MDEV-25080 Some fixes:

1. In TupleUnion::writeNull(), add the missing switch case for
   wide decimal with 16bytes column width.
2. MCOL-5432 Disable complete/partial pushdown of UNION operation
   if the query involves an ORDER BY or a LIMIT clause, until
   MCOL-5222 is fixed. Also add MTR test cases for this.

dbcon/joblist/tupleunion.cpp

@@ -1766,12 +1766,14 @@ void TupleUnion::writeNull(Row* out, uint32_t col)
       {
         case 1: out->setUintField<1>(joblist::TINYINTNULL, col); break;
 
-        case 2: out->setUintField<1>(joblist::SMALLINTNULL, col); break;
+        case 2: out->setUintField<2>(joblist::SMALLINTNULL, col); break;
 
         case 4: out->setUintField<4>(joblist::INTNULL, col); break;
 
         case 8: out->setUintField<8>(joblist::BIGINTNULL, col); break;
 
+        case 16: out->setInt128Field(datatypes::Decimal128Null, col); break;
+
         default:
         {
         }

dbcon/mysql/ha_mcs_pushdown.cpp

@@ -779,6 +779,15 @@ select_handler* create_columnstore_select_handler_(THD* thd, SELECT_LEX* sel_lex
     return nullptr;
   }
 
+  // MCOL-5432 Disable partial pushdown of the UNION operation if the query
+  // involves an order by or a limit clause.
+  if (sel_lex && sel_unit &&
+      (sel_unit->global_parameters()->limit_params.explicit_limit == true ||
+       sel_unit->global_parameters()->order_list.elements != 0))
+  {
+    return nullptr;
+  }
+
   std::vector<SELECT_LEX*> select_lex_vec;
 
   if (sel_unit && !sel_lex)
@@ -1036,13 +1045,23 @@ select_handler* create_columnstore_select_handler(THD* thd, SELECT_LEX* select_l
 select_handler* create_columnstore_unit_handler(THD* thd, SELECT_LEX_UNIT* sel_unit)
 {
   if (thd->lex->sql_command == SQLCOM_CREATE_VIEW)
+  {
     return nullptr;
+  }
 
   if (thd->stmt_arena && thd->stmt_arena->is_stmt_prepare())
   {
     return nullptr;
   }
 
+  // MCOL-5432 Disable UNION pushdown if the query involves an order by
+  // or a limit clause.
+  if (sel_unit->global_parameters()->limit_params.explicit_limit == true ||
+      sel_unit->global_parameters()->order_list.elements != 0)
+  {
+    return nullptr;
+  }
+
   return create_columnstore_select_handler_(thd, 0, sel_unit);
 }
 

mysql-test/columnstore/basic/r/mdev-25080.result

@@ -261,6 +261,53 @@ NULL	PUSHED UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL
 EXECUTE stmt;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 NULL	PUSHED UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL
+# MCOL-5432 Disable UNION pushdown if an ORDER BY or a LIMIT
+# clause is involved, until MCOL-5222 is fixed.
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a;
+a
+abc
+bcd
+bcd
+cde
+cde
+def
+efg
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2000	
+2	UNION	t2	ALL	NULL	NULL	NULL	NULL	2000	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 LIMIT 3;
+a
+abc
+bcd
+cde
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 LIMIT 3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2000	
+2	UNION	t2	ALL	NULL	NULL	NULL	NULL	2000	
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a DESC LIMIT 5;
+a
+efg
+def
+cde
+cde
+bcd
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a DESC LIMIT 5;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2000	
+2	UNION	t2	ALL	NULL	NULL	NULL	NULL	2000	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a LIMIT 3 OFFSET 2;
+a
+bcd
+cde
+cde
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a LIMIT 3 OFFSET 2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2000	
+2	UNION	t2	ALL	NULL	NULL	NULL	NULL	2000	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
 DROP USER 'cejuser'@'localhost';
 DROP TABLE t1, t2, t3, t4;
 DROP DATABASE mdev25080;

mysql-test/columnstore/basic/t/mdev-25080.test

@@ -169,6 +169,17 @@ PREPARE stmt FROM "EXPLAIN (SELECT * FROM t1 UNION
 EXECUTE stmt;
 EXECUTE stmt;
 
+--echo # MCOL-5432 Disable UNION pushdown if an ORDER BY or a LIMIT
+--echo # clause is involved, until MCOL-5222 is fixed.
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a;
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a;
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 LIMIT 3;
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 LIMIT 3;
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a DESC LIMIT 5;
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a DESC LIMIT 5;
+SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a LIMIT 3 OFFSET 2;
+EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2 ORDER BY a LIMIT 3 OFFSET 2;
+
 DROP USER 'cejuser'@'localhost';
 DROP TABLE t1, t2, t3, t4;
 DROP DATABASE mdev25080;