value, int32 header_size,
+					  int32 slicelength)
 {
 #ifndef HAVE_LIBLZ4
 	ereport(ERROR,
@@ -112,9 +113,9 @@ lz4_cmdecompress_slice(const struct varlena *value, int32 slicelength)
 
 	result = (struct varlena *) palloc(TOAST_COMPRESS_RAWSIZE(value) + VARHDRSZ);
 
-	rawsize = LZ4_decompress_safe_partial(TOAST_COMPRESS_RAWDATA(value),
+	rawsize = LZ4_decompress_safe_partial((char *) value + header_size,
 										  VARDATA(result),
-										  VARSIZE(value) - TOAST_COMPRESS_HDRSZ,
+										  VARSIZE(value) - header_size,
 										  slicelength,
 										  TOAST_COMPRESS_RAWSIZE(value));
 	if (rawsize < 0)
diff --git a/src/backend/access/compression/compress_pglz.c b/src/backend/access/compression/compress_pglz.c
index 578be6f..d33ea3d 100644
--- a/src/backend/access/compression/compress_pglz.c
+++ b/src/backend/access/compression/compress_pglz.c
@@ -27,7 +27,7 @@
  * compressed varlena, or NULL if compression fails.
  */
 static struct varlena *
-pglz_cmcompress(const struct varlena *value)
+pglz_cmcompress(const struct varlena *value, int32 header_size)
 {
 	int32		valsize,
 				len;
@@ -44,16 +44,16 @@ pglz_cmcompress(const struct varlena *value)
 		return NULL;
 
 	tmp = (struct varlena *) palloc(PGLZ_MAX_OUTPUT(valsize) +
-									TOAST_COMPRESS_HDRSZ);
+									header_size);
 
 	len = pglz_compress(VARDATA_ANY(DatumGetPointer(value)),
 						valsize,
-						TOAST_COMPRESS_RAWDATA(tmp),
+						(char *) tmp + header_size,
 						NULL);
 
 	if (len >= 0)
 	{
-		SET_VARSIZE_COMPRESSED(tmp, len + TOAST_COMPRESS_HDRSZ);
+		SET_VARSIZE_COMPRESSED(tmp, len + header_size);
 		return tmp;
 	}
 
@@ -68,7 +68,7 @@ pglz_cmcompress(const struct varlena *value)
  * Returns the decompressed varlena.
  */
 static struct varlena *
-pglz_cmdecompress(const struct varlena *value)
+pglz_cmdecompress(const struct varlena *value, int32 header_size)
 {
 	struct varlena *result;
 	int32		rawsize;
@@ -76,8 +76,8 @@ pglz_cmdecompress(const struct varlena *value)
 	result = (struct varlena *) palloc(TOAST_COMPRESS_RAWSIZE(value) + VARHDRSZ);
 	SET_VARSIZE(result, TOAST_COMPRESS_RAWSIZE(value) + VARHDRSZ);
 
-	rawsize = pglz_decompress(TOAST_COMPRESS_RAWDATA(value),
-							  TOAST_COMPRESS_SIZE(value),
+	rawsize = pglz_decompress((char *) value + header_size,
+							  VARSIZE(value) - header_size,
 							  VARDATA(result),
 							  TOAST_COMPRESS_RAWSIZE(value), true);
 
@@ -95,7 +95,7 @@ pglz_cmdecompress(const struct varlena *value)
  * Decompresses part of the data. Returns the decompressed varlena.
  */
 static struct varlena *
-pglz_cmdecompress_slice(const struct varlena *value,
+pglz_cmdecompress_slice(const struct varlena *value, int32 header_size,
 						int32 slicelength)
 {
 	struct varlena *result;
@@ -103,8 +103,8 @@ pglz_cmdecompress_slice(const struct varlena *value,
 
 	result = (struct varlena *) palloc(slicelength + VARHDRSZ);
 
