From: carex@skynet.be (carex)
Subject: select using regexp does not use indexscan
Date: 9 Nov 2004 12:02:26 -0800
Organization: http://groups.google.com
Lines: 64
Message-ID: <3a0a211c.0411091202.223f1c02@posting.google.com>
Content-Type: text/plain; charset=ISO-8859-1
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To: pgsql-sql@postgresql.org

This is what I get with postgres-7.3 (from Redhat Enterprise !!)

Here below a select with a regexp
ansroc=# explain select * from s12hwdb where host~'^tna2582t';
                         QUERY PLAN
-------------------------------------------------------------
 Seq Scan on s12hwdb  (cost=0.00..30660.35 rows=1 width=128)
   Filter: (host ~ '^tna2582t'::text)
(2 rows)

Here below the same select without regexp
ansroc=# explain select * from s12hwdb where host='tna2582t';
                                         QUERY PLAN
---------------------------------------------------------------------------------------------
 Index Scan using s12hwdb_host_rit_idx on s12hwdb 
(cost=0.00..18123.85 rows=4828 width=128)
   Index Cond: (host = 'tna2582t'::bpchar)
(2 rows)

ansroc=#

As you can see, the index is not use when a regexp is used in the
select.
I did the same test with postgres-7.3.6 & postgres-7.4.6 (compiled
from sources) but the results where the same.
 (index is NEVER used with regexp on a RHE)
I even tried with a 'set enable_seqscan to off', but the result is the
same.


BUT, with Debian (woody & sarge) everything is ok.
      (has always been with debian-:)
I did try with a postgres debian pachage, and also with a postgres
compiled from source, and even with different version (7.3.4, 7.4.6).
 Index is always used ! 

ansroc=# explain SELECT * FROM s12hwdb where host~'^tna2582t';
                                      QUERY PLAN
--------------------------------------------------------------------------------------
 Index Scan using s12hwdb_host_rit_idx on s12hwdb  (cost=0.00..4.41
rows=1 width=128)
   Index Cond: ((host >= 'tna2582t'::bpchar) AND (host <
'tna2582u'::bpchar))
   Filter: (host ~ '^tna2582t'::text)
(3 rows)

ansroc=# explain SELECT * FROM s12hwdb where host='tna2582t';
                                       QUERY PLAN
----------------------------------------------------------------------------------------
 Index Scan using s12hwdb_host_rit_idx on s12hwdb  (cost=0.00..76.02
rows=17 width=128)
   Index Cond: (host = 'tna2582t'::bpchar)
(2 rows)

ansroc=# 

And it works also perfectly with Gentoo.

So,is this a typical "Redhat Enterprise" problem ?
Or do I overlook something ??
Has someone experienced the same problem ??
Thanks.

carex.