v32-0003-Alter-table-set-compression.patch

From 5f7eeb3a21160e59841a57d03854f6ca56bdc24d Mon Sep 17 00:00:00 2001
From: Dilip Kumar <dilipkumar@localhost.localdomain>
Date: Sat, 6 Mar 2021 13:10:23 +0530
Subject: [PATCH v32 3/4] Alter table set compression

Add support for changing the compression method associated
with a column.  There are only built-in methods so we don't
need to rewrite the table, only the new tuples will be
compressed with the new compression method.

Dilip Kumar based on the patches from Ildus Kurbangaliev.
Design input from Robert Haas and Tomas Vondra.
Reviewed by Robert Haas, Tomas Vondra, Alexander Korotkov and Justin Pryzby

Discussions:
https://www.postgresql.org/message-id/20171213151818.75a20259@postgrespro.ru
https://www.postgresql.org/message-id/CA%2BTgmoaKDW1Oi9V%3Djc9hOGyf77NbkNEABuqgHD1Cq%3D%3D1QsOcxg%40mail.gmail.com
https://www.postgresql.org/message-id/CA%2BTgmobSDVgUage9qQ5P_%3DF_9jaMkCgyKxUQGtFQU7oN4kX-AA%40mail.gmail.com
https://www.postgresql.org/message-id/20201005160355.byp74sh3ejsv7wrj%40development
https://www.postgresql.org/message-id/CAFiTN-tzTTT2oqWdRGLv1dvvS5MC1W%2BLE%2B3bqWPJUZj4GnHOJg%40mail.gmail.com
---
 doc/src/sgml/ref/alter_table.sgml           |  16 +++
 src/backend/commands/tablecmds.c            | 198 ++++++++++++++++++++++------
 src/backend/parser/gram.y                   |   9 ++
 src/bin/psql/tab-complete.c                 |   2 +-
 src/include/nodes/parsenodes.h              |   3 +-
 src/test/regress/expected/compression.out   |  47 ++++++-
 src/test/regress/expected/compression_1.out |  48 ++++++-
 src/test/regress/sql/compression.sql        |  20 +++
 8 files changed, 296 insertions(+), 47 deletions(-)

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index c25ef5a..0bd0c1a 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -54,6 +54,7 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET ( <replaceable class="parameter">attribute_option</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> RESET ( <replaceable class="parameter">attribute_option</replaceable> [, ... ] )
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
+    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET COMPRESSION <replaceable class="parameter">compression_method</replaceable>
     ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]
     ADD <replaceable class="parameter">table_constraint_using_index</replaceable>
     ALTER CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
@@ -103,6 +104,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
   GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
   UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
+  COMPRESSION <replaceable class="parameter">compression_method</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> ] }
 [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
@@ -384,6 +386,20 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    </varlistentry>
 
    <varlistentry>
+    <term>
+     <literal>SET COMPRESSION <replaceable class="parameter">compression_method</replaceable></literal>
+    </term>
+    <listitem>
+     <para>
+      This sets the compression method for a column.  The supported compression
+      methods are <literal>pglz</literal> and <literal>lz4</literal>.
+      <literal>lz4</literal> is available only if <literal>--with-lz4</literal>
+      was used when building <productname>PostgreSQL</productname>.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><literal>ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]</literal></term>
     <listitem>
      <para>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 45d3496..c75c94a 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -530,6 +530,8 @@ static void ATExecReplicaIdentity(Relation rel, ReplicaIdentityStmt *stmt, LOCKM
 static void ATExecGenericOptions(Relation rel, List *options);
 static void ATExecSetRowSecurity(Relation rel, bool rls);
 static void ATExecForceNoForceRowSecurity(Relation rel, bool force_rls);
+static ObjectAddress ATExecSetCompression(AlteredTableInfo *tab, Relation rel,
+					 const char *column, Node *newValue, LOCKMODE lockmode);
 
 static void index_copy_data(Relation rel, RelFileNode newrnode);
 static const char *storage_name(char c);
@@ -3974,6 +3976,7 @@ AlterTableGetLockLevel(List *cmds)
 				 */
 			case AT_GenericOptions:
 			case AT_AlterColumnGenericOptions:
+			case AT_SetCompression:
 				cmd_lockmode = AccessExclusiveLock;
 				break;
 