-	rawsize = pglz_decompress(TOAST_COMPRESS_RAWDATA(value),
-							  VARSIZE(value) - TOAST_COMPRESS_HDRSZ,
+	rawsize = pglz_decompress((char *) value + header_size,
+							  VARSIZE(value) - header_size,
 							  VARDATA(result),
 							  slicelength, false);
 
diff --git a/src/backend/access/compression/compressamapi.c b/src/backend/access/compression/compressamapi.c
index 663102c..7aea8aa 100644
--- a/src/backend/access/compression/compressamapi.c
+++ b/src/backend/access/compression/compressamapi.c
@@ -19,6 +19,7 @@
 #include "access/htup_details.h"
 #include "access/reloptions.h"
 #include "access/table.h"
+#include "catalog/pg_am.h"
 #include "utils/fmgroids.h"
 #include "utils/syscache.h"
 
@@ -59,3 +60,29 @@ CompressionIdToOid(CompressionId cmid)
 			elog(ERROR, "Invalid compression method id %d", cmid);
 	}
 }
+
+/*
+ * GetCompressionAmRoutineByAmId - look up the handler of the compression access
+ * method with the given OID, and get its CompressionAmRoutine struct.
+ */
+CompressionAmRoutine *
+GetCompressionAmRoutineByAmId(Oid amoid)
+{
+	regproc		amhandler;
+	Datum		datum;
+	CompressionAmRoutine *routine;
+
+	/* Get handler function OID for the access method */
+	amhandler = GetAmHandlerByAmId(amoid, AMTYPE_COMPRESSION, false);
+	Assert(OidIsValid(amhandler));
+
+	/* And finally, call the handler function to get the API struct */
+	datum = OidFunctionCall0(amhandler);
+	routine = (CompressionAmRoutine *) DatumGetPointer(datum);
+
+	if (routine == NULL || !IsA(routine, CompressionAmRoutine))
+		elog(ERROR, "compression access method handler function %u did not return an CompressionAmRoutine struct",
+			 amhandler);
+
+	return routine;
+}
diff --git a/src/backend/access/index/amapi.c b/src/backend/access/index/amapi.c
index d30bc43..89a4b01 100644
--- a/src/backend/access/index/amapi.c
+++ b/src/backend/access/index/amapi.c
@@ -46,14 +46,14 @@ GetIndexAmRoutine(Oid amhandler)
 }
 
 /*
- * GetIndexAmRoutineByAmId - look up the handler of the index access method
- * with the given OID, and get its IndexAmRoutine struct.
+ * GetAmHandlerByAmId - look up the handler of the index/compression access
+ * method with the given OID, and get its handler function.
  *
- * If the given OID isn't a valid index access method, returns NULL if
- * noerror is true, else throws error.
+ * If the given OID isn't a valid index/compression access method, returns
+ * Invalid Oid if noerror is true, else throws error.
  */
-IndexAmRoutine *
-GetIndexAmRoutineByAmId(Oid amoid, bool noerror)
+regproc
+GetAmHandlerByAmId(Oid amoid, char amtype, bool noerror)
 {
 	HeapTuple	tuple;
 	Form_pg_am	amform;
@@ -64,24 +64,25 @@ GetIndexAmRoutineByAmId(Oid amoid, bool noerror)
 	if (!HeapTupleIsValid(tuple))
 	{
 		if (noerror)
-			return NULL;
+			return InvalidOid;
 		elog(ERROR, "cache lookup failed for access method %u",
 			 amoid);
 	}
 	amform = (Form_pg_am) GETSTRUCT(tuple);
 
 	/* Check if it's an index access method as opposed to some other AM */
-	if (amform->amtype != AMTYPE_INDEX)
+	if (amform->amtype != amtype)
 	{
 		if (noerror)
 		{
 			ReleaseSysCache(tuple);
-			return NULL;
+			return InvalidOid;
 		}
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("access method \"%s\" is not of type %s",
-						NameStr(amform->amname), "INDEX")));
+						NameStr(amform->amname), (amtype == AMTYPE_INDEX ?
+						"INDEX" : "COMPRESSION"))));
 	}
 
 	amhandler = amform->amhandler;
@@ -92,16 +93,41 @@ GetIndexAmRoutineByAmId(Oid amoid, bool noerror)
 		if (noerror)
 		{
 			ReleaseSysCache(tuple);
-			return NULL;
+			return InvalidOid;
 		}
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("index access method \"%s\" does not have a handler",
+				 errmsg("access method \"%s\" does not have a handler",
 						NameStr(amform->amname))));
 	}
 
