| From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
|---|---|
| To: | Sarah Officer <officers(at)aries(dot)tucson(dot)saic(dot)com> |
| Cc: | PostgreSQL-general <pgsql-general(at)postgreSQL(dot)org> |
| Subject: | Re: [GENERAL] Simulating an outer join |
| Date: | 2000-01-12 21:00:40 |
| Message-ID: | 200001122100.QAA14618@candle.pha.pa.us |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-general |
> Can somebody comment on using EXISTS vs. IN in a subselect? I have
> some statements with subselects, and I'd like to understand the
> ramifications of choosing EXISTS or IN.
We have some brain-damaged code that is faster with EXISTS than IN.
With IN, the subquery is evaluated and the result put in a temp
relation. Every test for IN sequentially scans the subquery result
looking for a match. EXISTS uses an index on the subquery result. Not
sure why we can't just fix this, but I don't understand enough to know
the reason. People who do understand say there is no good solution
until we redesign the query tree in 7.1.
--
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
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Arthur M. Kang | 2000-01-12 22:07:55 | Stumped. Somebody Help me please! |
| Previous Message | Bruce Momjian | 2000-01-12 20:55:57 | Re: [GENERAL] Simulating an outer join |