Add support for SET ACCESS METHOD in ALTER TABLE

The logic used to support a change of access method for a table is
similar to changes for tablespace or relation persistence, requiring a
table rewrite with an exclusive lock of the relation changed.  Table
rewrites done in ALTER TABLE already go through the table AM layer when
scanning tuples from the old relation and inserting them into the new
one, making this implementation straight-forward.

Note that partitioned tables are not supported as these have no access
methods defined.

Author: Justin Pryzby, Jeff Davis
Reviewed-by: Michael Paquier, Vignesh C
Discussion: https://postgr.es/m/20210228222530.GD20769@telsasoft.com

doc/src/sgml/ref/alter_table.sgml

@@ -75,6 +75,7 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
     CLUSTER ON <replaceable class="parameter">index_name</replaceable>
     SET WITHOUT CLUSTER
     SET WITHOUT OIDS
+    SET ACCESS METHOD <replaceable class="parameter">new_access_method</replaceable>
     SET TABLESPACE <replaceable class="parameter">new_tablespace</replaceable>
     SET { LOGGED | UNLOGGED }
     SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )
@@ -692,6 +693,16 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>SET ACCESS METHOD</literal></term>
+    <listitem>
+     <para>
+      This form changes the access method of the table by rewriting it. See
+      <xref linkend="tableam"/> for more information.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>SET TABLESPACE</literal></term>
     <listitem>
@@ -1228,6 +1239,15 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><replaceable class="parameter">new_access_method</replaceable></term>
+      <listitem>
+       <para>
+        The name of the access method to which the table will be converted.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><replaceable class="parameter">new_tablespace</replaceable></term>
       <listitem>

src/backend/commands/cluster.c

@@ -576,6 +576,7 @@ static void
 rebuild_relation(Relation OldHeap, Oid indexOid, bool verbose)
 {
 	Oid			tableOid = RelationGetRelid(OldHeap);
+	Oid			accessMethod = OldHeap->rd_rel->relam;
 	Oid			tableSpace = OldHeap->rd_rel->reltablespace;
 	Oid			OIDNewHeap;
 	char		relpersistence;
@@ -597,6 +598,7 @@ rebuild_relation(Relation OldHeap, Oid indexOid, bool verbose)
 
 	/* Create the transient table that will receive the re-ordered data */
 	OIDNewHeap = make_new_heap(tableOid, tableSpace,
+							   accessMethod,
 							   relpersistence,
 							   AccessExclusiveLock);
 
@@ -618,16 +620,16 @@ rebuild_relation(Relation OldHeap, Oid indexOid, bool verbose)
 /*
  * Create the transient table that will be filled with new data during
  * CLUSTER, ALTER TABLE, and similar operations.  The transient table
- * duplicates the logical structure of the OldHeap, but is placed in
- * NewTableSpace which might be different from OldHeap's.  Also, it's built
- * with the specified persistence, which might differ from the original's.
+ * duplicates the logical structure of the OldHeap; but will have the
+ * specified physical storage properties NewTableSpace, NewAccessMethod, and
+ * relpersistence.
  *
  * After this, the caller should load the new heap with transferred/modified
  * data, then call finish_heap_swap to complete the operation.
  */
 Oid
-make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, char relpersistence,
-			  LOCKMODE lockmode)
+make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, Oid NewAccessMethod,
+			  char relpersistence, LOCKMODE lockmode)
 {
 	TupleDesc	OldHeapDesc;
 	char		NewHeapName[NAMEDATALEN];
@@ -686,7 +688,7 @@ make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, char relpersistence,
 										  InvalidOid,
 										  InvalidOid,
 										  OldHeap->rd_rel->relowner,
-										  OldHeap->rd_rel->relam,
+										  NewAccessMethod,
 										  OldHeapDesc,
 										  NIL,
 										  RELKIND_RELATION,
@@ -1036,6 +1038,10 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class,
 		relform1->reltablespace = relform2->reltablespace;
 		relform2->reltablespace = swaptemp;
 
+		swaptemp = relform1->relam;
+		relform1->relam = relform2->relam;
+		relform2->relam = swaptemp;
+
 		swptmpchr = relform1->relpersistence;
 		relform1->relpersistence = relform2->relpersistence;
 		relform2->relpersistence = swptmpchr;
@@ -1071,6 +1077,9 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class,
 		if (relform1->relpersistence != relform2->relpersistence)
 			elog(ERROR, "cannot change persistence of mapped relation \"%s\"",
 				 NameStr(relform1->relname));
+		if (relform1->relam != relform2->relam)
+			elog(ERROR, "cannot change access method of mapped relation \"%s\"",
+				 NameStr(relform1->relname));
 		if (!swap_toast_by_content &&
 			(relform1->reltoastrelid || relform2->reltoastrelid))
 			elog(ERROR, "cannot swap toast by links for mapped relation \"%s\"",