 	ReleaseSysCache(tuple);
 
+	return amhandler;
+}
+
+/*
+ * GetIndexAmRoutineByAmId - look up the handler of the index access method
+ * with the given OID, and get its IndexAmRoutine struct.
+ *
+ * If the given OID isn't a valid index access method, returns NULL if
+ * noerror is true, else throws error.
+ */
+IndexAmRoutine *
+GetIndexAmRoutineByAmId(Oid amoid, bool noerror)
+{
+	regproc		amhandler;
+
+	/* Get handler function OID for the access method */
+	amhandler = GetAmHandlerByAmId(amoid, AMTYPE_INDEX, noerror);
+
+	/* Complain if handler OID is invalid */
+	if (!OidIsValid(amhandler))
+	{
+		Assert(noerror);
+		return NULL;
+	}
+
 	/* And finally, call the handler function to get the API struct. */
 	return GetIndexAmRoutine(amhandler);
 }
diff --git a/src/backend/commands/amcmds.c b/src/backend/commands/amcmds.c
index 668e43f..31d4a3e 100644
--- a/src/backend/commands/amcmds.c
+++ b/src/backend/commands/amcmds.c
@@ -241,6 +241,8 @@ get_am_type_string(char amtype)
 			return "INDEX";
 		case AMTYPE_TABLE:
 			return "TABLE";
+		case AMTYPE_COMPRESSION:
+			return "TABLE";
 		default:
 			/* shouldn't happen */
 			elog(ERROR, "invalid access method type '%c'", amtype);
@@ -278,6 +280,9 @@ lookup_am_handler_func(List *handler_name, char amtype)
 		case AMTYPE_TABLE:
 			expectedType = TABLE_AM_HANDLEROID;
 			break;
+		case AMTYPE_COMPRESSION:
+			expectedType = COMPRESSION_AM_HANDLEROID;
+			break;
 		default:
 			elog(ERROR, "unrecognized access method type \"%c\"", amtype);
 	}
diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 916f654..a637372 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -2089,7 +2089,7 @@ CompareCompressionMethodAndDecompress(TupleTableSlot *slot,
 			 * compression method is not supported by the target attribute then
 			 * we need to decompress it.
 			 */
-			cmoid = CompressionIdToOid(TOAST_COMPRESS_METHOD(new_value));
+			cmoid = toast_get_compression_oid(new_value);
 			if (!IsCompressionSupported(&targetTupDesc->attrs[i], cmoid))
 			{
 				new_value = detoast_attr(new_value);
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 332e60f..0a0ea05 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -5304,6 +5304,7 @@ CreateAmStmt: CREATE ACCESS METHOD name TYPE_P am_type HANDLER handler_name
 am_type:
 			INDEX			{ $$ = AMTYPE_INDEX; }
 		|	TABLE			{ $$ = AMTYPE_TABLE; }
+		|	COMPRESSION		{ $$ = AMTYPE_COMPRESSION; }
 		;
 
 /*****************************************************************************
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index aff5b7e..ebd7971 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -5309,7 +5309,6 @@ Datum
 pg_column_compression(PG_FUNCTION_ARGS)
 {
 	Datum		value = PG_GETARG_DATUM(0);
-	char	   *compression;
 	int			typlen;
 	struct varlena *varvalue;
 
@@ -5339,10 +5338,8 @@ pg_column_compression(PG_FUNCTION_ARGS)
 
 	varvalue = (struct varlena *) DatumGetPointer(value);
 
-	compression =
-		get_am_name(CompressionIdToOid(TOAST_COMPRESS_METHOD(varvalue)));
-
-	PG_RETURN_TEXT_P(cstring_to_text(compression));
+	PG_RETURN_TEXT_P(cstring_to_text(get_am_name(
+								toast_get_compression_oid(varvalue))));
 }
 
