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Re: Automatic function replanning

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Lukas Smith <smith(at)pooteeweet(dot)org>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Automatic function replanning
Date: 2005-12-22 20:53:03
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Lukas Smith wrote:
> Jim C. Nasby wrote:
> > Now, if both of these are done using a prepared statement, it's going to
> > look like:
> > 
> > SELECT * FROM queue WHERE status='?';
> > 
> > If the first one to run is the queue processing one, the planner will
> > probably choose the index. This means that when we're searching on 'N',
> > there will be a fairly small number of tuples read to execute the query,
> > but when searching for 'D' a very large number of tuples will be read.
> I do not know how exactly how pg handles this internally, however while 
> skimming the oracle tuning pocket guide I picked up for 2 euros I 
> noticed that it mentioned that since oracle 9i bound parameter values 
> are evaluated before the execution plan is determined.
> Maybe I am mixing up separate concepts (are bound variables and prepared 
> statements different concepts?) here. I also do not really understand if 
> that means that oracle does not store a query plan for a prepared query 
> or if it just does some special handling in case it knows that a 
> prepared statement column is known to have a highly varying selectivity 
> per value.

What the Oralce manual means I think is that the plan of the query is
delayed until the _first_ EXECUTE, so it has some values to use in the
optimizer.  The problem is that later queries might use constants of
greatly different cardinality.

  Bruce Momjian                        |
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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