Comments on: Getting MySQL to use full key length

By: chad

chad — Thu, 15 Feb 2007 20:13:21 +0000

This seems to still be a weakness of MySQL. I am running 5.0.32 enterprise and I have the same experience. I am working with MySQL support who really have not been much help up to this point. The workaround mentioned by Peter above does indeed work for me too, and cuts my query time from about 120 seconds to 4 seconds. Hopefully MySQL will address this soon.

mysql> select version();
+—————————+
| version() |
+—————————+
| 5.0.32-enterprise-gpl-log |
+—————————+
1 row in set (0.06 sec)

mysql> explain SELECT SUM(IF(FACILITY=’failedsell’,1,0)), SUM(IF(FACILITY=’faile
dbuy’,1,0)) FROM XCORE.LOGS WHERE PARTITIONID=1 AND TIME>=1171515600 AND TIME explain SELECT SUM(IF(FACILITY=’failedsell’,1,0)), SUM(IF(FACILITY=’faile
dbuy’,1,0)) FROM XCORE.LOGS AS LOGS FORCE INDEX(PARTITIONIDTIME) WHERE PARTITION
ID=1 AND TIME>=1171515600 AND TIME

By: araki

araki — Sun, 04 Feb 2007 10:19:01 +0000

typo: “worse it can can select something”

By: peter

peter — Thu, 01 Feb 2007 09:16:12 +0000

Sergey,

This case is from MySQL 4.1.22 so it might be fixed in later versions.
In later versions it might be fixed but in older you need to use workaround as described.

Regarding choosing wrong key like (A,D) instead of (A,B) I’m confident the bug happens in MySQL 5.0 I’ll get you the test case as soon as we see it again.

By: Sergey Petrunia

Sergey Petrunia — Wed, 31 Jan 2007 23:52:42 +0000

Peter, this ought to work. The EXPLAIN should be like in the second case whether FORCE INDEX clause is present or not.
The “use range(index) instead of ref(the-same-index)” heuristics has been in the code since long time. IIRC it had some bug but we’ve fixed quite some time ago.
Could you please post the table DDL and data sample, or file a bug? I’m interested to look.