src/backend/commands/matview.c

@@ -298,8 +298,9 @@ ExecRefreshMatView(RefreshMatViewStmt *stmt, const char *queryString,
 	 * it against access by any other process until commit (by which time it
 	 * will be gone).
 	 */
-	OIDNewHeap = make_new_heap(matviewOid, tableSpace, relpersistence,
-							   ExclusiveLock);
+	OIDNewHeap = make_new_heap(matviewOid, tableSpace,
+							   matviewRel->rd_rel->relam,
+							   relpersistence, ExclusiveLock);
 	LockRelationOid(OIDNewHeap, AccessExclusiveLock);
 	dest = CreateTransientRelDestReceiver(OIDNewHeap);
 

src/backend/commands/tablecmds.c

@@ -176,6 +176,7 @@ typedef struct AlteredTableInfo
 	List	   *afterStmts;		/* List of utility command parsetrees */
 	bool		verify_new_notnull; /* T if we should recheck NOT NULL */
 	int			rewrite;		/* Reason for forced rewrite, if any */
+	Oid			newAccessMethod;	/* new access method; 0 means no change */
 	Oid			newTableSpace;	/* new tablespace; 0 means no change */
 	bool		chgPersistence; /* T if SET LOGGED/UNLOGGED is used */
 	char		newrelpersistence;	/* if above is true */
@@ -538,6 +539,7 @@ static void change_owner_recurse_to_sequences(Oid relationOid,
 static ObjectAddress ATExecClusterOn(Relation rel, const char *indexName,
 									 LOCKMODE lockmode);
 static void ATExecDropCluster(Relation rel, LOCKMODE lockmode);
+static void ATPrepSetAccessMethod(AlteredTableInfo *tab, Relation rel, const char *amname);
 static bool ATPrepChangePersistence(Relation rel, bool toLogged);
 static void ATPrepSetTableSpace(AlteredTableInfo *tab, Relation rel,
 								const char *tablespacename, LOCKMODE lockmode);
@@ -4096,6 +4098,7 @@ AlterTableGetLockLevel(List *cmds)
 				 */
 			case AT_AddColumn:	/* may rewrite heap, in some cases and visible
 								 * to SELECT */
+			case AT_SetAccessMethod:	/* must rewrite heap */
 			case AT_SetTableSpace:	/* must rewrite heap */
 			case AT_AlterColumnType:	/* must rewrite heap */
 				cmd_lockmode = AccessExclusiveLock;
@@ -4622,6 +4625,24 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			ATSimplePermissions(cmd->subtype, rel, ATT_TABLE | ATT_FOREIGN_TABLE);
 			pass = AT_PASS_DROP;
 			break;
+		case AT_SetAccessMethod:	/* SET ACCESS METHOD */
+			ATSimplePermissions(cmd->subtype, rel, ATT_TABLE | ATT_MATVIEW);
+
+			/* partitioned tables don't have an access method */
+			if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+				ereport(ERROR,
+						(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+						 errmsg("cannot change access method of a partitioned table")));
+
+			/* check if another access method change was already requested */
+			if (OidIsValid(tab->newAccessMethod))
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("cannot have multiple SET ACCESS METHOD subcommands")));
+
+			ATPrepSetAccessMethod(tab, rel, cmd->name);
+			pass = AT_PASS_MISC;	/* does not matter; no work in Phase 2 */
+			break;
 		case AT_SetTableSpace:	/* SET TABLESPACE */
 			ATSimplePermissions(cmd->subtype, rel, ATT_TABLE | ATT_MATVIEW | ATT_INDEX |
 								ATT_PARTITIONED_INDEX);
@@ -4997,6 +5018,9 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab,
 		case AT_DropOids:		/* SET WITHOUT OIDS */
 			/* nothing to do here, oid columns don't exist anymore */
 			break;
+		case AT_SetAccessMethod:	/* SET ACCESS METHOD */
+			/* handled specially in Phase 3 */
+			break;
 		case AT_SetTableSpace:	/* SET TABLESPACE */
 
 			/*
@@ -5324,7 +5348,7 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 
 		/*
 		 * We only need to rewrite the table if at least one column needs to
-		 * be recomputed, or we are changing its persistence.
+		 * be recomputed, or we are changing its persistence or access method.
 		 *
 		 * There are two reasons for requiring a rewrite when changing
 		 * persistence: on one hand, we need to ensure that the buffers
@@ -5338,6 +5362,7 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 			/* Build a temporary relation and copy data */
 			Relation	OldHeap;
 			Oid			OIDNewHeap;
+			Oid			NewAccessMethod;
 			Oid			NewTableSpace;
 			char		persistence;
 
