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

1 files changed, 15 insertions, 6 deletions

diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 6487a9e3fcb..b3d8b6deb03 100644
--- a/src/backend/commands/cluster.c
+++ b/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\"",