ALTER TABLE: rework determination of access method ID

Avoid setting an access method OID for relation kinds that don't take
one.  Code review for new feature added in 374c7a2290.

Author: Justin Pryzby <pryzby@telsasoft.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/e5516ac1-5264-c3c0-d822-9e6f614ea93b@gmail.com

src/backend/commands/tablecmds.c

@@ -958,22 +958,26 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	}
 
 	/*
-	 * Select access method to use: an explicitly indicated one, or (in the
-	 * case of a partitioned table) the parent's, if it has one.
+	 * For relations with table AM and partitioned tables, select access
+	 * method to use: an explicitly indicated one, or (in the case of a
+	 * partitioned table) the parent's, if it has one.
 	 */
 	if (stmt->accessMethod != NULL)
+	{
+		Assert(RELKIND_HAS_TABLE_AM(relkind) || relkind == RELKIND_PARTITIONED_TABLE);
 		accessMethodId = get_table_am_oid(stmt->accessMethod, false);
-	else if (stmt->partbound)
+	}
+	else if (RELKIND_HAS_TABLE_AM(relkind) || relkind == RELKIND_PARTITIONED_TABLE)
+	{
+		if (stmt->partbound)
 		{
 			Assert(list_length(inheritOids) == 1);
 			accessMethodId = get_rel_relam(linitial_oid(inheritOids));
 		}
-	else
-		accessMethodId = InvalidOid;
 
-	/* still nothing? use the default */
 		if (RELKIND_HAS_TABLE_AM(relkind) && !OidIsValid(accessMethodId))
 			accessMethodId = get_table_am_oid(default_table_access_method, false);
+	}
 
 	/*
 	 * Create the relation.  Inherited defaults and constraints are passed in

src/test/regress/expected/create_am.out

@@ -547,6 +547,9 @@ CREATE TABLE i_am_a_failure() USING "I do not exist AM";
 ERROR:  access method "I do not exist AM" does not exist
 CREATE TABLE i_am_a_failure() USING "btree";
 ERROR:  access method "btree" is not of type TABLE
+-- Other weird invalid cases that cause problems
+CREATE FOREIGN TABLE fp PARTITION OF pg_am DEFAULT SERVER x;
+ERROR:  "pg_am" is not partitioned
 -- Drop table access method, which fails as objects depends on it
 DROP ACCESS METHOD heap2;
 ERROR:  cannot drop access method heap2 because other objects depend on it

src/test/regress/sql/create_am.sql

@@ -348,6 +348,9 @@ CREATE TABLE i_am_a_failure() USING i_do_not_exist_am;
 CREATE TABLE i_am_a_failure() USING "I do not exist AM";
 CREATE TABLE i_am_a_failure() USING "btree";
 
+-- Other weird invalid cases that cause problems
+CREATE FOREIGN TABLE fp PARTITION OF pg_am DEFAULT SERVER x;
+
 -- Drop table access method, which fails as objects depends on it
 DROP ACCESS METHOD heap2;