Question

I have encountered the following query as part of a PHP application. The query runs quickly (less than 1 second) in mysql 5.0, but in mysql 5.1 and later it takes about 7 minutes to complete.

The returned result is only 3 rows, but when I do an strace on the mysqld process on the 5.1 server as it’s running this query, I can see it continuously reading data from the Event table for several minutes (a table with only 7000 rows at 200 bytes per row) — presumably re-reading it multiple times.

So the question is, what’s different here that I’m running into, and how can I modify things (either the query or better yet some MySQL settings) so that this will run under 5.1 as fast as it runs under 5.0.

---

Referenced Data

The query to blame

SELECT S.Sections_ID, S.Sections_Name, S.Sections_CustomURL
FROM Sections S
WHERE S.Sections_Status = 'Active'
    AND S.Sections_Name!='Hidden'
    AND S.Sections_ParentID = 0
    AND S.Sections_MainSection = 1
    AND (
        SELECT COUNT(MainEvent.Event_ID) AS tot
        FROM Event MainEvent, Event_Section ES
        WHERE ES.EventSection_EventID=MainEvent.Event_ID
            AND ES.EventSection_SectionID=S.Sections_ID
            AND (
                (MainEvent.Event_DateTime > '2011-12-27 18:05:00')
                OR
                    (
                        (
                        SELECT ChildEvent.Event_DateTime
                        FROM Event ChildEvent
                        WHERE ChildEvent.Event_ParentEventID=MainEvent.Event_ID
                        ORDER BY ChildEvent.Event_DateTime DESC LIMIT 1
                        ) > '2011-12-27 18:05:00'
                    )
                )
            AND (MainEvent.Event_ParentEventID=0 or MainEvent.Event_ParentEventID IS NULL) 
            AND (MainEvent.Event_Status='Active' or MainEvent.Event_Status='Canceled') 
            AND MainEvent.Event_ID IN (
                SELECT
                Event_Website.EventWebsite_EventID
                FROM Event_Website 
                WHERE Event_Website.EventWebsite_CompanyID='3'
            )
        )>0
ORDER BY S.Sections_Order ASC, S.Sections_Name ASC

The referenced tables have the following number of rows

Sections: 60
Event: 7000
Event_Section: 7000
Event_Website: 15000

Below is the EXPLAIN for the above query from the 5.0 (fast) and 5.1 (slow) server.
Clipped for space; hopefully I didn’t clip out anything useful.

SLOW (5.1)

+----+---------------+-------------+----------------------------+------------------------+---------+----------------------------------+------+-----------------------------+
| id | table         | type        | possible_keys              | key                    | key_len | ref                              | rows | Extra                       |
+----+---------------+-------------+----------------------------+------------------------+---------+----------------------------------+------+-----------------------------+
|  1 | S             | ref         | Sections_ParentID          | Sections_ParentID      | 5       | const                            |   10 | Using where; Using filesort |
|  2 | MainEvent     | ref_or_null | PRIMARY,Event_DateTime,... | Event_ParentID         | 5       | const                            | 4582 | Using where                 |
|  2 | ES            | ref         | EventSection_EventID       | EventSection_EventID   | 10      | MainEvent.Event_ID,S.Sections_ID |    1 | Using where; Using index    |
|  4 | Event_Website | ref         | EventWebsite_CompanyID     | EventWebsite_CompanyID | 4       | const                            | 4421 | Using where                 |
|  3 | ChildEvent    | index       | Event_ParentID             | Event_DateTime         | 8       | NULL                             |    1 | Using where                 |
+----+---------------+-------------+----------------------------+------------------------+---------+----------------------------------+------+-----------------------------+

FAST (5.0)

+----+---------------+--------+---------------------------+------------------------+---------+-------------------------+------+-----------------------------+
| id | table         | type   | possible_keys             | key                    | key_len | ref                     | rows | Extra                       |
+----+---------------+--------+---------------------------+------------------------+---------+-------------------------+------+-----------------------------+
|  1 | S             | ref    | Sections_ParentID         | Sections_ParentID      | 5       | const                   |   10 | Using where; Using filesort | 
|  2 | ES            | index  | EventSection_EventID      | EventSection_EventID   | 10      | NULL                    | 5610 | Using where; Using index    | 
|  2 | MainEvent     | eq_ref | PRIMARY,Event_DateTime,...| PRIMARY                | 4       | ES.EventSection_EventID |    1 | Using where                 | 
|  4 | Event_Website | ref    | EventWebsite_CompanyID    | EventWebsite_CompanyID | 4       | const                   | 5809 | Using where                 | 
|  3 | ChildEvent    | ref    | Event_ParentID            | Event_ParentID         | 5       | MainEvent.Event_ID      |    4 | Using where; Using filesort | 
+----+---------------+--------+---------------------------+------------------------+---------+-------------------------+------+-----------------------------+

Answer 1

Two suggestions for rewriting:

1. Change IN (SELECT ...) to a JOIN query.

2. Change (SELECT COUNT(MainEvent.Event_ID) ...) > 0 to EXISTS (SELECT * ...).