Re: immutable functions vs. join for lookups ? - Mailing list pgsql-performance

From Tom Lane
Subject Re: immutable functions vs. join for lookups ?
Date
Msg-id 6776.1113599567@sss.pgh.pa.us
Whole thread Raw
In response to immutable functions vs. join for lookups ?  (Enrico Weigelt <weigelt@metux.de>)
Responses Re: immutable functions vs. join for lookups ?  (Enrico Weigelt <weigelt@metux.de>)
List pgsql-performance
Enrico Weigelt <weigelt@metux.de> writes:
> c) CREATE FUNCTION id2username(oid) RETURNS text
>     LANGUAGE 'SQL' IMMUTABLE AS '
>     SELECT username AS RESULT FROM users WHERE uid = $1';

This is simply dangerous.  The function is *NOT* immutable (it is
stable though).  When ... not if ... your application breaks because
you got the wrong answers, you'll get no sympathy from anyone.

The correct question to ask was "if I make a stable function like
this, is it likely to be faster than the join?".  The answer is
"probably not; at best it will be equal to the join".  The best the
planner is likely to be able to do with the function-based query
is equivalent to a nestloop with inner indexscan (assuming there is
an index on users.uid).  If that's the best plan then the join case
should find it too ... but if you are selecting a lot of items rows
then it won't be the best plan.

            regards, tom lane

pgsql-performance by date:

Previous
From: Thomas F.O'Connell
Date:
Subject: pgbench Comparison of 7.4.7 to 8.0.2
Next
From: Tom Lane
Date:
Subject: Re: pgbench Comparison of 7.4.7 to 8.0.2