Handle identity columns properly in the router planner (#6802)

DESCRIPTION: Fixes a bug with insert..select queries with identity
columns
Fixes #6798

src/backend/distributed/deparser/citus_ruleutils.c

@@ -1391,7 +1391,7 @@ convert_aclright_to_string(int aclright)
 
 /*
  * contain_nextval_expression_walker walks over expression tree and returns
- * true if it contains call to 'nextval' function.
+ * true if it contains call to 'nextval' function or it has an identity column.
  */
 bool
 contain_nextval_expression_walker(Node *node, void *context)
@@ -1401,6 +1401,13 @@ contain_nextval_expression_walker(Node *node, void *context)
 		return false;
 	}
 
+	/* check if the node contains an identity column */
+	if (IsA(node, NextValueExpr))
+	{
+		return true;
+	}
+
+	/* check if the node contains call to 'nextval' */
 	if (IsA(node, FuncExpr))
 	{
 		FuncExpr *funcExpr = (FuncExpr *) node;

src/test/regress/expected/generated_identity.out

@@ -522,4 +522,21 @@ INSERT INTO color(color_name) VALUES ('Red');
 \c - - - :master_port
 SET search_path TO generated_identities;
 SET client_min_messages to ERROR;
+DROP TABLE IF EXISTS test;
+CREATE TABLE test (x int, y int, z bigint generated by default as identity);
+SELECT create_distributed_table('test', 'x', colocate_with := 'none');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+INSERT INTO test VALUES (1,2);
+INSERT INTO test SELECT x, y FROM test WHERE x = 1;
+SELECT * FROM test;
+ x | y | z
+---------------------------------------------------------------------
+ 1 | 2 | 1
+ 1 | 2 | 2
+(2 rows)
+
 DROP SCHEMA generated_identities CASCADE;

src/test/regress/sql/generated_identity.sql

@@ -263,4 +263,11 @@ INSERT INTO color(color_name) VALUES ('Red');
 SET search_path TO generated_identities;
 SET client_min_messages to ERROR;
 
+DROP TABLE IF EXISTS test;
+CREATE TABLE test (x int, y int, z bigint generated by default as identity);
+SELECT create_distributed_table('test', 'x', colocate_with := 'none');
+INSERT INTO test VALUES (1,2);
+INSERT INTO test SELECT x, y FROM test WHERE x = 1;
+SELECT * FROM test;
+
 DROP SCHEMA generated_identities CASCADE;