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.


mysql-test/r/status.result:
  Bug #30377: test case
mysql-test/t/status.test:
  Bug #30377: test case
sql/sql_lex.h:
  Bug #30377: helper function
sql/sql_select.cc:
  Bug #30377: don't assign cost if not on single level statement

1 files changed, 32 insertions, 0 deletions

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