PostgreSQL does CAST implicitely between int and a domain derived from int

[Jean-Michel Pouré <jm@xxxxxxxxx>]

Dear friends,

I contact on Postgresql hackers request.

I am running into a systemic
problem using Drupal under PostgreSQL 8.4

Drupal relies heavily on a domain derived from int:

CREATE DOMAIN int_unsigned
  AS integer
   CONSTRAINT int_unsigned_check CHECK ((VALUE >= 0));

Analysing slow queries, I noticed that PostgreSQL 8.4 would cast data
from int4 to int_unsigned. Some queries range between 400ms and 700ms.

This provides some large sequential scans.
Could you help understand why a cast happens?

Details, query plan and database: 
http://drupal.org/node/559986

Postgresql.conf has no special settings for optimizing queries other
than PostgreSQL 8.4 syntax. Only shared memory is much larger.

Kind regards,
Jean-Michel

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