@@ -5378,6 +5403,15 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 			else
 				NewTableSpace = OldHeap->rd_rel->reltablespace;
 
+			/*
+			 * Select destination access method (same as original unless user
+			 * requested a change)
+			 */
+			if (OidIsValid(tab->newAccessMethod))
+				NewAccessMethod = tab->newAccessMethod;
+			else
+				NewAccessMethod = OldHeap->rd_rel->relam;
+
 			/*
 			 * Select persistence of transient table (same as original unless
 			 * user requested a change)
@@ -5417,8 +5451,8 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 			 * persistence. That wouldn't work for pg_class, but that can't be
 			 * unlogged anyway.
 			 */
-			OIDNewHeap = make_new_heap(tab->relid, NewTableSpace, persistence,
-									   lockmode);
+			OIDNewHeap = make_new_heap(tab->relid, NewTableSpace, NewAccessMethod,
+									   persistence, lockmode);
 
 			/*
 			 * Copy the heap data into the new table with the desired
@@ -5933,6 +5967,8 @@ ATGetQueueEntry(List **wqueue, Relation rel)
 	tab->rel = NULL;			/* set later */
 	tab->relkind = rel->rd_rel->relkind;
 	tab->oldDesc = CreateTupleDescCopyConstr(RelationGetDescr(rel));
+	tab->newAccessMethod = InvalidOid;
+	tab->newTableSpace = InvalidOid;
 	tab->newrelpersistence = RELPERSISTENCE_PERMANENT;
 	tab->chgPersistence = false;
 
@@ -6003,6 +6039,8 @@ alter_table_type_to_string(AlterTableType cmdtype)
 			return "CLUSTER ON";
 		case AT_DropCluster:
 			return "SET WITHOUT CLUSTER";
+		case AT_SetAccessMethod:
+			return "SET ACCESS METHOD";
 		case AT_SetLogged:
 			return "SET LOGGED";
 		case AT_SetUnLogged:
@@ -13609,6 +13647,28 @@ ATExecDropCluster(Relation rel, LOCKMODE lockmode)
 	mark_index_clustered(rel, InvalidOid, false);
 }
 
+/*
+ * Preparation phase for SET ACCESS METHOD
+ *
+ * Check that access method exists.  If it is the same as the table's current
+ * access method, it is a no-op.  Otherwise, a table rewrite is necessary.
+ */
+static void
+ATPrepSetAccessMethod(AlteredTableInfo *tab, Relation rel, const char *amname)
+{
+	Oid			amoid;
+
+	/* Check that the table access method exists */
+	amoid = get_table_am_oid(amname, false);
+
+	if (rel->rd_rel->relam == amoid)
+		return;
+
+	/* Save info for Phase 3 to do the real work */
+	tab->rewrite |= AT_REWRITE_ACCESS_METHOD;
+	tab->newAccessMethod = amoid;
+}
+
 /*
  * ALTER TABLE SET TABLESPACE
  */

src/backend/parser/gram.y

@@ -2623,6 +2623,14 @@ alter_table_cmd:
 					n->newowner = $3;
 					$$ = (Node *)n;
 				}
+			/* ALTER TABLE <name> SET ACCESS METHOD <amname> */
+			| SET ACCESS METHOD name
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					n->subtype = AT_SetAccessMethod;
+					n->name = $4;
+					$$ = (Node *)n;
+				}
 			/* ALTER TABLE <name> SET TABLESPACE <tablespacename> */
 			| SET TABLESPACE name
 				{

src/bin/psql/tab-complete.c

@@ -2141,8 +2141,15 @@ psql_completion(const char *text, int start, int end)
 	}
 	/* If we have ALTER TABLE <sth> SET, provide list of attributes and '(' */
 	else if (Matches("ALTER", "TABLE", MatchAny, "SET"))
-		COMPLETE_WITH("(", "LOGGED", "SCHEMA", "TABLESPACE", "UNLOGGED",
-					  "WITH", "WITHOUT");
+		COMPLETE_WITH("(", "ACCESS METHOD", "LOGGED", "SCHEMA",
+					  "TABLESPACE", "UNLOGGED", "WITH", "WITHOUT");
+
+	/*
+	 * If we have ALTER TABLE <smt> SET ACCESS METHOD provide a list of table
+	 * AMs.
+	 */
+	else if (Matches("ALTER", "TABLE", MatchAny, "SET", "ACCESS", "METHOD"))
+		COMPLETE_WITH_QUERY(Query_for_list_of_table_access_methods);
 
 	/*
 	 * If we have ALTER TABLE <sth> SET TABLESPACE provide a list of

src/include/commands/cluster.h

@@ -35,8 +35,8 @@ extern void check_index_is_clusterable(Relation OldHeap, Oid indexOid,
 									   bool recheck, LOCKMODE lockmode);
 extern void mark_index_clustered(Relation rel, Oid indexOid, bool is_internal);
 