@@ -4501,7 +4504,8 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 		case AT_DisableRowSecurity:
 		case AT_ForceRowSecurity:
 		case AT_NoForceRowSecurity:
-			ATSimplePermissions(rel, ATT_TABLE);
+		case AT_SetCompression:
+			ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW);
 			/* These commands never recurse */
 			/* No command-specific prep needed */
 			pass = AT_PASS_MISC;
@@ -4909,6 +4913,10 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			Assert(rel->rd_rel->relkind == RELKIND_INDEX);
 			ATExecAlterCollationRefreshVersion(rel, cmd->object);
 			break;
+		case AT_SetCompression:
+			address = ATExecSetCompression(tab, rel, cmd->name, cmd->def,
+										   lockmode);
+			break;
 		default:				/* oops */
 			elog(ERROR, "unrecognized alter table type: %d",
 				 (int) cmd->subtype);
@@ -7774,6 +7782,67 @@ ATExecSetOptions(Relation rel, const char *colName, Node *options,
 }
 
 /*
+ * Helper function for ATExecSetStorage and ATExecSetCompression
+ *
+ * Set the attcompression and/or attstorage for the respective index attribute
+ * if the respective input values are valid.
+ */
+static void
+ApplyChangesToIndexes(Relation rel, Relation attrelation, AttrNumber attnum,
+					  char newcompression, char newstorage, LOCKMODE lockmode)
+{
+	HeapTuple	tuple;
+	ListCell   *lc;
+	Form_pg_attribute attrtuple;
+
+	foreach(lc, RelationGetIndexList(rel))
+	{
+		Oid			indexoid = lfirst_oid(lc);
+		Relation	indrel;
+		AttrNumber	indattnum = 0;
+
+		indrel = index_open(indexoid, lockmode);
+
+		for (int i = 0; i < indrel->rd_index->indnatts; i++)
+		{
+			if (indrel->rd_index->indkey.values[i] == attnum)
+			{
+				indattnum = i + 1;
+				break;
+			}
+		}
+
+		if (indattnum == 0)
+		{
+			index_close(indrel, lockmode);
+			continue;
+		}
+
+		tuple = SearchSysCacheCopyAttNum(RelationGetRelid(indrel), indattnum);
+
+		if (HeapTupleIsValid(tuple))
+		{
+			attrtuple = (Form_pg_attribute) GETSTRUCT(tuple);
+
+			if (CompressionMethodIsValid(newcompression))
+				attrtuple->attcompression = newcompression;
+
+			if (newstorage != '\0')
+				attrtuple->attstorage = newstorage;
+
+			CatalogTupleUpdate(attrelation, &tuple->t_self, tuple);
+
+			InvokeObjectPostAlterHook(RelationRelationId,
+									  RelationGetRelid(rel),
+									  attrtuple->attnum);
+
+			heap_freetuple(tuple);
+		}
+
+		index_close(indrel, lockmode);
+	}
+}
+/*
  * ALTER TABLE ALTER COLUMN SET STORAGE
  *
  * Return value is the address of the modified column
@@ -7788,7 +7857,6 @@ ATExecSetStorage(Relation rel, const char *colName, Node *newValue, LOCKMODE loc
 	Form_pg_attribute attrtuple;
 	AttrNumber	attnum;
 	ObjectAddress address;
-	ListCell   *lc;
 
 	Assert(IsA(newValue, String));
 	storagemode = strVal(newValue);
@@ -7852,47 +7920,8 @@ ATExecSetStorage(Relation rel, const char *colName, Node *newValue, LOCKMODE loc
 	 * Apply the change to indexes as well (only for simple index columns,
 	 * matching behavior of index.c ConstructTupleDescriptor()).
 	 */
-	foreach(lc, RelationGetIndexList(rel))
-	{
-		Oid			indexoid = lfirst_oid(lc);
-		Relation	indrel;
-		AttrNumber	indattnum = 0;
-
-		indrel = index_open(indexoid, lockmode);
-
-		for (int i = 0; i < indrel->rd_index->indnatts; i++)
-		{
-			if (indrel->rd_index->indkey.values[i] == attnum)
-			{
-				indattnum = i + 1;
-				break;
-			}
-		}
-
-		if (indattnum == 0)
-		{
-			index_close(indrel, lockmode);
-			continue;
-		}
-
-		tuple = SearchSysCacheCopyAttNum(RelationGetRelid(indrel), indattnum);
-
-		if (HeapTupleIsValid(tuple))
-		{
-			attrtuple = (Form_pg_attribute) GETSTRUCT(tuple);
-			attrtuple->attstorage = newstorage;
-
-			CatalogTupleUpdate(attrelation, &tuple->t_self, tuple);
-
-			InvokeObjectPostAlterHook(RelationRelationId,
-									  RelationGetRelid(rel),
-									  attrtuple->attnum);
-
-			heap_freetuple(tuple);
-		}
-
-		index_close(indrel, lockmode);
-	}
+	ApplyChangesToIndexes(rel, attrelation, attnum, InvalidCompressionMethod,
+						  newstorage, lockmode);
 