 /*
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index 57df7b9..ca2063e 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -13124,6 +13124,9 @@ dumpAccessMethod(Archive *fout, AccessMethodInfo *aminfo)
 		case AMTYPE_TABLE:
 			appendPQExpBufferStr(q, "TYPE TABLE ");
 			break;
+		case AMTYPE_COMPRESSION:
+			appendPQExpBufferStr(q, "TYPE COMPRESSION ");
+			break;
 		default:
 			pg_log_warning("invalid type \"%c\" of access method \"%s\"",
 						   aminfo->amtype, qamname);
diff --git a/src/bin/psql/tab-complete.c b/src/bin/psql/tab-complete.c
index c40d498..5de2a6a 100644
--- a/src/bin/psql/tab-complete.c
+++ b/src/bin/psql/tab-complete.c
@@ -883,6 +883,12 @@ static const SchemaQuery Query_for_list_of_statistics = {
 "  WHERE substring(pg_catalog.quote_ident(amname),1,%d)='%s' AND "\
 "   amtype=" CppAsString2(AMTYPE_TABLE)
 
+#define Query_for_list_of_compression_access_methods \
+" SELECT pg_catalog.quote_ident(amname) "\
+"   FROM pg_catalog.pg_am "\
+"  WHERE substring(pg_catalog.quote_ident(amname),1,%d)='%s' AND "\
+"   amtype=" CppAsString2(AMTYPE_COMPRESSION)
+
 /* the silly-looking length condition is just to eat up the current word */
 #define Query_for_list_of_arguments \
 "SELECT pg_catalog.oidvectortypes(proargtypes)||')' "\
diff --git a/src/include/access/amapi.h b/src/include/access/amapi.h
index d357ebb..0f7b5b0 100644
--- a/src/include/access/amapi.h
+++ b/src/include/access/amapi.h
@@ -285,6 +285,7 @@ typedef struct IndexAmRoutine
 
 /* Functions in access/index/amapi.c */
 extern IndexAmRoutine *GetIndexAmRoutine(Oid amhandler);
+extern regproc GetAmHandlerByAmId(Oid amoid, char amtype, bool noerror);
 extern IndexAmRoutine *GetIndexAmRoutineByAmId(Oid amoid, bool noerror);
 
 #endif							/* AMAPI_H */
diff --git a/src/include/access/compressamapi.h b/src/include/access/compressamapi.h
index 8226ae0..913a633 100644
--- a/src/include/access/compressamapi.h
+++ b/src/include/access/compressamapi.h
@@ -26,18 +26,25 @@
 typedef enum CompressionId
 {
 	PGLZ_COMPRESSION_ID = 0,
-	LZ4_COMPRESSION_ID = 1
+	LZ4_COMPRESSION_ID = 1,
+	/* one free slot for the future built-in method */
+	CUSTOM_COMPRESSION_ID = 3
 } CompressionId;
 
 /* Use default compression method if it is not specified. */
 #define DefaultCompressionOid	PGLZ_COMPRESSION_AM_OID
+#define IsCustomCompression(cmid)     ((cmid) == CUSTOM_COMPRESSION_ID)
 #define IsStorageCompressible(storage) ((storage) != TYPSTORAGE_PLAIN && \
 										(storage) != TYPSTORAGE_EXTERNAL)
 /* compression handler routines */
-typedef struct varlena *(*cmcompress_function) (const struct varlena *value);
-typedef struct varlena *(*cmdecompress_function) (const struct varlena *value);
+typedef struct varlena *(*cmcompress_function) (const struct varlena *value,
+												int32 toast_header_size);
+typedef struct varlena *(*cmdecompress_function) (const struct varlena *value,
+												  int32 toast_header_size);
 typedef struct varlena *(*cmdecompress_slice_function)
-			(const struct varlena *value, int32 slicelength);
+												(const struct varlena *value,
+												 int32 toast_header_size,
+												 int32 slicelength);
 
