Re: Automatic function replanning - Mailing list pgsql-hackers

Good idea, TODO updated:
* Flush cached query plans when the dependent objects change or  when the cardinality of parameters changes
dramatically


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Jim C. Nasby wrote:
> On Tue, Dec 13, 2005 at 04:49:10PM -0500, Neil Conway wrote:
> > On Tue, 2005-12-13 at 22:32 +0100, Joachim Wieland wrote:
> > > there's a topic that comes up from time to time on the lists, the problem
> > > that pgsql functions get planned only once and thereafter the same query
> > > plan is used until server shutdown or explicit recreation of the function.
> > 
> > The problem really has nothing to do with functions, per se: whenever a
> > plan is created and then stored for future use, the assumptions made by
> > that plan may be invalidated by the time the plan is executed. This
> > applies to PREPARE, pl/pgsql functions, perhaps the plan caching done by
> > the RI triggers, and so forth.
> > 
> > I also think that invalidating cached plans on a periodic basis is the
> > wrong approach -- we can use sinval to invalidate plans as soon as a
> > dependent database object changes and not before. This thread contains
> > some ideas on how to do this:
> > 
> >     http://archives.postgresql.org/pgsql-hackers/2005-03/msg00426.php
> > 
> > I got somewhat sidetracked by the complexities of the "central plan
> > caching module" that Tom would like to see, but I'm still hoping to take
> > a look at this for 8.2.
> 
> As for predicate-driven plan changes (ie: query is planned the first
> time with a predicate that has high cardinality, but there are also low
> cardinality values that will be queried on), it would make more sense to
> track the amount of work (probably tuples fetched) normally required to
> execute a prepared statement. Any time that prepared statement is
> executed with a set of predicates that substantially changes the amount
> of work required it should be remembered and considered for re-planning
> the next time the query is executed with those predicates.
> -- 
> Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
> Pervasive Software      http://pervasive.com    work: 512-231-6117
> vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461
> 
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