Re: Optimizing AGE()

On Fri, Jun 6, 2008 at 7:20 AM, Volkan YAZICI <yazicivo(at)ttmail(dot)com> wrote:
> Hi,
>
> Am I wrong or AGE() always gets directed to a sequential scan?
>
> # BEGIN;
> ] SET enable_seqscan TO off;
> ] EXPLAIN ANALYZE
> ] SELECT count(1)
> ] FROM incomingmessageslog
> ] WHERE AGE(time) < '1 year';
> ] ROLLBACK;
> BEGIN
> SET
> QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------------------
> Aggregate (cost=100000528.33..100000528.34 rows=1 width=0) (actual time=13.789..13.790 rows=1 loops=1)
> -> Seq Scan on incomingmessageslog (cost=100000000.00..100000520.00 rows=3333 width=0) (actual time=13.783..13.783 rows=0 loops=1)
> Filter: (age((('now'::text)::date)::timestamp without time zone, "time") < '1 year'::interval)
> Total runtime: 13.852 ms
> (4 rows)
>
> ROLLBACK
>
> As far as I know, AGE() can take advantage of a very simple equation for
> constant comparisons:
>
> = AGE(field) < constant_criteria
> = AGE(field, constant_ts) < constant_criteria
> = AGE(field) < constant_criteria + constant_ts
> = AGE(field) < CONSTANT_CRITERIA
>
> How much does such a hack into optimizer cost? I don't know about its
> implications but I'll really appreciate such a functionality. At the
> moment, I'm trying replace every AGE() usage in my code and it really
> feels a PITA.

Yeah, age() isn't real performent in such situations. I generally
stick to simpler date math like:

where timestampvalue < now() - interval '2 year'

which can use an index on timestampvalue

The problem with age is it's always compared to now, so there's always
gonna be some math.