-extern Oid	make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, char relpersistence,
-						  LOCKMODE lockmode);
+extern Oid	make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, Oid NewAccessMethod,
+						  char relpersistence, LOCKMODE lockmode);
 extern void finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap,
 							 bool is_system_catalog,
 							 bool swap_toast_by_content,

src/include/commands/event_trigger.h

@@ -32,6 +32,7 @@ typedef struct EventTriggerData
 #define AT_REWRITE_ALTER_PERSISTENCE	0x01
 #define AT_REWRITE_DEFAULT_VAL			0x02
 #define AT_REWRITE_COLUMN_REWRITE		0x04
+#define AT_REWRITE_ACCESS_METHOD		0x08
 
 /*
  * EventTriggerData is the node type that is passed as fmgr "context" info

src/include/nodes/parsenodes.h

@@ -1901,6 +1901,7 @@ typedef enum AlterTableType
 	AT_SetLogged,				/* SET LOGGED */
 	AT_SetUnLogged,				/* SET UNLOGGED */
 	AT_DropOids,				/* SET WITHOUT OIDS */
+	AT_SetAccessMethod,			/* SET ACCESS METHOD */
 	AT_SetTableSpace,			/* SET TABLESPACE */
 	AT_SetRelOptions,			/* SET (...) -- AM specific parameters */
 	AT_ResetRelOptions,			/* RESET (...) -- AM specific parameters */

src/test/regress/expected/create_am.out

@@ -230,6 +230,40 @@ ORDER BY classid, objid, objsubid;
  table tableam_parted_d_heap2
 (5 rows)
 
+-- ALTER TABLE SET ACCESS METHOD
+CREATE TABLE heaptable USING heap AS
+  SELECT a, repeat(a::text, 100) FROM generate_series(1,9) AS a;
+SELECT amname FROM pg_class c, pg_am am
+  WHERE c.relam = am.oid AND c.oid = 'heaptable'::regclass;
+ amname 
+--------
+ heap
+(1 row)
+
+ALTER TABLE heaptable SET ACCESS METHOD heap2;
+SELECT amname FROM pg_class c, pg_am am
+  WHERE c.relam = am.oid AND c.oid = 'heaptable'::regclass;
+ amname 
+--------
+ heap2
+(1 row)
+
+SELECT COUNT(a), COUNT(1) FILTER(WHERE a=1) FROM heaptable;
+ count | count 
+-------+-------
+     9 |     1
+(1 row)
+
+-- No support for multiple subcommands
+ALTER TABLE heaptable SET ACCESS METHOD heap, SET ACCESS METHOD heap2;
+ERROR:  cannot have multiple SET ACCESS METHOD subcommands
+DROP TABLE heaptable;
+-- No support for partitioned tables.
+CREATE TABLE am_partitioned(x INT, y INT)
+  PARTITION BY hash (x);
+ALTER TABLE am_partitioned SET ACCESS METHOD heap2;
+ERROR:  cannot change access method of a partitioned table
+DROP TABLE am_partitioned;
 -- Second, create objects in the new AM by changing the default AM
 BEGIN;
 SET LOCAL default_table_access_method = 'heap2';

src/test/regress/sql/create_am.sql

@@ -161,6 +161,23 @@ WHERE pg_depend.refclassid = 'pg_am'::regclass
     AND pg_am.amname = 'heap2'
 ORDER BY classid, objid, objsubid;
 
+-- ALTER TABLE SET ACCESS METHOD
+CREATE TABLE heaptable USING heap AS
+  SELECT a, repeat(a::text, 100) FROM generate_series(1,9) AS a;
+SELECT amname FROM pg_class c, pg_am am
+  WHERE c.relam = am.oid AND c.oid = 'heaptable'::regclass;
+ALTER TABLE heaptable SET ACCESS METHOD heap2;
+SELECT amname FROM pg_class c, pg_am am
+  WHERE c.relam = am.oid AND c.oid = 'heaptable'::regclass;
+SELECT COUNT(a), COUNT(1) FILTER(WHERE a=1) FROM heaptable;
+-- No support for multiple subcommands
+ALTER TABLE heaptable SET ACCESS METHOD heap, SET ACCESS METHOD heap2;
+DROP TABLE heaptable;
+-- No support for partitioned tables.
+CREATE TABLE am_partitioned(x INT, y INT)
+  PARTITION BY hash (x);
+ALTER TABLE am_partitioned SET ACCESS METHOD heap2;
+DROP TABLE am_partitioned;
 
 -- Second, create objects in the new AM by changing the default AM
 BEGIN;