SET STATEMENT queries are put into query cache but never hit it

[Elena Stepanova]

# Start server with query-cache-type = 1

MySQL [test]> create table t1 (i int);
Query OK, 0 rows affected (0.96 sec)

MySQL [test]> select @@query_cache_size, @@query_cache_type;
+--------------------+--------------------+
| @@query_cache_size | @@query_cache_type |
+--------------------+--------------------+
| 1048576 | ON |
+--------------------+--------------------+
1 row in set (0.00 sec)

MySQL [test]> # Make sure that query cache works

MySQL [test]> show status like 'QCache%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 1031200 |
| Qcache_hits | 0 |
| Qcache_inserts | 0 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 2 |
| Qcache_queries_in_cache | 0 |
| Qcache_total_blocks | 1 |
+-------------------------+---------+
8 rows in set (0.00 sec)

MySQL [test]> select * from t1;
Empty set (0.00 sec)

MySQL [test]> show status like 'QCache%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 1029664 |
| Qcache_hits | 0 |
| Qcache_inserts | 1 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 2 |
| Qcache_queries_in_cache | 1 |
| Qcache_total_blocks | 4 |
+-------------------------+---------+
8 rows in set (0.00 sec)

MySQL [test]> select * from t1;
Empty set (0.00 sec)

MySQL [test]> show status like 'QCache%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 1029664 |
| Qcache_hits | 1 |
| Qcache_inserts | 1 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 2 |
| Qcache_queries_in_cache | 1 |
| Qcache_total_blocks | 4 |
+-------------------------+---------+
8 rows in set (0.00 sec)

MySQL [test]> # Query cache works

MySQL [test]> set statement lock_wait_timeout=1 for select * from t1;
Empty set (0.00 sec)

MySQL [test]> show status like 'QCache%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 1028640 |
| Qcache_hits | 1 |
| Qcache_inserts | 2 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 2 |
| Qcache_queries_in_cache | 2 |
| Qcache_total_blocks | 6 |
+-------------------------+---------+
8 rows in set (0.00 sec)

MySQL [test]> # The query was put into the query cache

MySQL [test]> set statement lock_wait_timeout=1 for select * from t1;
Empty set (0.00 sec)

MySQL [test]> show status like 'QCache%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 1028640 |
| Qcache_hits | 1 |
| Qcache_inserts | 2 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 3 |
| Qcache_queries_in_cache | 2 |
| Qcache_total_blocks | 6 |
+-------------------------+---------+
8 rows in set (0.00 sec)

MySQL [test]> # ... but didn't hit it.

MySQL [test]> # Why put the query into cache if can't use it?

[Nilnandan Joshi (nilnandan-joshi)]

Able to reproduce the same with PS 5.6.21

nilnandan@Dell-XPS:~$ mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 37
Server version: 5.6.21-69.0 Percona Server (GPL), Release 69.0, Revision 675

Copyright (c) 2009-2014 Percona LLC and/or its affiliates
Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>
mysql> select @@query_cache_size, @@query_cache_type;
+--------------------+--------------------+
| @@query_cache_size | @@query_cache_type |
+--------------------+--------------------+
| 16777216 | ON |
+--------------------+--------------------+
1 row in set (0.00 sec)

mysql> use test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> create table t1 (i int);
Query OK, 0 rows affected (0.04 sec)

mysql> show status like 'QCache%';
+-------------------------+----------+
| Variable_name | Value |
+-------------------------+----------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 16759648 |
| Qcache_hits | 0 |
| Qcache_inserts | 0 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 70 |
| Qcache_queries_in_cache | 0 |
| Qcache_total_blocks | 1 |
+-------------------------+----------+
8 rows in set (0.01 sec)

mysql>
mysql> select * from t1;
Empty set (0.00 sec)

mysql> show status like 'QCache%';
+-------------------------+----------+
| Variable_name | Value |
+-------------------------+----------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 16758112 |
| Qcache_hits | 0 |
| Qcache_inserts | 1 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 70 |
| Qcache_queries_in_cache | 1 |
| Qcache_total_blocks | 4 |
+-------------------------+----------+
8 rows in set (0.00 sec)

mysql> select * from t1;
Empty set (0.00 sec)

mysql> show status like 'QCache%';
+-------------------------+----------+
| Variable_name | Value |
+-------------------------+----------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 16758112 |
| Qcache_hits | 1 |
| Qcache_inserts | 1 |
| Qcache_lowmem_prunes | 0 |
| Qcache_not_cached | 70 |
| Qcache_queries_in_cache | 1 |
| Qcache_total_blocks | 4 |
+-------------------------+----------+
8 rows in set (0.00 sec)

mysql> set statement lock_wait_timeout=1 for select * from t1;
Empty set (0.00 sec)

mysql> show status like 'QCache%';
+-------------------------+----------+
| Variable_name | Value |
+-------------------------+----------+
| Qcache_free_blocks | 1 |
| Qcache_free_memory | 16757088 |
| Qcache_hits | 1 ...

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-1573