 /*
  * API struct for a compression AM.
@@ -61,5 +68,6 @@ extern const CompressionAmRoutine lz4_compress_methods;
 /* access/compression/compressamapi.c */
 extern CompressionId CompressionOidToId(Oid cmoid);
 extern Oid CompressionIdToOid(CompressionId cmid);
+extern CompressionAmRoutine *GetCompressionAmRoutineByAmId(Oid amoid);
 
 #endif							/* COMPRESSAMAPI_H */
diff --git a/src/include/access/detoast.h b/src/include/access/detoast.h
index 0adf53c..6cdc375 100644
--- a/src/include/access/detoast.h
+++ b/src/include/access/detoast.h
@@ -89,4 +89,12 @@ extern Size toast_raw_datum_size(Datum value);
  */
 extern Size toast_datum_size(Datum value);
 
+/* ----------
+ * toast_get_compression_oid -
+ *
+ *	Return the compression method oid from the compressed value
+ * ----------
+ */
+extern Oid toast_get_compression_oid(struct varlena *attr);
+
 #endif							/* DETOAST_H */
diff --git a/src/include/access/toast_internals.h b/src/include/access/toast_internals.h
index 84ba303..c158354 100644
--- a/src/include/access/toast_internals.h
+++ b/src/include/access/toast_internals.h
@@ -27,6 +27,17 @@ typedef struct toast_compress_header
 								 * rawsize */
 } toast_compress_header;
 
+/*
+ * If the compression method were used, then data also contains
+ * Oid of compression options
+ */
+typedef struct toast_compress_header_custom
+{
+	int32		vl_len_;		/* varlena header (do not touch directly!) */
+	uint32		info;			/*  2 bits for compression method + rawsize */
+	Oid			cmoid;			/* Oid from pg_am */
+} toast_compress_header_custom;
+
 #define RAWSIZEMASK (0x3FFFFFFFU)
 
 /*
@@ -38,6 +49,7 @@ typedef struct toast_compress_header
  * two highest bits.
  */
 #define TOAST_COMPRESS_HDRSZ		((int32) sizeof(toast_compress_header))
+#define TOAST_CUSTOM_COMPRESS_HDRSZ ((int32)sizeof(toast_compress_header_custom))
 #define TOAST_COMPRESS_RAWSIZE(ptr) (((toast_compress_header *) (ptr))->info & RAWSIZEMASK)
 #define TOAST_COMPRESS_METHOD(ptr)  (((toast_compress_header *) (ptr))->info >> 30)
 #define TOAST_COMPRESS_SIZE(ptr)	((int32) VARSIZE_ANY(ptr) - TOAST_COMPRESS_HDRSZ)
@@ -49,6 +61,9 @@ typedef struct toast_compress_header
 		((toast_compress_header *) (ptr))->info = ((len) | (cm_method) << 30); \
 	} while (0)
 
+#define TOAST_COMPRESS_SET_CMOID(ptr, oid) \
+	(((toast_compress_header_custom *)(ptr))->cmoid = (oid))
+
 extern Datum toast_compress_datum(Datum value, Oid cmoid);
 extern Oid	toast_get_valid_index(Oid toastoid, LOCKMODE lock);
 
diff --git a/src/test/regress/expected/compression.out b/src/test/regress/expected/compression.out
index 151478b..636ab40 100644
--- a/src/test/regress/expected/compression.out
+++ b/src/test/regress/expected/compression.out
@@ -235,13 +235,51 @@ SELECT pg_column_compression(f1) FROM cmdata;
  pglz
 (2 rows)
 
