Re: Query slowing down significantly??

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