Bug #30377: EXPLAIN loses last_query_cost when used with UNION

Currently the Last_query_cost session status variable shows
only the cost of a single flat subselect. For complex queries
(with subselects or unions etc) Last_query_cost is not valid
as it was showing the cost for the last optimized subselect.
Fixed by reseting to zero Last_query_cost when the complete
cost of the query cannot be determined.
Last_query_cost will be non-zero only for single flat queries.

4 files changed, 108 insertions, 2 deletions

diff --git a/mysql-test/r/status.result b/mysql-test/r/status.result
index ca21b333a6a..36857e22aaf 100644
--- a/mysql-test/r/status.result
+++ b/mysql-test/r/status.result
@@ -43,3 +43,51 @@ SHOW STATUS LIKE 'max_used_connections';
 Variable_name	Value
 Max_used_connections	4
 SET GLOBAL thread_cache_size=@save_thread_cache_size;
+CREATE TABLE t1 ( a INT );
+INSERT INTO t1 VALUES (1), (2);
+SELECT a FROM t1 LIMIT 1;
+a
+1
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	2.402418
+EXPLAIN SELECT a FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	2.402418
+SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY a;
+a
+1
+2
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000
+EXPLAIN SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+2	UNION	t1	ALL	NULL	NULL	NULL	NULL	2	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000
+SELECT a IN (SELECT a FROM t1) FROM t1 LIMIT 1;
+a IN (SELECT a FROM t1)
+1
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000
+SELECT (SELECT a FROM t1 LIMIT 1) x FROM t1 LIMIT 1;
+x
+1
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000
+SELECT * FROM t1 a, t1 b LIMIT 1;
+a	a
+1	1
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	4.805836
+DROP TABLE t1;
diff --git a/mysql-test/t/status.test b/mysql-test/t/status.test
index 6fcb82e160d..33bba3a626a 100644
--- a/mysql-test/t/status.test
+++ b/mysql-test/t/status.test
@@ -139,4 +139,36 @@ disconnect con3;
 disconnect con2;
 disconnect con1;
 
+
+#
+# Bug #30377: EXPLAIN loses last_query_cost when used with UNION
+#
+
+CREATE TABLE t1 ( a INT );
+INSERT INTO t1 VALUES (1), (2);
+
+SELECT a FROM t1 LIMIT 1;
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+
+EXPLAIN SELECT a FROM t1;
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+
+SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY a;
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+
+EXPLAIN SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY a;
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+
+SELECT a IN (SELECT a FROM t1) FROM t1 LIMIT 1;
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+
+SELECT (SELECT a FROM t1 LIMIT 1) x FROM t1 LIMIT 1;
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+
+SELECT * FROM t1 a, t1 b LIMIT 1;
+SHOW SESSION STATUS LIKE 'Last_query_cost';
+
+DROP TABLE t1;
+
+
 # End of 5.0 tests
diff --git a/sql/sql_lex.h b/sql/sql_lex.h
index 94015a9fe07..ce56be79744 100644
--- a/sql/sql_lex.h
+++ b/sql/sql_lex.h
@@ -1257,6 +1257,28 @@ typedef struct st_lex : public Query_tables_list
 
   void reset_n_backup_query_tables_list(Query_tables_list *backup);
   void restore_backup_query_tables_list(Query_tables_list *backup);
+
+  /**
+    @brief check if the statement is a single-level join
+    @return result of the check
+      @retval TRUE  The statement doesn't contain subqueries, unions and 
+                    stored procedure calls.
+      @retval FALSE There are subqueries, UNIONs or stored procedure calls.
+  */
+  bool is_single_level_stmt() 
+  { 
+    /* 
+      This check exploits the fact that the last added to all_select_list is
+      on its top. So select_lex (as the first added) will be at the tail 
+      of the list.
+    */ 
+    if (&select_lex == all_selects_list && !sroutines.records)
+    {
+      DBUG_ASSERT(!all_selects_list->next_select_in_list());
+      return TRUE;
+    }
+    return FALSE;
+  }
 } LEX;
 
 struct st_lex_local: public st_lex
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index b7846a7433d..7e9cb4cfbec 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -4369,9 +4369,13 @@ choose_plan(JOIN *join, table_map join_tables)
 
   /* 
     Store the cost of this query into a user variable
-    Don't update last_query_cost for 'show status' command
+    Don't update last_query_cost for 'show status' command.
+    Don't update last_query_cost for statements that are not "flat joins" :
+    i.e. they have subqueries, unions or call stored procedures.
+    TODO: calculate a correct cost for a query with subqueries and UNIONs.
   */
-  if (join->thd->lex->orig_sql_command != SQLCOM_SHOW_STATUS)
+  if (join->thd->lex->orig_sql_command != SQLCOM_SHOW_STATUS &&
+      join->thd->lex->is_single_level_stmt())
     join->thd->status_var.last_query_cost= join->best_read;
   DBUG_RETURN(FALSE);
 }