Level down the lock of creating indexes on AO tables if blkdir exists

Appendoptimized tables need block directory relation for index access. Creating
and maintaining block directory is expensive. We delay the block directory
creation until it is really needed - the first index creation. Once created,
all indexes share the same block directory. We need stronger lock
(ShareRowExclusiveLock) if the block directory needs to be created. If the
block directory already exists, we can use the same lock as heap tables.

src/backend/commands/indexcmds.c

@@ -72,6 +72,7 @@
 #include "catalog/aoblkdir.h"
 #include "catalog/pg_constraint.h"
 #include "catalog/oid_dispatch.h"
+#include "catalog/pg_appendonly_fn.h"
 #include "cdb/cdbcat.h"
 #include "cdb/cdbdisp_query.h"
 #include "cdb/cdbdispatchresult.h"
@@ -680,28 +681,32 @@ DefineIndex(Oid relationId,
 	 * parallel workers under the control of certain particular ambuild
 	 * functions will need to be updated, too.
 	 */
+	lockmode = stmt->concurrent ? ShareUpdateExclusiveLock : ShareLock;
+
 	/*
-	 * GPDB_12_MERGE_FIXME: Appendoptimized tables need block directory
-	 * relation for index access.  Creating and maintaining block directory is
-	 * expensive, because it needs to be kept up to date whenever new data is
-	 * inserted in the table.  We delay the block directory creation until it
-	 * is really needed - the first index creation.  Once created, all indexes
-	 * share the same block directory.  We need stronger lock
+	 * Appendoptimized tables need block directory relation for index
+	 * access. Creating and maintaining block directory is expensive,
+	 * because it needs to be kept up to date whenever new data is inserted
+	 * in the table. We delay the block directory creation until it is
+	 * really needed - the first index creation. Once created, all indexes
+	 * share the same block directory. We need stronger lock
 	 * (ShareRowExclusiveLock) that blocks index creation from another
-	 * transaction (not to be confused with create index concurrently) as well
-	 * as concurrent insert for appendoptimized tables, if the block directory
-	 * needs to be created.  If the block directory already exists, we can use
-	 * the same lock as heap tables but this is not currently implemented
-	 * (hence the fixme).
+	 * transaction (not to be confused with create index concurrently) as
+	 * well as concurrent insert for appendoptimized tables, if the block
+	 * directory needs to be created. If the block directory already exists,
+	 * we can use the same lock as heap tables.
 	 */
+	rel = table_open(relationId, NoLock);
+	if (RelationIsAppendOptimized(rel))
 	{
-		rel = table_open(relationId, NoLock);
-		if (RelationIsAppendOptimized(rel))
-			lockmode = ShareRowExclusiveLock;
-		else
-			lockmode = stmt->concurrent ? ShareUpdateExclusiveLock : ShareLock;
-		table_close(rel, NoLock);
+		Oid blkdirrelid = InvalidOid;
+		GetAppendOnlyEntryAuxOids(relationId, NULL, NULL, &blkdirrelid, NULL, NULL, NULL);
+
+		if (!OidIsValid(blkdirrelid))
+			lockmode = ShareRowExclusiveLock; /* Relation is AO, and has no block directory */
 	}
+	table_close(rel, NoLock);
+
 	rel = table_open(relationId, lockmode);
 
 	namespaceId = RelationGetNamespace(rel);