Postgres Pro
Downloads
Home   >   mailing lists

Thread:

From
"Michael Rothschild"
Date:
I consider using PostgreSQL for a project we have in our company and, to get a better picture of the product, I started scanning its source code and internal documentation.
Based on what I saw (and maybe I didn't see enough) it seems that the optimizer will always decide to repeatedly scan the whole row set returned by sub selects in the context of an IN clause sequentially, as opposed to what I would expect it to do (which is to create some index or hash structure to improve performance).
For example, if I have the following query:
Select * from a where x in (select y from b where z=7)
Then I would expect an index or hash structure to be created for b.y when it is first scanned and brought into the cache but I couldn't see it happening in the source.
As I said, I only inferred it from reading the source - not from actual experiments - so I may be wrong.
1. Am I wrong?
2. If I'm right, is there any plan to change it (after all, in the context of an IN clause, an index on the returned row set is all that is needed - the row set itself does not seem to matter).
 
Thank you,
 
Michael Rothschild

Re:

From
Christopher Kings-Lynne
Date:
> For example, if I have the following query:
> Select * from a where x in (select y from b where z=7)
> Then I would expect an index or hash structure to be created for b.y
> when it is first scanned and brought into the cache but I couldn't see
> it happening in the source.
> As I said, I only inferred it from reading the source - not from actual
> experiments - so I may be wrong.
> 1. Am I wrong?

You are wrong - this is old behaviour and one of the major speed
improvements of PostgreSQL 7.4 is that IN subqueries now use a hash index
and hence they are much faster.

Chris

Re:

From
Tom Lane
Date:
"Michael Rothschild" <michael.r@corigin.co.il> writes:
> Based on what I saw (and maybe I didn't see enough) it seems that the
> optimizer will always decide to repeatedly scan the whole row set
> returned by sub selects in the context of an IN clause sequentially,

What version were you looking at?

            regards, tom lane