Re: Query slowing down significantly??

От: Tom Lane
Тема: Re: Query slowing down significantly??
Дата: ,
Msg-id: 17910.1267460132@sss.pgh.pa.us
(см: обсуждение, исходный текст)
Ответ на: Query slowing down significantly??  (Rainer Pruy)
Ответы: Re: Query slowing down significantly??  (Rainer Pruy)
Список: pgsql-performance

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Query slowing down significantly??  (Rainer Pruy, )
 Re: Query slowing down significantly??  (Tom Lane, )
  Re: Query slowing down significantly??  (Rainer Pruy, )
   Re: Query slowing down significantly??  (Tom Lane, )
    Re: Query slowing down significantly??  (Rainer Pruy, )
     Re: Query slowing down significantly??  (Kris Jurka, )
   Re: Query slowing down significantly??  (Yeb Havinga, )

Rainer Pruy <> writes:
> Normally the following Query behaves well:

> select c.*, h.*
> from Context c, Context_Hierarchy h
> where c.Idx = h.ContextIdx and c.ContextId='testID' and h.HierarchyName='InsuranceHierarchy' and h.ParentIdx=49292395
> ;
>                                                                       QUERY PLAN
>
------------------------------------------------------------------------------------------------------------------------------------------------------
>  Nested Loop  (cost=0.00..43.57 rows=4 width=175) (actual time=0.291..0.293 rows=1 loops=1)
>    ->  Index Scan using uk_context_hierarchy_01 on context_hierarchy h  (cost=0.00..14.76 rows=4 width=108) (actual
time=0.169..0.169
> rows=1 loops=1)
>          Index Cond: (((hierarchyname)::text = 'InsuranceHierarchy'::text) AND (parentidx = 49292395))
>    ->  Index Scan using pk_context on context c  (cost=0.00..7.20 rows=1 width=67) (actual time=0.110..0.111 rows=1
loops=1)
>          Index Cond: (c.idx = h.contextidx)
>          Filter: ((c.contextid)::text = 'testID'::text)
>  Total runtime: 0.388 ms
> (7 rows)

> (From a freshly started PG)

> However during a long term read-only transaction (actually just bout 15min)
> (the transaction is issuing about 10k-20k of such queries among others)
> PG is logging a number of the following:

> Mar  1 09:58:09 gaia postgres[20126]: [25-1] LOG:  00000: duration: 343.663 ms  execute S_5: select c.*, h.Idx as
h_Idx,h.WbuIdx as 
> h_WbuIdx, h.OrigWbuIdx as h_OrigWbuIdx, h.Ts as h_Ts, h.
> UserId as h_UserId, h.ParentIdx as h_ParentIdx, h.ContextIdx as h_ContextIdx, h.HierarchyName as h_HierarchyName,
h.HierarchyPathas 
> h_HierarchyPath from Context c, Context_Hierarchy h wher
> e c.Idx = h.ContextIdx and c.ContextId=$1 and h.HierarchyName=$2 and h.ParentIdx=$3
> Mar  1 09:58:09 gaia postgres[20126]: [25-2] DETAIL:  parameters: $1 = 'testID', $2 = 'InsuranceHierarchy', $3 =
'49292395'
> Mar  1 09:58:09 gaia postgres[20126]: [25-3] LOCATION:  exec_execute_message, postgres.c:1988

That's not the same query at all, and it may not be getting the same
plan.  What you need to do to check the plan is to try PREPARE-ing
and EXPLAIN EXECUTE-ing the query with the same parameter symbols
as are actually used in the application-issued query.

You might be entertained by the recent thread on -hackers about
"Avoiding bad prepared-statement plans" ...

            regards, tom lane


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