Re: Automatic function replanning

Jim C. Nasby wrote:
> Is cardinality the only thing we'd need to worry about? My idea was
> actually to track the amount of work normally required by a stored query
> plan, and if a query uses that plan but requires a very different amount
> of work it's a good indication that we either need to replan or store
> multiple plans for that query. Though if we're certain that cardinality
> is the only thing that could make a cached plan go bad it would
> certainly simplify things greatly.

This gets into another area of re-optimizing when the executor finds
that the actual tables don't match the optimizer estimates. I think we
decided that was too hard/risky, but invalidating the plan might help,
though I am thinking re-planning might just generate the same plan as
before. I think something would need to have happened since the last
planning, like ANALYZE or something.

Updated TODO:

* Flush cached query plans when the dependent objects change,
when the cardinality of parameters changes dramatically, or
when new ANALYZE statistics are available

---------------------------------------------------------------------------

>
> On Fri, Dec 16, 2005 at 11:10:43PM -0500, Bruce Momjian wrote:
> >
> > Good idea, TODO updated:
> >
> > * Flush cached query plans when the dependent objects change or
> > when the cardinality of parameters changes dramatically
> >
> >
> > ---------------------------------------------------------------------------
> >
> > 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(at)pervasive(dot)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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> > >
> >
> > --
> > Bruce Momjian | http://candle.pha.pa.us
> > 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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>
> --
> Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
> Pervasive Software http://pervasive.com work: 512-231-6117
> vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
>

--
Bruce Momjian | http://candle.pha.pa.us
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