Re: Performance regarding LIKE searches

От: Tom Lane
Тема: Re: Performance regarding LIKE searches
Дата: ,
Msg-id: 16513.1269882003@sss.pgh.pa.us
(см: обсуждение, исходный текст)
Ответ на: Performance regarding LIKE searches  ()
Ответы: Re: Performance regarding LIKE searches  ()
Список: pgsql-performance

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Performance regarding LIKE searches  (, )
 Re: Performance regarding LIKE searches  (Tom Lane, )
  Re: Performance regarding LIKE searches  (, )
   Re: Performance regarding LIKE searches  (Andy Colson, )
 Re: Performance regarding LIKE searches  (Matthew Wakeling, )

 writes:
> I can see I am hitting an index using an index that I created using the varchar_pattern_ops setting.  This is very
fastand performs like I would expect.  However, when my application, GBrowse, access the database, I see in my slow
querylog this: 

> 2010-03-29 09:34:38.083
PDT,"gdr_gbrowse_live","gdr_gbrowse_live",11649,"10.0.0.235:59043",4bb0399d.2d81,8,"SELECT",2010-03-2822:24:45
PDT,4/118607,0,LOG,00000,"duration:21467.467 ms  execute dbdpg_p25965_9: SELECT
f.id,f.object,f.typeid,f.seqid,f.start,f.end,f.strand
>   FROM feature as f, name as n
>   WHERE (n.id=f.id AND lower(n.name) LIKE $1)

> ","parameters: $1 = 'Scaffold:scaffold\_163:1000..1199%'",,,,,,,

> GBrowse is a perl based application.  Looking at the duration for this query is around 21 seconds.  That is a bit
long. Does anyone have any ideas why the query duration is so different? 

You're not going to get an index optimization when the LIKE pattern
isn't a constant (and left-anchored, but this is).

It is possible to get the planner to treat a query parameter as a
constant (implying a re-plan on each execution instead of having a
cached plan).  I believe what you have to do at the moment is use
unnamed rather than named prepared statements.  The practicality of
this would depend a lot on your client-side software stack, which
you didn't mention.

            regards, tom lane


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