7.2.1 optimises very badly against 7.2

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От Sam Liddicott
Тема 7.2.1 optimises very badly against 7.2
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Msg-id D38A0FCD5830E848992DF2D4AF5F6F4F58159E@conwy.leeds.ananova.internal
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Ответы Re: 7.2.1 optimises very badly against 7.2
Re: 7.2.1 optimises very badly against 7.2
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I have a 10GB database which serves up customised tv listings selections at www.ananova.com/tv  (see "Personalise Listings" in the left column)
 
We had it running very well under postgres 7.2 on a 4xPentium 700mhz with 8GB RAM
 
For a personalised selection from "start" to "end" of 7 channels [url1 below]
takes a fraction of a second to do the query and if I'm lucky enough to spot it on "top" it uses low CPU percentage.
 
Under 7.2.1 it takes 99% CPU for between 5-9 seconds.
Rolling back to 7.2 is also very slow unless we vacuum analyse after rolling back, then it is very fast again.
[We normally vacuum analyse every 24 hours]
 
We have max connections=128, 4520 shared buffers 91268 sortmem and collect row level and block level stats.
 
I also have this anecodotal information;
we installed 7.2.1 on a fresh box and then restored a pg_dump of the tv database and found it always very slow, stopping postgres and transferring the binary DB files and restarting was very fast but degraded slowly over a few days.
 
So I imagine there is something about stats gathering and use changed with 7.2.1 (I hear it has a new optimiser).
 
The query we do is complex and really does need a good optimiser (why we don't use mysql) as it has to join programmes against broadcasts (restricted by time) to channels (restricted to interested channels).  It has to be careful not to initially start with all broadcasts of a interested channel as well as not all broadcasts on the interested channels.
 
[url1, 7 channels from midnight to about 6:00am day after]
 
 

Samuel Liddicott
Support Consultant
sam@ananova.com
Direct Dial: +44 (0)113 367 4523
Fax: +44 (0)113 367 4680
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