+-- create compression method
+CREATE ACCESS METHOD pglz2 TYPE COMPRESSION HANDLER pglzhandler;
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION pglz2 PRESERVE ALL;
+INSERT INTO cmdata VALUES (repeat('1234567890',1004));
+\d+ cmdata
+                                        Table "public.cmdata"
+ Column | Type | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
+--------+------+-----------+----------+---------+----------+-------------+--------------+-------------
+ f1     | text |           |          |         | extended | pglz2       |              | 
+Indexes:
+    "idx" btree (f1)
+
+SELECT pg_column_compression(f1) FROM cmdata;
+ pg_column_compression 
+-----------------------
+ lz4
+ pglz
+ pglz2
+(3 rows)
+
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION lz4 PRESERVE (pglz2);
+SELECT pg_column_compression(f1) FROM cmdata;
+ pg_column_compression 
+-----------------------
+ lz4
+ lz4
+ pglz2
+(3 rows)
+
+\d+ cmdata
+                                        Table "public.cmdata"
+ Column | Type | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
+--------+------+-----------+----------+---------+----------+-------------+--------------+-------------
+ f1     | text |           |          |         | extended | lz4         |              | 
+Indexes:
+    "idx" btree (f1)
+
 -- check data is ok
 SELECT length(f1) FROM cmdata;
  length 
 --------
   10000
   10040
-(2 rows)
+  10040
+(3 rows)
 
 SELECT length(f1) FROM cmdata1;
  length 
diff --git a/src/test/regress/expected/compression_1.out b/src/test/regress/expected/compression_1.out
index 81182fe..3177ff9 100644
--- a/src/test/regress/expected/compression_1.out
+++ b/src/test/regress/expected/compression_1.out
@@ -232,13 +232,52 @@ SELECT pg_column_compression(f1) FROM cmdata;
  pglz
 (2 rows)
 
+-- create compression method
+CREATE ACCESS METHOD pglz2 TYPE COMPRESSION HANDLER pglzhandler;
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION pglz2 PRESERVE ALL;
+INSERT INTO cmdata VALUES (repeat('1234567890',1004));
+\d+ cmdata
+                                        Table "public.cmdata"
+ Column | Type | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
+--------+------+-----------+----------+---------+----------+-------------+--------------+-------------
+ f1     | text |           |          |         | extended | pglz2       |              | 
+Indexes:
+    "idx" btree (f1)
+
+SELECT pg_column_compression(f1) FROM cmdata;
+ pg_column_compression 
+-----------------------
+ pglz
+ pglz
+ pglz2
+(3 rows)
+
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION lz4 PRESERVE (pglz2);
+ERROR:  not built with lz4 support
+SELECT pg_column_compression(f1) FROM cmdata;
+ pg_column_compression 
+-----------------------
+ pglz
+ pglz
+ pglz2
+(3 rows)
+
+\d+ cmdata
+                                        Table "public.cmdata"
+ Column | Type | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
+--------+------+-----------+----------+---------+----------+-------------+--------------+-------------
+ f1     | text |           |          |         | extended | pglz2       |              | 
+Indexes:
+    "idx" btree (f1)
+
 -- check data is ok
 SELECT length(f1) FROM cmdata;
  length 
 --------
   10000
   10040
-(2 rows)
+  10040
+(3 rows)
 
 SELECT length(f1) FROM cmdata1;
  length 
diff --git a/src/test/regress/sql/compression.sql b/src/test/regress/sql/compression.sql
index e447316..0552eec 100644
--- a/src/test/regress/sql/compression.sql
+++ b/src/test/regress/sql/compression.sql
@@ -95,6 +95,16 @@ SELECT pg_column_compression(f1) FROM cmdata;
 ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION lz4 PRESERVE ALL;
 SELECT pg_column_compression(f1) FROM cmdata;
 
+-- create compression method
+CREATE ACCESS METHOD pglz2 TYPE COMPRESSION HANDLER pglzhandler;
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION pglz2 PRESERVE ALL;
+INSERT INTO cmdata VALUES (repeat('1234567890',1004));
+\d+ cmdata
+SELECT pg_column_compression(f1) FROM cmdata;
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION lz4 PRESERVE (pglz2);
+SELECT pg_column_compression(f1) FROM cmdata;
+\d+ cmdata
+
 -- check data is ok
 SELECT length(f1) FROM cmdata;
 SELECT length(f1) FROM cmdata1;
-- 
1.8.3.1