On Sun, May 29, 2005 at 08:27:26AM -0500, Michael Engelhart wrote:
> Hi -
> I have a table of about 3 million rows of city "aliases" that I need
> to query using LIKE - for example:
> select * from city_alias where city_name like '%FRANCISCO'
> When I do an EXPLAIN ANALYZE on the above query, the result is:
> Seq Scan on city_alias (cost=0.00..59282.31 rows=2 width=42)
> (actual time=73.369..3330.281 rows=407 loops=1)
> Filter: ((name)::text ~~ '%FRANCISCO'::text)
> Total runtime: 3330.524 ms
> (3 rows)
> this is a query that our system needs to do a LOT. Is there any way
> to improve the performance on this either with changes to our query
> or by configuring the database deployment? We have an index on
> city_name but when using the % operator on the front of the query
> string postgresql can't use the index .
If that's really what you're doing (the wildcard is always at the beginning)
then something like this
create index city_name_idx on foo (reverse(city_name));
select * from city_alias where reverse(city_name) like reverse('%FRANCISCO');
should do just what you need.
I use this, with a plpgsql implementation of reverse, and it works nicely.
CREATE OR REPLACE FUNCTION reverse(text) RETURNS text AS '
original alias for $1;
reverse_str = '''';
FOR i IN REVERSE LENGTH(original)..1 LOOP
reverse_str = reverse_str || substr(original,i,1);
LANGUAGE 'plpgsql' IMMUTABLE;
Someone will no doubt suggest using tsearch2, and you might want to
take a look at it if you actually need full-text search, but my
experience has been that it's too slow to be useful in production, and
it's not needed for the simple "leading wildcard" case.
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