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Group by Optimization
Posted by: Joe Genshlea
Date: December 01, 2005 08:27AM

Newbie here

This query is taking 15 minutes...The table is 27 Million Rows. As a matter of fact all GROUP by Statements that hit big tables ( >10 gig) take a long, long, time.

We also have this table on a DB2 box...and this SQL ran in 90 Seconds...the box is a little bigger than this one, but I don't think the hardware difference could possibly explain the difference in performance.

When I ran top during the SQL execution, the idle time was 73% , iowait was 25% and the cpu was 2%. I did not see any evidence of swapping in vmstat or the Created_tmp_disk_tables variable. My Guess is that MySQL is reading an index into RAM which is taking a long time.

mysql> explain SELECT PROP_CITY, COUNT(LOAN_ID) FROM LOAN_LOAN WHERE PROP_STATE = 'CA' GROUP BY PROP_CITY \G;
--------------
explain SELECT PROP_CITY, COUNT(LOAN_ID) FROM LOAN_LOAN WHERE PROP_STATE = 'CA' GROUP BY PROP_CITY
--------------

*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: LOAN_LOAN
type: ref
possible_keys: IDX3_PROPSTATE
key: IDX3_PROPSTATE
key_len: 3
ref: const
rows: 3644149
Extra: Using where; Using temporary; Using filesort
1 row in set (0.00 sec)


Environment: Suse Linux 9.3 (32 bit); IBM eServer 226; 8GB RAM; 1.2 TB RAID 5EE (6x300GB 10K RPM Ultra 320); Dual Xeon 3.4 Ghz (Single Core) ; MySQL 5.0.13;

See below for SHOW STATUS

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Group by Optimization
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December 01, 2005 08:27AM
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December 03, 2005 06:07AM
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December 05, 2005 11:32AM
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December 09, 2005 04:53PM
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