Date: Wed, 18 Dec 2013 12:12:29 -0800 (PST)
From: David Johnston <polobo@yahoo.com>
To: pgsql-performance@postgresql.org
Message-ID: <1387397549054-5783942.post@n5.nabble.com>
In-Reply-To: <1387394601821-5783927.post@n5.nabble.com>
References: <52A7A428.8070104@gmail.com>
 <1386746318622-5782822.post@n5.nabble.com>
 <1387394601821-5783927.post@n5.nabble.com>
Subject: Re: Problem with slow query with WHERE conditions with OR clause on
 primary keys
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kolszew73@gmail.com wrote
> Thanx for your answer
> 
> My example is trivial because i want to show strange (for me) postgres
> behavior with dealing with primary keys (extreme example), in real
> situation user put search condition e.g.  "Panas" and this generates query
> ...
> where gd.other_code like 'Panas%' OR g.code like 'Panas%'
> ..
> 
> both columns has very good indexes and selectivity for "like 'Panas%'" ...
> 
> I have experience from Oracle with this type of queries, and Oracle have
> no problem with it,
> executes select on index on other_code from gd and join g
> in next step executes select on index on code from g and join gd
> and this two results are connected in last step (like union)
> very fast on minimal cost
> 
> and in my opinion read whole huge tables only for 10 rows in result where
> conditions are very good  ... is strange

I suppose the equivalent query that you'd want would be:

SELECT ... FROM gd JOIN gd_data USING (id_gd)
WHERE id_gd IN (

SELECT id_gd FROM gd WHERE ...
UNION ALL -distinct not required in this situation
SELECT id_gd FROM gd_data WHERE ...

) --ignoring NULL implications

It does make sense conceptually...

David J.


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