Makes sense. It is NOT executing the subquery more than once is it?
On Tue, 19 Dec 2006 20:02:35 +0000, "Richard Huxton" <dev@archonet.com>
said:
> Jeremy Haile wrote:
> > Here's the query and explain analyze using the result of the sub-query
> > substituted:
> >
> > QUERY
> > explain analyze select min(nlogid) as start_nlogid,
> > max(nlogid) as end_nlogid,
> > min(dtCreateDate) as start_transaction_timestamp,
> > max(dtCreateDate) as end_transaction_timestamp
> > from activity_log_facts
> > where nlogid > 478287801
> > and dtCreateDate < '2006-12-18 9:10'
> >
> > EXPLAIN ANALYZE
> > Aggregate (cost=657.37..657.38 rows=1 width=12) (actual
> > time=0.018..0.019 rows=1 loops=1)
> > -> Index Scan using activity_log_facts_nlogid_idx on
> > activity_log_facts (cost=0.00..652.64 rows=472 width=12) (actual
> > time=0.014..0.014 rows=0 loops=1)
> > Index Cond: (nlogid > 478287801)
> > Filter: (dtcreatedate < '2006-12-18 09:10:00'::timestamp without
> > time zone)
> > Total runtime: 0.076 ms
> >
> >
> > Sorry if the reason should be obvious, but I'm not the best at
> > interpreting the explains. Why is this explain so much simpler than the
> > other query plan (with the subquery)?
>
> Because it's planning it with knowledge of what "nlogid"s it's filtering
> by. It knows it isn't going to get many rows back with nlogid >
> 478287801. In your previous explain it thought a large number of rows
> would match and was trying not to sequentially scan the
> activity_log_facts table.
>
> Ideally, the planner would evaluate the subquery in your original form
> (it should know it's only getting one row back from max()). Then it
> could plan the query as above. I'm not sure how tricky that is to do
> though.
>
> --
> Richard Huxton
> Archonet Ltd
