Re: [GENERAL] eliminating records not in (select id ... so SLOW?

On Thu, 31 Jul 2008 21:37:39 -0400
Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Ivan Sergio Borgonovo <mail@webthatworks.it> writes:
> > I'm doing something like:
> > delete from table1 where id not in (select id from table2).
> > table1 contains ~1M record table2 contains ~ 600K record and id
> > is unique.

> That's going to pretty much suck unless you've got work_mem set
> high enough to allow a "hashed subplan" plan --- which is likely
> to require tens of MB for this case, I don't recall exactly what

Thanks.

> the per-row overhead is. Experiment until EXPLAIN tells you it'll
> use a hashed subplan.

explain delete from catalog_categoryitem where ItemID not in (select
ItemID from catalog_items);

Well I reached 3Gb of work_mem and still I got:

"Seq Scan on catalog_categoryitem (cost=31747.84..4019284477.13
rows=475532 width=6)"
" Filter: (NOT (subplan))"
" SubPlan"
" -> Materialize (cost=31747.84..38509.51 rows=676167 width=8)"
" -> Seq Scan on catalog_items (cost=0.00..31071.67
rows=676167 width=8)"

I've this too:
alter table catalog_items cluster on catalog_items_pkey;
should I drop it?

This is just a dev box. I loaded the 2 tables with 2 not coherent
set of data just to play with, before adding all the pk/fk I need.
I could just truncate the tables and reload them from coherent
sources.

But what if I *really* had to execute that query?
Any other magic I could play to speed it up?

thanks

--
Ivan Sergio Borgonovo
http://www.webthatworks.it

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