Fix create_distributed_table on a table using GENERATED ALWAYS AS

If the generated column does not come at the end of the column list,
columnNameList doesn't line up with the column indexes. Seek past

CREATE TABLE test_table (
    test_id int PRIMARY KEY,
    gen_n int GENERATED ALWAYS AS (1) STORED,
    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
);
SELECT create_distributed_table('test_table', 'test_id');

Would raise ERROR: cannot cast 23 to 1184

(cherry picked from commit 34f241af16)

src/backend/distributed/commands/multi_copy.c

@@ -1490,8 +1490,8 @@ TypeForColumnName(Oid relationId, TupleDesc tupleDescriptor, char *columnName)
 
 
 /*
- * Walks a TupleDesc and returns an array of the types of each attribute. Will return
- * InvalidOid in the place of dropped attributes.
+ * Walks a TupleDesc and returns an array of the types of each attribute.
+ * Returns InvalidOid in the place of dropped or generated attributes.
  */
 static Oid *
 TypeArrayFromTupleDescriptor(TupleDesc tupleDescriptor)
@@ -1503,7 +1503,11 @@ TypeArrayFromTupleDescriptor(TupleDesc tupleDescriptor)
 	for (columnIndex = 0; columnIndex < columnCount; columnIndex++)
 	{
 		Form_pg_attribute attr = TupleDescAttr(tupleDescriptor, columnIndex);
-		if (attr->attisdropped)
+		if (attr->attisdropped
+#if PG_VERSION_NUM >= 120000
+			|| attr->attgenerated == ATTRIBUTE_GENERATED_STORED
+#endif
+			)
 		{
 			typeArray[columnIndex] = InvalidOid;
 		}
@@ -1541,7 +1545,7 @@ ColumnCoercionPaths(TupleDesc destTupleDescriptor, TupleDesc inputTupleDescripto
 
 		if (inputTupleType == InvalidOid)
 		{
-			/* this was a dropped column and will not be in the incoming tuples */
+			/* TypeArrayFromTupleDescriptor decided to skip this column */
 			continue;
 		}
 
@@ -1582,9 +1586,9 @@ TypeOutputFunctions(uint32 columnCount, Oid *typeIdArray, bool binaryFormat)
 		bool typeVariableLength = false;
 		Oid outputFunctionId = InvalidOid;
 
-		/* If there are any dropped columns it'll show up as a NULL */
 		if (columnTypeId == InvalidOid)
 		{
+			/* TypeArrayFromTupleDescriptor decided to skip this column */
 			continue;
 		}
 		else if (binaryFormat)

src/test/regress/expected/pg12.out

@@ -21,10 +21,11 @@ insert into test_am values (1, 1);
 select create_distributed_table('test_am','id');
 ERROR:  cannot distribute relations using non-heap access methods
 -- Test generated columns
+-- val1 after val2 to test https://github.com/citusdata/citus/issues/3538
 create table gen1 (
 	id int,
-	val1 int,
-	val2 int GENERATED ALWAYS AS (val1 + 2) STORED
+	val2 int GENERATED ALWAYS AS (val1 + 2) STORED,
+	val1 int
 );
 create table gen2 (
 	id int,
@@ -46,16 +47,16 @@ DETAIL:  Distribution column must not use GENERATED ALWAYS AS (...) STORED.
 insert into gen1 (id, val1) values (2,4),(4,6),(6,2),(8,2);
 insert into gen2 (id, val1) values (2,4),(4,6),(6,2),(8,2);
 select * from gen1 order by 1,2,3;
- id | val1 | val2 
+ id | val2 | val1 
 ----+------+------
-  1 |    4 |    6
-  2 |    4 |    6
-  3 |    6 |    8
-  4 |    6 |    8
-  5 |    2 |    4
-  6 |    2 |    4
-  7 |    2 |    4
-  8 |    2 |    4
+  1 |    6 |    4
+  2 |    6 |    4
+  3 |    8 |    6
+  4 |    8 |    6
+  5 |    4 |    2
+  6 |    4 |    2
+  7 |    4 |    2
+  8 |    4 |    2
 (8 rows)
 
 select * from gen2 order by 1,2,3;

src/test/regress/sql/pg12.sql

@@ -25,10 +25,11 @@ insert into test_am values (1, 1);
 select create_distributed_table('test_am','id');
 
 -- Test generated columns
+-- val1 after val2 to test https://github.com/citusdata/citus/issues/3538
 create table gen1 (
 	id int,
-	val1 int,
-	val2 int GENERATED ALWAYS AS (val1 + 2) STORED
+	val2 int GENERATED ALWAYS AS (val1 + 2) STORED,
+	val1 int
 );
 create table gen2 (
 	id int,