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Re: Speed up a function?CREATE TABLE readings ( "when" TIMESTAMP DEFAULT timeofday()::timestamp NOT NULL PRIMARY KEY, "barometer" FLOAT DEFAULT NULL,

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Creager <Robert_Creager(at)LogicalChaos(dot)org>
Cc: PGPerformance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Speed up a function?CREATE TABLE readings ( "when" TIMESTAMP DEFAULT timeofday()::timestamp NOT NULL PRIMARY KEY, "barometer" FLOAT DEFAULT NULL,
Date: 2004-02-25 06:04:48
Message-ID: 938.1077689088@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Robert Creager <Robert_Creager(at)LogicalChaos(dot)org> writes:
> I've implemented a couple of functions ala date_trunc (listed at the bottom)
> [ and they're too slow ]

Well, it's hardly surprising that a function that invokes date_trunc and
half a dozen other comparably-expensive operations should be half a
dozen times as expensive as date_trunc.  Not to mention that plpgsql is
inherently far slower than C.

Assuming that you don't want to descend to writing C, I'd suggest doing
arithmetic on the Unix-epoch version of the timestamp.  Perhaps
something along the lines of

select 'epoch'::timestamptz +
  trunc(extract(epoch from now())/(3600*24*7))*(3600*24*7) * '1sec'::interval;

This doesn't have the same roundoff behavior as what you posted, but I
think it could be adjusted to do so with a couple more additions and
subtractions, unless there's some magic I'm not seeing about the year
boundary behavior.  Certainly the five-minute-trunc problem could be
done this way.

If you do feel like descending to C, I don't see any fundamental reason
why we accept date_part('week',...) but not date_trunc('week',...).
Feel free to submit a patch.

			regards, tom lane

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