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Re: Tsearch2 - bad performance with concatenated ts-vectors

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Jan Wielgus <jan_w(at)tlen(dot)pl>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Tsearch2 - bad performance with concatenated ts-vectors
Date: 2011-10-24 19:31:43
Message-ID: CA+Tgmobw--pbT_zVWxNNHEzLSnNj7qVhqY9UwqCfvejM-Ob2ow@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, Aug 2, 2011 at 2:22 AM, Jan Wielgus <jan_w(at)tlen(dot)pl> wrote:
> So, there is apparently a problem with vector concatenating - the indexes don't work then. I tried to use the vectors separately and to make 'OR' comparison between single vector @@ ts_query checks,
> but it didn't help very much (performance was better, but still over 20 sec):
> ...
> (participant.participant_tsv @@ to_tsquery('simple',to_tsquerystring('Abigail'))) OR (person.person_tsv @@ to_tsquery('simple',to_tsquerystring('Abigail')))
> ...
>
> Is there a way to make this work with better performance? Or is it necessary to create a single vector that contains data from multiple tables and then add an index on it? It would be so far problematic for us,
> because we are using multiple complex queries with variable number of selected columns. I know that another solution might be an union among multiple queries, every of which uses a single vector,
> but this solution is inconvenient too.

Only something of the form 'indexed-column indexable-operator value'
is going to be indexable.  So when you concatenate two columns from
different tables - as you say - not indexable.

In general, OR-based conditions that cross table boundaries tend to be
expensive, because they have to be applied only after performing the
join.  You can't know for sure looking only at a row from one table
whether or not it will be needed, so you have to join them all and
then filter the results.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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