Bug #18899860: EXPLAIN .. SELECT .. FOR UPDATE TAKES LOCKS

Problem:
Explain for a statement with "For update" takes locks. Flags to take the lock
are set while parsing itself for a "SELECT ..FOR UPDATE" statement.

Solution:
Do not set the flag for explain in parser. We do this by checking the lex->describe
flag.

Author: Chaithra Gopalareddy

mysql-test/r/explain.result

@@ -554,3 +554,18 @@ NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	Using temporary
 DROP TABLE t1, t2;
 # End WL#4897
 End of 6.0 tests.
+#
+# Bug #18899860: EXPLAIN .. SELECT .. FOR UPDATE TAKES LOCKS
+#
+CREATE TABLE t1(c1 INT PRIMARY KEY) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1),(2),(3);
+START TRANSACTION;
+EXPLAIN SELECT * FROM t1 WHERE c1 = 1 FOR UPDATE;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	const	PRIMARY	PRIMARY	4	const	1	Using index
+START TRANSACTION;
+EXPLAIN SELECT * FROM t1 WHERE c1 = 1 FOR UPDATE;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	const	PRIMARY	PRIMARY	4	const	1	Using index
+DROP TABLE t1;
+# End of test for Bug#18899860

mysql-test/t/explain.test

@@ -400,3 +400,25 @@ DROP TABLE t1, t2;
 --echo # End WL#4897
 
 --echo End of 6.0 tests.
+
+--echo #
+--echo # Bug #18899860: EXPLAIN .. SELECT .. FOR UPDATE TAKES LOCKS
+--echo #
+
+CREATE TABLE t1(c1 INT PRIMARY KEY) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1),(2),(3);
+CONNECT (c1,localhost,root,,);
+CONNECT (c2,localhost,root,,);
+CONNECTION c1;
+START TRANSACTION;
+EXPLAIN SELECT * FROM t1 WHERE c1 = 1 FOR UPDATE;
+CONNECTION c2;
+START TRANSACTION;
+EXPLAIN SELECT * FROM t1 WHERE c1 = 1 FOR UPDATE;
+CONNECTION default;
+DISCONNECT c1;
+DISCONNECT c2;
+DROP TABLE t1;
+
+--echo # End of test for Bug#18899860
+

sql/sql_yacc.yy

@@ -1,5 +1,5 @@
 /*
-   Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -8775,15 +8775,21 @@ select_lock_type:
         | FOR_SYM UPDATE_SYM
           {
             LEX *lex=Lex;
-            lex->current_select->set_lock_for_tables(TL_WRITE);
-            lex->safe_to_cache_query=0;
+            if (!lex->describe)
+            {
+              lex->current_select->set_lock_for_tables(TL_WRITE);
+              lex->safe_to_cache_query=0;
+            }
           }
         | LOCK_SYM IN_SYM SHARE_SYM MODE_SYM
           {
             LEX *lex=Lex;
-            lex->current_select->
-              set_lock_for_tables(TL_READ_WITH_SHARED_LOCKS);
-            lex->safe_to_cache_query=0;
+            if (!lex->describe)
+            {
+              lex->current_select->
+                set_lock_for_tables(TL_READ_WITH_SHARED_LOCKS);
+              lex->safe_to_cache_query=0;
+            }
           }
         ;