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=12706 loops=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(dot)Pruy(at)Acrys(dot)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.HierarchyPath as
>> 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