Adds error message to AlterTableSetAccessMethod for below PG12

src/backend/distributed/commands/alter_table.c

@@ -428,6 +428,11 @@ AlterDistributedTable(TableConversionParameters *params)
 TableConversionReturn *
 AlterTableSetAccessMethod(TableConversionParameters *params)
 {
+#if PG_VERSION_NUM < PG_VERSION_12
+	ereport(ERROR, (errmsg("table access methods are not supported "
+						   "for Postgres versions earlier than 12")));
+#endif
+
 	EnsureRelationExists(params->relationId);
 	EnsureTableOwner(params->relationId);
 

src/test/regress/expected/alter_table_set_access_method.out

@@ -1,3 +1,17 @@
+-- test for Postgres version
+-- should error before PG12
+CREATE TABLE alter_am_pg_version_table (a INT);
+SELECT alter_table_set_access_method('alter_am_pg_version_table', 'columnar');
+NOTICE:  creating a new table for public.alter_am_pg_version_table
+NOTICE:  Moving the data of public.alter_am_pg_version_table
+NOTICE:  Dropping the old public.alter_am_pg_version_table
+NOTICE:  Renaming the new table to public.alter_am_pg_version_table
+ alter_table_set_access_method
+---------------------------------------------------------------------
+
+(1 row)
+
+DROP TABLE alter_am_pg_version_table;
 SHOW server_version \gset
 SELECT substring(:'server_version', '\d+')::int > 11 AS server_version_above_eleven
 \gset

src/test/regress/expected/alter_table_set_access_method_0.out

@@ -1,3 +1,9 @@
+-- test for Postgres version
+-- should error before PG12
+CREATE TABLE alter_am_pg_version_table (a INT);
+SELECT alter_table_set_access_method('alter_am_pg_version_table', 'columnar');
+ERROR:  table access methods are not supported for Postgres versions earlier than 12
+DROP TABLE alter_am_pg_version_table;
 SHOW server_version \gset
 SELECT substring(:'server_version', '\d+')::int > 11 AS server_version_above_eleven
 \gset

src/test/regress/sql/alter_table_set_access_method.sql

@@ -1,3 +1,9 @@
+-- test for Postgres version
+-- should error before PG12
+CREATE TABLE alter_am_pg_version_table (a INT);
+SELECT alter_table_set_access_method('alter_am_pg_version_table', 'columnar');
+DROP TABLE alter_am_pg_version_table;
+
 SHOW server_version \gset
 SELECT substring(:'server_version', '\d+')::int > 11 AS server_version_above_eleven
 \gset