Re: explain shows lots-o-preliminary sorting

On Wed, Mar 28, 2001 at 07:40:21PM -0500, Joseph Shraibman wrote:
> Merge joins sorta need their input to be sorted.

okay. what other kinds of joins are there that do NOT need
pre-sorting of subqueries?

more generally, what changes are possible to enhance performance
of select queries? forcing indexes vs. seqential scans, using
different joins that don't need sorting, etc. what's available?

and is there a /usr/share/doc/postgresql-doc/*/*/* file somewhere
that delineates all this?

> will trillich wrote:
> >
> > CREATE VIEW course AS
> > SELECT
> > e.code AS educode,
> > e.name AS eduname,
> > t.code AS topiccode,
> > t.name AS topicname,
> > c.name,
> > c.descr
> > FROM
> > _edu e,
> > _topic t,
> > _course c
> > WHERE
> > c.topic = t.id -- maybe this should be swapped
> > AND
> > t.edu = e.id -- with this ??
> > ;
> >
> > psql=> explain select * from course;
> > NOTICE: QUERY PLAN:
> >
> > Merge Join (cost=4.14..4.42 rows=8 width=238)
> > -> Sort (cost=2.63..2.63 rows=5 width=60)
> > -> Merge Join (cost=2.38..2.57 rows=5 width=60)
> > -> Sort (cost=1.30..1.30 rows=11 width=32)
> > -> Seq Scan on _topic (cost=0.00..1.11 rows=11 width=32)
> > -> Sort (cost=1.08..1.08 rows=4 width=28)
> > -> Seq Scan on _edu (cost=0.00..1.04 rows=4 width=28)
> > -> Sort (cost=1.52..1.52 rows=17 width=178)
> > -> Seq Scan on _course (cost=0.00..1.17 rows=17 width=178)
> >
> > EXPLAIN
> >
> > there's FOUR sort items mentioned there, and that's before the
> > merge join (results will not be sorted in any particular order).
> >
> > which document will allay my 'holy cow is this ever gonna slow
> > down my database performance' concerns? (perhaps by saying that
> > sorting is just a myth, or by telling me how to get this puppy to
> > not sort at all -- and to use the indexes that i've defined for
> > all these joined fields...!)

--
It is always hazardous to ask "Why?" in science, but it is often
interesting to do so just the same.
-- Isaac Asimov, 'The Genetic Code'

will(at)serensoft(dot)com
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