Optimizer differences between 7.2 and 7.3 - Mailing list pgsql-performance

Our production database is running under 7.2.4; our test database
running almost the same data is at 7.3.3.  One table has about 400,000
rows in each schema. A query against an indexed column uses an index
scan under 7.2.4, but a sequential scan under 7.3.3. A count of the
table in question shows that they have comparable numbers of matching
rows.

On 7.2.4:

select count(*) from articles;
 count
--------
 420213

select count(*) from articles
 where path_base like 'http://news.findlaw.com/hdocs%';
 count
-------
    38

(and it returns this nearly instantaneously)

explain select count(*) from articles
   where path_base like 'http://news.findlaw.com/hdocs%'

Aggregate  (cost=6.02..6.02 rows=1 width=0)
  ->  Index Scan using ix_articles_3 on articles  (cost=0.00..6.01
rows=1 width=0)

On 7.3.3:

select count(*) from articles;
 count
--------
 406319

select count(*) from articles
   where path_base like 'http://news.findlaw.com/hdocs%'
 count
-------
    23

(and it takes many seconds to return)

explain select count(*) from articles
   where path_base like 'http://news.findlaw.com/hdocs%'

 Aggregate  (cost=205946.65..205946.65 rows=1 width=0)
   ->  Seq Scan on articles  (cost=0.00..205946.65 rows=1 width=0)
         Filter: (path_base ~~ 'http://news.findlaw.com/hdocs%'::text)

I can't find any differences between the indexes (ix_articles_3 exists
in both schemas); the column statistics are set up the same (the
default); and the optimizer settings (costs in postgresql.conf) are the
same.

--
Jeff Boes                                      vox 269.226.9550 ext 24
Database Engineer                                     fax 269.349.9076
Nexcerpt, Inc.                                 http://www.nexcerpt.com
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