Re: [HACKERS] 6.5 beta and ORDER BY patch

>
>
> Jan, is this implemented in 6.5 beta?

It is still the simple suppressing of the sort if the choosen
index scan has already the requested sort order. The possible
enhancements of the optimizer (mainly taking LIMIT into
account and use index scan if sort order can be obtained from
that) aren't implemented AFAIK.

I have too less knowledge in the planner/optimizer corner to
get my hands on it at this stage! And there are things left
in the rewrite system. It might be better to leave this all
for v6.6.

Jan

>
> > > > > As we are again approaching the beta (feature freeze),
> > > > > I will ask my ordinary question ;)
> > > > >
> > > > > Is the patch by Jan that eliminated the duplicate sort node in case it
> > > > > was redundant included in 6.5 ?
> > > >
> > > > Sorry,
> > > >
> > > > I missed to put it into after v6.4 release. And since it
> > > > wasn't there during v6.5 development, I would not put it in
> > > > now.
> > > >
> > > > Note that it wasn't in the v6.4 feature patches either, so it
> > > > isn't tested enough to get released.
> > >
> > > But if it is not relesed it will _never_ be tested enough ...
> > >
> > > As we are just going into beta, not relese, I would suggest to put
> > > it in now, and back out if it relly breaks anything.
> > >
> > > I have been using it with 6.4 almost since the relese an have
> > > seen no problems - in fact it solved a big problem and provided about
> > > 1000X speedup for certain queries (a fraction of second instead of
> > > 6 minutes) , not to mention avoiding backend crashes due to disk space
> > > exhaustion.
> > >
> > > And it did not break anything in regression tests either, the only
> > > argument then was that there is nothing in regression tests that
> > > could possibly be broken by it ;)
> > >
> > > I greatly prefer it over my previous method of doing the same on the
> > > client side (issuing an EXPLAIN, parsing it to see if it is SORT on
> > > INDEX SCAN, and omitting the ORDER BY if it is)
> > >
> > > Also, not having it greatly diminishes the value of LIMIT.
> >
> > Ok ok ok - OK. You got me, I'll go ahead and put it in.
> >
> > >
> > > I agree that it is a hack and only a partial solution and that in
> > > ideal world the optimiser would also know about sort nodes.
> >
> > First the executor must know better how to handle LIMIT's
> > OFFSET. For now it processes the query until OFFSET is
> > reached, simply suppressing the in fact produced result
> > tuples in the output. The it stops sending if the LIMIT count
> > is reached. For joins or other complex things, it has no
> > chance to do something different. But for an indexed single
> > table scan, where ALL the qualifications are done on the
> > index, it should handle the OFFSET by skipping index tuples
> > only.
> >
> > Second the optimizer must take LIMIT into account and
> > depending on the known number of tuples, LIMIT and OFFSET
> > produce an index scan even if the query isn't qualified at
> > all but has an ORDER BY clause matched by the index.
> >
> > These two features would finally solve your huge table
> > problems.
> >
>
>
> --
> Bruce Momjian | http://www.op.net/~candle
> maillist(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
> + If your life is a hard drive, | 830 Blythe Avenue
> + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
>

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