From: Bruce Momjian <maillist@candle.pha.pa.us>
Message-Id: <199807152339.TAA07145@candle.pha.pa.us>
Subject: Re: [INTERFACES] Re: [HACKERS] changes in 6.4
In-Reply-To: <35AD2A22.A5C2D5A1@insightdist.com> from David Hartwig at "Jul
 15,
	1998  6:16: 2 pm"
To: daveh@insightdist.com (David Hartwig)
Date: Wed, 15 Jul 1998 19:39:05 -0400 (EDT)
Cc: hannu@trust.ee, pgsql-interfaces@postgreSQL.org, hackers@postgreSQL.org
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> The results are profound.    Queries that used to scan tables because of the
> ORs, now make use of any indexes.   Thus, the size of the table has virtually
> no effect on performance.  Furthermore, queries that used to crash the
> backend, now run in under a second.
> 
> Currently the down sides are:
>     1. If there is no usable index, performance is significantly worse.  The
> patch does not check to make sure that there is a usable index.  I could use
> some pointers on this.
> 
>     2. Small tables are actually a bit slower than without the patch.
> 
>     3.  Not very elegant.    I am looking for a more generalized solution.
> I have lots of ideas, but I would need to know the backend much better before
> attempting any of them.   My favorite idea is before cnfify(), to factor the
> OR terms and pull out the constants into a virtual (temporary) table spaces.
> Then rewrite the query as a join.   The optimizer will (should) treat the new
> query accordingly.  This assumes that an efficient factoring algorithm exists
> and that temporary tables can exist in the heap.

OK, I have an idea.  Just today, we allow:

	select *
	from tab1
	where val in (
		select x from tab2
		union
		select y from tab3
	)

How about if instead of doing:

	select * from tab1 where val = 3
	union
	select * from tab1 where val = 4
	...

you change it to:
	
	select * from tab1 where val in (
		select 3
		union
		select 4
	)

This may be a big win.  You aren't running the same query over and over
again, with the same joins, and just a different constant.

Let me know.

If it fails for some reason, it is possible my subselect union code has
a bug, so let me know.


-- 
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