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

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

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

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

			regards, tom lane