 	table_close(attrelation, RowExclusiveLock);
 
@@ -15018,6 +15047,89 @@ ATExecGenericOptions(Relation rel, List *options)
 }
 
 /*
+ * ALTER TABLE ALTER COLUMN SET COMPRESSION
+ *
+ * Return value is the address of the modified column
+ */
+static ObjectAddress
+ATExecSetCompression(AlteredTableInfo *tab,
+					 Relation rel,
+					 const char *column,
+					 Node *newValue,
+					 LOCKMODE lockmode)
+{
+	Relation	attrel;
+	HeapTuple	tuple;
+	Form_pg_attribute atttableform;
+	AttrNumber	attnum;
+	char	   *compression;
+	char		typstorage;
+	Oid			cmoid;
+	Datum		values[Natts_pg_attribute];
+	bool		nulls[Natts_pg_attribute];
+	bool		replace[Natts_pg_attribute];
+	ObjectAddress address;
+
+	Assert(IsA(newValue, String));
+	compression = strVal(newValue);
+
+	attrel = table_open(AttributeRelationId, RowExclusiveLock);
+
+	tuple = SearchSysCacheAttName(RelationGetRelid(rel), column);
+	if (!HeapTupleIsValid(tuple))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_COLUMN),
+				 errmsg("column \"%s\" of relation \"%s\" does not exist",
+						column, RelationGetRelationName(rel))));
+
+	/* prevent them from altering a system attribute */
+	atttableform = (Form_pg_attribute) GETSTRUCT(tuple);
+	attnum = atttableform->attnum;
+	if (attnum <= 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot alter system column \"%s\"", column)));
+
+	typstorage = get_typstorage(atttableform->atttypid);
+
+	/* prevent from setting compression methods for uncompressible type */
+	if (!IsStorageCompressible(typstorage))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					errmsg("column data type %s does not support compression",
+						format_type_be(atttableform->atttypid))));
+
+	/* initialize buffers for new tuple values */
+	memset(values, 0, sizeof(values));
+	memset(nulls, false, sizeof(nulls));
+	memset(replace, false, sizeof(replace));
+
+	/* get the attribute compression method. */
+	cmoid = GetAttributeCompression(atttableform, compression);
+
+	atttableform->attcompression = cmoid;
+	CatalogTupleUpdate(attrel, &tuple->t_self, tuple);
+
+	InvokeObjectPostAlterHook(RelationRelationId,
+							  RelationGetRelid(rel),
+							  atttableform->attnum);
+
+	ReleaseSysCache(tuple);
+
+	/* apply changes to the index column as well */
+	ApplyChangesToIndexes(rel, attrel, attnum, cmoid, '\0', lockmode);
+	table_close(attrel, RowExclusiveLock);
+
+	/* make changes visible */
+	CommandCounterIncrement();
+
+	ObjectAddressSubSet(address, RelationRelationId,
+						RelationGetRelid(rel), atttableform->attnum);
+	return address;
+}
+
+
+/*
  * Preparation phase for SET LOGGED/UNLOGGED
  *
  * This verifies that we're not trying to change a temp table.  Also,
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 9d923b5..5c4e779 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -2308,6 +2308,15 @@ alter_table_cmd:
 					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
+			/* ALTER TABLE <name> ALTER [COLUMN] <colname> SET (COMPRESSION <cm>) */
+			| ALTER opt_column ColId SET optColumnCompression
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_SetCompression;
+					n->name = $3;
+					n->def = (Node *) makeString($5);
+					$$ = (Node *)n;
+				}
 			/* ALTER TABLE <name> DROP [COLUMN] IF EXISTS <colname> [RESTRICT|CASCADE] */
 			| DROP opt_column IF_P EXISTS ColId opt_drop_behavior
 				{
diff --git a/src/bin/psql/tab-complete.c b/src/bin/psql/tab-complete.c
index 9f0208a..07ba55f 100644
--- a/src/bin/psql/tab-complete.c
+++ b/src/bin/psql/tab-complete.c
@@ -2115,7 +2115,7 @@ psql_completion(const char *text, int start, int end)
 	/* ALTER TABLE ALTER [COLUMN] <foo> SET */
 	else if (Matches("ALTER", "TABLE", MatchAny, "ALTER", "COLUMN", MatchAny, "SET") ||
 			 Matches("ALTER", "TABLE", MatchAny, "ALTER", MatchAny, "SET"))
-		COMPLETE_WITH("(", "DEFAULT", "NOT NULL", "STATISTICS", "STORAGE");
+		COMPLETE_WITH("(", "COMPRESSION", "DEFAULT", "NOT NULL", "STATISTICS", "STORAGE");
 	/* ALTER TABLE ALTER [COLUMN] <foo> SET ( */
 	else if (Matches("ALTER", "TABLE", MatchAny, "ALTER", "COLUMN", MatchAny, "SET", "(") ||
 			 Matches("ALTER", "TABLE", MatchAny, "ALTER", MatchAny, "SET", "("))
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 19d2ba2..f9a87de 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1901,7 +1901,8 @@ typedef enum AlterTableType
 	AT_AddIdentity,				/* ADD IDENTITY */
 	AT_SetIdentity,				/* SET identity column options */
 	AT_DropIdentity,			/* DROP IDENTITY */
-	AT_AlterCollationRefreshVersion /* ALTER COLLATION ... REFRESH VERSION */
+	AT_AlterCollationRefreshVersion, /* ALTER COLLATION ... REFRESH VERSION */
+	AT_SetCompression			/* SET COMPRESSION */
 } AlterTableType;
 
