Fix motion toast error.

src/backend/cdb/motion/tupser.c

@@ -412,7 +412,11 @@ SerializeTuple(TupleTableSlot *slot, SerTupInfo *pSerInfo, struct directTranspor
 	{
 		Form_pg_attribute attr = TupleDescAttr(slot->tts_tupleDescriptor, i);
 
-		if (!attr->attisdropped && attr->attlen == -1 && !slot->tts_isnull[i])
+		/*
+		 * Cannot access slot->tts_isnull before invoking slot_getallattrs.
+		 * See Github Issue 16906.
+		 */
+		if (!attr->attisdropped && attr->attlen == -1)
 		{
 			hasExternalAttr = true;
 			break;

src/test/regress/expected/toast.out

@@ -180,3 +180,19 @@ SELECT encode(substring(a from 521*26+1 for 26), 'escape') FROM toast_chunk_test
  abcdefghijklmnopqrstuvwxyz
 (1 row)
 
+-- Test for Github Issue 16906
+create table t_16906(a int, b text) distributed by(a);
+-- Insert two rows and make sure they are in the same segment (same dist key)
+-- the 1st row's column b must be NULL;
+-- the 2nd row's column b must be a long string even after toast compression
+-- for details please refer to the issue page.
+insert into t_16906 values(1, null);
+insert into t_16906 values(1, randomtext(10240));
+-- Don't want actually fetch all data just need to test
+-- it does not hit assert fail or error. Using explain
+-- analyze might introduce a new ansfile for ORCA so here
+-- I decide to use \o.
+\o /tmp/t_16906.tmp
+select * from t_16906;
+\o
+drop table t_16906;

src/test/regress/sql/toast.sql

@@ -95,3 +95,23 @@ SELECT * FROM toast_chunk_test WHERE a <> repeat('abcdefghijklmnopqrstuvwxyz', 1
 
 -- Random access into the toast table should work equally well.
 SELECT encode(substring(a from 521*26+1 for 26), 'escape') FROM toast_chunk_test;
+
+-- Test for Github Issue 16906
+create table t_16906(a int, b text) distributed by(a);
+
+-- Insert two rows and make sure they are in the same segment (same dist key)
+-- the 1st row's column b must be NULL;
+-- the 2nd row's column b must be a long string even after toast compression
+-- for details please refer to the issue page.
+insert into t_16906 values(1, null);
+insert into t_16906 values(1, randomtext(10240));
+
+-- Don't want actually fetch all data just need to test
+-- it does not hit assert fail or error. Using explain
+-- analyze might introduce a new ansfile for ORCA so here
+-- I decide to use \o.
+\o /tmp/t_16906.tmp
+select * from t_16906;
+\o
+
+drop table t_16906;