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Why does this query take 2 minutes?
Posted by: Mike Petit
Date: February 02, 2005 10:39AM

Following output for your consideration (mySQL 4.0.20 on fast Linux server):

mysql> SELECT COUNT(*) as preCount from hapresults.hresult
-> WHERE hresultid>0
-> AND hresult.hcreationtimestamp>'2003-10-17 14:01:17'
-> AND hid='petit';

+----------+
| preCount |
+----------+
| 298784 |
+----------+
1 row in set (1 min 57.17 sec)

mysql> explain SELECT COUNT(*) as preCount from hapresults.hresult
-> WHERE hresultid>0
-> AND hresult.hcreationtimestamp>'2003-10-17 14:01:17'
-> AND hid='petit';
+---------+------+--------------------------------+------+---------+-------+--------+-------------+
| table | type | possible_keys | key | key_len | ref | rows | Extra |
+---------+------+--------------------------------+------+---------+-------+--------+-------------+
| hresult | ref | PRIMARY,hcreationtimestamp,hid | hid | 64 | const | 268172 | Using where |
+---------+------+--------------------------------+------+---------+-------+--------+-------------+
1 row in set (0.00 sec)

Primary key on hresult is hresultid. Both hcreationtimestamp and hid are keys. Table hresult is about 6 million rows. Especially given the fact that this is a count-only operation, I'm mystified why it performs so poorly. Anyone have any ideas? Any help greatly appreciated.

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