Re: Functionscan estimates

From: Jim C. Nasby
Subject: Re: Functionscan estimates
Date: ,
Msg-id: 20050409152257.GG93835@decibel.org
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In response to: Re: Functionscan estimates  (Tom Lane)
Responses: Re: Functionscan estimates  (Tom Lane)
List: pgsql-performance

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Functionscan estimates  (Josh Berkus, )
 Re: Functionscan estimates  (Michael Fuhr, )
  Re: Functionscan estimates  (Alvaro Herrera, )
   Re: Functionscan estimates  (Josh Berkus, )
    Re: Functionscan estimates  (Alvaro Herrera, )
     Re: Functionscan estimates  (Tom Lane, )
      Re: Functionscan estimates  (PFC, )
      Re: Functionscan estimates  ("Jim C. Nasby", )
       Re: Functionscan estimates  (Tom Lane, )
        Re: Functionscan estimates  (Neil Conway, )
         Re: Functionscan estimates  (Josh Berkus, )
      Re: Functionscan estimates  (Neil Conway, )
    Re: Functionscan estimates  (PFC, )

On Sat, Apr 09, 2005 at 12:00:56AM -0400, Tom Lane wrote:
> Not too many releases ago, there were several columns in pg_proc that
> were intended to support estimation of the runtime cost and number of
> result rows of set-returning functions.  I believe in fact that these
> were the remains of Joe Hellerstein's thesis on expensive-function
> evaluation, and are exactly what he was talking about here:
> http://archives.postgresql.org/pgsql-hackers/2002-06/msg00085.php
>
> But with all due respect to Joe, I think the reason that stuff got
> trimmed is that it didn't work very well.  In most cases it's
> *hard* to write an estimator for a SRF.  Let's see you produce
> one for dblink() for instance ...

Actually, if the remote database supported a way to get a rows estimate
from the query passed to db_link, it would be trivial, since you'd just
pass that back.

In fact, having such a function (estimate_rows_for_sql(text)) would
probably be very useful to functions that wanted to support returning a
rows estimate.
--
Jim C. Nasby, Database Consultant               
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