 typedef struct ReplicaIdentityStmt
diff --git a/src/test/regress/expected/compression.out b/src/test/regress/expected/compression.out
index 3b546da..36b6376 100644
--- a/src/test/regress/expected/compression.out
+++ b/src/test/regress/expected/compression.out
@@ -165,12 +165,57 @@ CREATE TABLE cminh(f1 TEXT COMPRESSION lz4) INHERITS(cmdata);
 NOTICE:  merging column "f1" with inherited definition
 ERROR:  column "f1" has a compression method conflict
 DETAIL:  pglz versus lz4
+-- test alter compression method
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION lz4;
+INSERT INTO cmdata VALUES (repeat('123456789',4004));
+\d+ cmdata
+                                        Table "public.cmdata"
+ Column | Type | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
+--------+------+-----------+----------+---------+----------+-------------+--------------+-------------
+ f1     | text |           |          |         | extended | lz4         |              | 
+Indexes:
+    "idx" btree (f1)
+
+SELECT pg_column_compression(f1) FROM cmdata;
+ pg_column_compression 
+-----------------------
+ pglz
+ lz4
+(2 rows)
+
+-- test alter compression method for the materialized view
+ALTER MATERIALIZED VIEW mv ALTER COLUMN x SET COMPRESSION lz4;
+\d+ mv
+                                    Materialized view "public.mv"
+ Column | Type | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
+--------+------+-----------+----------+---------+----------+-------------+--------------+-------------
+ x      | text |           |          |         | extended | lz4         |              | 
+View definition:
+ SELECT cmdata1.f1 AS x
+   FROM cmdata1;
+
+-- test alter compression method for the partitioned table
+ALTER TABLE cmpart1 ALTER COLUMN f1 SET COMPRESSION pglz;
+ALTER TABLE cmpart2 ALTER COLUMN f1 SET COMPRESSION lz4;
+-- new data should be compressed with the current compression method
+INSERT INTO cmpart VALUES (repeat('123456789',1004));
+INSERT INTO cmpart VALUES (repeat('123456789',4004));
+SELECT pg_column_compression(f1) FROM cmpart;
+ pg_column_compression 
+-----------------------
+ lz4
+ pglz
+ pglz
+ lz4
+(4 rows)
+
 -- check data is ok
 SELECT length(f1) FROM cmdata;
  length 
 --------
   10000
-(1 row)
+  36036
+(2 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 c7fd3cc..62ce8cd 100644
--- a/src/test/regress/expected/compression_1.out
+++ b/src/test/regress/expected/compression_1.out
@@ -161,12 +161,58 @@ CREATE TABLE cminh(f1 TEXT COMPRESSION lz4) INHERITS(cmdata);
 NOTICE:  merging column "f1" with inherited definition
 ERROR:  column "f1" has a compression method conflict
 DETAIL:  pglz versus lz4
+-- test alter compression method
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION lz4;
+ERROR:  unsupported LZ4 compression method
+DETAIL:  This functionality requires the server to be built with lz4 support.
+HINT:  You need to rebuild PostgreSQL using --with-lz4.
+INSERT INTO cmdata VALUES (repeat('123456789',4004));
+\d+ cmdata
+                                        Table "public.cmdata"
+ Column | Type | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
+--------+------+-----------+----------+---------+----------+-------------+--------------+-------------
+ f1     | text |           |          |         | extended | pglz        |              | 
+Indexes:
+    "idx" btree (f1)
+
+SELECT pg_column_compression(f1) FROM cmdata;
+ pg_column_compression 
+-----------------------
+ pglz
+ pglz
+(2 rows)
+
+-- test alter compression method for the materialized view
+ALTER MATERIALIZED VIEW mv ALTER COLUMN x SET COMPRESSION lz4;
+ERROR:  relation "mv" does not exist
+\d+ mv
+-- test alter compression method for the partitioned table
+ALTER TABLE cmpart1 ALTER COLUMN f1 SET COMPRESSION pglz;
+ERROR:  relation "cmpart1" does not exist
+ALTER TABLE cmpart2 ALTER COLUMN f1 SET COMPRESSION lz4;
+ERROR:  unsupported LZ4 compression method
+DETAIL:  This functionality requires the server to be built with lz4 support.
+HINT:  You need to rebuild PostgreSQL using --with-lz4.
+-- new data should be compressed with the current compression method
+INSERT INTO cmpart VALUES (repeat('123456789',1004));
+ERROR:  relation "cmpart" does not exist
+LINE 1: INSERT INTO cmpart VALUES (repeat('123456789',1004));
+                    ^
+INSERT INTO cmpart VALUES (repeat('123456789',4004));
+ERROR:  relation "cmpart" does not exist
+LINE 1: INSERT INTO cmpart VALUES (repeat('123456789',4004));
+                    ^
+SELECT pg_column_compression(f1) FROM cmpart;
+ERROR:  relation "cmpart" does not exist
+LINE 1: SELECT pg_column_compression(f1) FROM cmpart;
+                                              ^
 -- check data is ok
 SELECT length(f1) FROM cmdata;
  length 
 --------
   10000
-(1 row)
+  36036
+(2 rows)
 
