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Slow query log 'Rows_examined' vs 'explain select' rows
Posted by: Ramon Rockx ()
Date: January 09, 2013 05:26AM

Hello,

I'm wondering if someone can help me out with the following thing which is unclear to me:
On our production environment we are using a large table (157 million records, MySQL 5.5, InnoDB). The slow query log is turned on, and although the following query is executed a lot (once every 10 seconds) a few times a day the query will pop up in this log, with stats simular like these:

# Query_time: 742.048022 Lock_time: 0.000071 Rows_sent: 42 Rows_examined: 157244068
SET timestamp=1357683797;
select vss_id from visitorsession where (vss_dateexpired is null) and vss_datelastvisited<'2013-01-08 23:07:55' and vss_datelastvisited>'2013-01-08 21:10:55' order by vss_id desc limit 100;

As you can see the query takes a long time, but that's not so strange since it looks like a full table scan (see rows_examined).

However, when I query the following:

explain select vss_id from visitorsession where (vss_dateexpired is null) and vss_datelastvisited<'2013-01-08 23:07:55' and vss_datelastvisited>'2013-01-08 21:10:55' order by vss_id desc limit 100;

this results in:

id: 1
select_type: SIMPLE
table: visitorsession
type: index
possible_keys: indx_dateexpired,indx_datelastvisited
key: PRIMARY
key_len: 8
rows: 147128
ref: NULL
Extra: Using where


So, as you can see 'rows' is a lot smaller than Rows_examined in the slow query log.
What I expected was that these values should be a lot closer to each other. And why the table scan?

I hope somebody can help me out.
Thanks!

Regards,
Ramon Rockx

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Subject Views Written By Posted
Slow query log 'Rows_examined' vs 'explain select' rows 2743 Ramon Rockx 01/09/2013 05:26AM
Re: Slow query log 'Rows_examined' vs 'explain select' rows 1375 Rick James 01/10/2013 08:56PM
Re: Slow query log 'Rows_examined' vs 'explain select' rows 1233 Ramon Rockx 01/15/2013 09:18AM
Re: Slow query log 'Rows_examined' vs 'explain select' rows 977 Rick James 01/16/2013 09:16AM


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