Re: Query slowing down significantly??

Thanks for the hint.
I should have been considering that in the first place.
(But the obvious is easily left unrecognised..)

The prepared statement gives:

                                                                        QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.00..25.18 rows=2 width=175) (actual time=36.116..49.998 rows=1 loops=1)
   ->  Index Scan using x_context_01 on context c  (cost=0.00..10.76 rows=2 width=67) (actual time=0.029..6.947
rows=12706loops=1) 
         Index Cond: ((contextid)::text = $1)
   ->  Index Scan using x_fk_context_hierarchy_02 on context_hierarchy h  (cost=0.00..7.20 rows=1 width=108) (actual
time=0.003..0.003
rows=0 loops=12706)
         Index Cond: (h.contextidx = c.idx)
         Filter: (((h.hierarchyname)::text = $2) AND (h.parentidx = $3))
 Total runtime: 50.064 ms
(7 rows)


And that is quite a bad plan given the current distribution of values.

Regards,
Rainer

Am 01.03.2010 17:15, schrieb Tom Lane:
> Rainer Pruy <Rainer.Pruy@Acrys.COM> 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