Re: a JOIN on same table, but 'slided over'

On 6/26/07, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> "news.gmane.org" <nis(at)superlativ(dot)dk> writes:
> > Gurjeet Singh skrev:
> >> Also note that this query is much cheaper that the 'distinct on' query
> >> by more than two orders on magnitude ( 217.86 vs. 98040.67):
>
> > No it isn't. The estimate is much lower, but the actual times are very
> > close:
>
> > [explain of distinct on]
> >> Time: 5.003 ms
>
> > [explain of correlated subquery]
> >> Time: 4.125 ms
>
> You're both confused:

???

the planner estimate certainly should not be taken
> as gospel,

true

but the actual runtime of an EXPLAIN (not EXPLAIN ANALYZE)
> only reflects planning effort.

Agree completely

EXPLAIN ANALYZE output would be a lot more suitable to settle the
> question which one is faster.

Agree again. I was using the EXPLAIN output just to make a point that
optimizer thinks the query utilizing a subquery is much cheaper (and hence
maybe faster) than the 'distinct on' query.

In a later mail I posted the analyze o/p too...

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