>>> On Mon, Oct 22, 2007 at  1:30 PM, in message
> On Mon, 2007-10-22 at 09:31 -0500, Kevin Grittner wrote:
>> I've requested this before without response, but I'm asking again
>> because it just caused me pain again: could we get a TODO added to
>> have the planner recognize equivalent IN and EXISTS constructs and
>> have them compete on cost estimates?  I know it's not a trivial
>> improvement, but if it's on the list maybe someone will pick it up,
>> and I see it as the single biggest weakness in PostgreSQL
>> performance.
> I'll pick it up as a default unless someone requests they have it from
> me.
Thanks, Simon.
One more logically equivalent, PostgreSQL-specific form which
costs out even better was suggested off-list:
step=# explain DELETE FROM "Body" USING "Message" WHERE "Message"."bodySeqNo" = 
                                            QUERY PLAN
 Merge Join  (cost=0.00..696766.20 rows=4048543 width=6)
   Merge Cond: (("Body"."bodySeqNo")::numeric = 
   ->  Index Scan using "Body_pkey" on "Body"  (cost=0.00..326108.11 
rows=4048543 width=18)
   ->  Index Scan using "Message_Body" on "Message"  (cost=0.00..310085.16 
rows=4048847 width=12)
(4 rows)
If both of the other syntaxes could compete against that, it would
be fantastic.  (If that's feasible.)

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