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Re: query using incorrect index

From: Russell Keane <Russell(dot)Keane(at)inps(dot)co(dot)uk>
To: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: query using incorrect index
Date: 2012-08-03 15:57:50
Message-ID: 8D0E5D045E36124A8F1DDDB463D548557CEF4D2FF2@mxsvr1.is.inps.co.uk (view raw or flat)
Thread:
Lists: pgsql-performance
I tried creating the following index:

CREATE INDEX messageq17
  ON messageq_table
  USING btree
  (staff_ty, staff_id, entity_id)
  WHERE inactive = false;

'inactive = false' (active would be much easy but this is legacy) records should make up a smaller proportion of the overall dataset (and much more of the queries will specify this clause) and the results are very promising.

I will also try changing the settings and report back.

Thanks again guys,



-----Original Message-----
From: Kevin Grittner [mailto:Kevin(dot)Grittner(at)wicourts(dot)gov] 
Sent: 03 August 2012 15:34
To: Russell Keane; pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] query using incorrect index

Russell Keane <Russell(dot)Keane(at)inps(dot)co(dot)uk> wrote:
 
> "log_min_duration_statement";"1ms"
 
> "shared_buffers";"32MB"
> "work_mem";"1MB"
 
Those are pretty low values even for a 4GB machine.  I suggest the following changes and additions, based on the fact that you seem to have the active portion of the database fully cached.
 
shared_buffers = '160MB'
work_mem = '8MB'
seq_page_cost = 0.1
random_page_cost = 0.1
cpu_tuple_cost = 0.03
effective_cache_size = '2GB'
 
> Explain analyse with both indexes present but without the limit (uses 
> the correct index):
 
> "Total runtime: 0.092 ms"
 
Part of problem is that it thinks it will find a matching row fairly quickly, and having done so using the index it chose will mean it is the *right* row.  The problem is that there are no matching rows, so it has to scan the entire index.  More fine-grained statistics
*might* help.  If other techniques don't help, you can rewrite the query slightly to create an optimization fence, but that should be a last resort.  I agree with Robert that if you have a lot of queries that select on "incoming" and/or "inactive", a conditional index (with a WHERE clause in its definition) is likely to be very helpful.
 
-Kevin

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