Using MySQL 5.6 to find queries creating disk temporary tables

arubin — Sun, 01 May 2011 23:58:10 +0000

In my previous post, I’ve showed how to use Dtrace to find queries creating disk temporary tables (only available for OS with dtrace: solaris, freebsd, etc).

In MySQL 5.6 (which is not released yet, use “labs” version for now) we can use new performance_schema table events_statements_history or events_statements_history_long to find all performance metrics for all queries including created disk/memory tables, use of index, etc. WOW! This is what I have been waiting for a long time!

To illustrate, I have grabbed mysql-5.6.3-labs-performance-schema-linux2.6-x86_64.tar.gz from labs.mysql.com (this feature is only in labs version) and run sysbench readonly test (you need to disable prepared statements in sysbench, seems to be not working with prepared statements, I will check it later).

Here are the results:

mysql> select * from events_statements_history_long where  CREATED_TMP_DISK_TABLES > 0 limit 10\G
*************************** 10. row ***************************
              THREAD_ID: 74
               EVENT_ID: 3295633
             EVENT_NAME: statement/sql/select
                 SOURCE: sql_parse.cc:935
            TIMER_START: 633828149000000
              TIMER_END: 633843868000000
             TIMER_WAIT: 15719000000
              LOCK_TIME: 53000000
               SQL_TEXT: SELECT DISTINCT c from sbtest where id between 847399 and 847499 order by c
         CURRENT_SCHEMA: sbtest
            OBJECT_TYPE: NULL
          OBJECT_SCHEMA: NULL
            OBJECT_NAME: NULL
  OBJECT_INSTANCE_BEGIN: NULL
            MYSQL_ERRNO: 0
      RETURNED_SQLSTATE: NULL
           MESSAGE_TEXT: NULL
                 ERRORS: 0
               WARNINGS: 0
          ROWS_AFFECTED: 0
              ROWS_SENT: 1
          ROWS_EXAMINED: 103
CREATED_TMP_DISK_TABLES: 1
     CREATED_TMP_TABLES: 1
       SELECT_FULL_JOIN: 0
 SELECT_FULL_RANGE_JOIN: 0
           SELECT_RANGE: 1
     SELECT_RANGE_CHECK: 0
            SELECT_SCAN: 0
      SORT_MERGE_PASSES: 0
             SORT_RANGE: 0
              SORT_ROWS: 1
              SORT_SCAN: 1
          NO_INDEX_USED: 0
     NO_GOOD_INDEX_USED: 0
       NESTING_EVENT_ID: NULL
     NESTING_EVENT_TYPE: NULL
10 rows in set (0.00 sec)

Or if you need only list of queries:

mysql> select sql_text, count(*) as cnt  from events_statements_history_long
where  CREATED_TMP_DISK_TABLES > 0
group by sql_text order by cnt desc  limit 10;
+-----------------------------------------------------------------------------+-----+
| sql_text                                                                    | cnt |
+-----------------------------------------------------------------------------+-----+
| SELECT DISTINCT c from sbtest where id between 242012 and 242112 order by c |   2 |
| SELECT DISTINCT c from sbtest where id between 797388 and 797488 order by c |   2 |
| SELECT DISTINCT c from sbtest where id between 973150 and 973250 order by c |   1 |
| SELECT DISTINCT c from sbtest where id between 478783 and 478883 order by c |   1 |
| SELECT DISTINCT c from sbtest where id between 967035 and 967135 order by c |   1 |
| SELECT DISTINCT c from sbtest where id between 602102 and 602202 order by c |   1 |
| SELECT DISTINCT c from sbtest where id between 123827 and 123927 order by c |   1 |
| SELECT DISTINCT c from sbtest where id between 980527 and 980627 order by c |   1 |
| SELECT DISTINCT c from sbtest where id between 450354 and 450454 order by c |   1 |
| SELECT DISTINCT c from sbtest where id between 674804 and 674904 order by c |   1 |
+-----------------------------------------------------------------------------+-----+
10 rows in set (0.04 sec)

We can filter and order by rows_examined, SORT_MERGE_PASSES, NO_INDEX_USED, NO_GOOD_INDEX_USED, etc.

Links:

What is new in MySQL 5.6: all long waited great features of MySQL 5.6 (btw: Multi-Threaded Slaves are coming up, now in labs only)
A Big Bag of Epic Awesomeness, by Mark Leith

Using Dtrace to find queries creating disk temporary tables

arubin — Fri, 02 Oct 2009 18:44:52 +0000

Sometimes we have a lots of small and rather fast queries which use group by/order by, thus creating temporary tables. Some of those queries are retrieving text fields and mysql have to use disk (myisam) temporary tables. Those queries usually run for less than 1-2 seconds, so they did not get into slow query log, however, they sometimes add serious load on the system.

Here is the stat example:

bash-3.00$  /usr/local/mysql/bin/mysqladmin -uroot -p -i 2 -r extended-status|grep tmp_disk
...
| Created_tmp_disk_tables           | 109           |
| Created_tmp_disk_tables           | 101           |
| Created_tmp_disk_tables           | 122           |
...

40-50 tmp_disk_tables created per second

So, how can we grab those queries? Usually we have to temporary enable general log, filter out queries with “group by/order by” and profile them all. On solaris/mac we can use dtrace instead.

Here is the simple script, which will find the list of queries creating tmp_disk_tables:

#pragma D option quiet
dtrace:::BEGIN
{
printf("Tracing... Hit Ctrl-C to end.\n");
}

pid$target::*mysql_parse*:entry
{
self->query = copyinstr(arg1);
}

pid$target::*create_myisam_tmp_table*:return
{
@query[self->query] = count();
}

put it into tmpdisktable.d, chmod +x tmpdisktable.d and run it with
./tmpdisktable.d -p `pgrep -x mysqld`

Ctrl+C after 5 seconds whatever and you will see the queries:

# ./tmpdisktable.d -p `pgrep -x mysqld`
Tracing... Hit Ctrl-C to end.
^C

Queries are stripped by the “strsize”, which is can be tweaked:

#pragma D option strsize=N

We can increase the “strsize” length now and run the script again to get the real queries examples.

Please note: running dtrace for a while can decrease performance, so do not run it for more than couple minutes on production systems.

Alexander Rubin's Blog on MySQL » temporary tables

Using MySQL 5.6 to find queries creating disk temporary tables

Using Dtrace to find queries creating disk temporary tables