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08.19.2008 at 01:49AM PDT, ID: 23658852
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6.8

VERY slow simple SQL query

Asked by BaliDave in MySQL Server

Tags:

Here's a SQL query running on my Linux server under MySQL 5.2 that is VERY slow compared to other queries:

SELECT COUNT(*) FROM OMDaily WHERE actSubP = 1 AND BINARY username NOT IN (SELECT username from OMPrevious)

It's taking about 120 seconds!!!  Here both OMDaily & OMPrevious have 17,000 records, are both indexed on username and actSubP, and both have username as NOT NULL,  CHAR(16),  latin1_swedish_ci and actSubP as TINYINT. Each table has about 30 columns (no TEXT or BLOBs), and about 12 columns indexed individually.. Even if I take out the actSubP =1 part, the query is abysmally slow.  Same if I do a SELECT BINARY username in the subclause.  Here's the EXPLAIN from this query:

id       select_type       table       type       possible_keys       key       key_len       ref       rows       Extra
1       PRIMARY       OMDaily       ref       actSubP       actSubP       2       const       4174       Using where
2       DEPENDENT SUBQUERY       OMPrevious       index       username       username       16       NULL       17301       Using where; Using index

I've run into a few such cases before and usually manage to work around them by redoing them somehow, but I'd like to understand why these occur so I can avoid them altogether.

All help appreciated!Start Free Trial
[+][-]08.19.2008 at 01:55AM PDT, ID: 22258099

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[+][-]08.19.2008 at 02:06AM PDT, ID: 22258152

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[+][-]08.19.2008 at 02:10AM PDT, ID: 22258175

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[+][-]08.19.2008 at 02:11AM PDT, ID: 22258176

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[+][-]08.19.2008 at 02:20AM PDT, ID: 22258211

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[+][-]08.19.2008 at 02:22AM PDT, ID: 22258225

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About this solution

Zone: MySQL Server
Tags: MySQL SQL query performance
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Solution Provided By: BaliDave
Participating Experts: 3
Solution Grade: A
 
 
[+][-]08.20.2008 at 10:29AM PDT, ID: 22272003

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[+][-]08.22.2008 at 11:47AM PDT, ID: 22292948

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