 SELECT length(f1) FROM cmdata1;
 ERROR:  relation "cmdata1" does not exist
diff --git a/src/test/regress/sql/compression.sql b/src/test/regress/sql/compression.sql
index ee46ba0..d2478b4 100644
--- a/src/test/regress/sql/compression.sql
+++ b/src/test/regress/sql/compression.sql
@@ -74,6 +74,26 @@ SELECT pg_column_compression(f1) FROM cmpart;
 CREATE TABLE cminh() INHERITS(cmdata, cmdata1);
 CREATE TABLE cminh(f1 TEXT COMPRESSION lz4) INHERITS(cmdata);
 
+-- test alter compression method
+ALTER TABLE cmdata ALTER COLUMN f1 SET COMPRESSION lz4;
+INSERT INTO cmdata VALUES (repeat('123456789',4004));
+\d+ cmdata
+SELECT pg_column_compression(f1) FROM cmdata;
+
+-- test alter compression method for the materialized view
+ALTER MATERIALIZED VIEW mv ALTER COLUMN x SET COMPRESSION lz4;
+\d+ mv
+
+-- test alter compression method for the partitioned table
+ALTER TABLE cmpart1 ALTER COLUMN f1 SET COMPRESSION pglz;
+ALTER TABLE cmpart2 ALTER COLUMN f1 SET COMPRESSION lz4;
+
+-- new data should be compressed with the current compression method
+INSERT INTO cmpart VALUES (repeat('123456789',1004));
+INSERT INTO cmpart VALUES (repeat('123456789',4004));
+
+SELECT pg_column_compression(f1) FROM cmpart;
+
 -- check data is ok
 SELECT length(f1) FROM cmdata;
 SELECT length(f1) FROM cmdata1;
-- 
1.8.3.1