Re: Thinking about IN/EXISTS optimization

Mike Benoit <mikeb(at)netnation(dot)com> writes:
> I'm just curious, will your proposed in/exists optimizations help for
> queries like:

> db=# explain delete from dns_expired_domains where domain_id in (select
> domain_id from dns_expired_domains group by domain_id having count(*)=14
> );

Probably, but I'm more than a tad curious about why you're concerned
about the efficiency of this particular example. Why would "count=14"
be an interesting condition for deleting groups?

> Seq Scan on dns_expired_domains (cost=0.00..55448724329.92 rows=324754
> width=6)
> SubPlan
> -> Materialize (cost=85370.33..85370.33 rows=64951 width=4)
> -> Aggregate (cost=82122.79..85370.33 rows=64951 width=4)
> -> Group (cost=82122.79..83746.56 rows=649508 width=4)
> -> Sort (cost=82122.79..82122.79 rows=649508
> width=4)
> -> Seq Scan on dns_expired_domains
> (cost=0.00..10316.08 rows=649508 width=4)

What are the *actual*, not estimated, row counts here --- ie, how many
rows in the table, and how many distinct domain_ids are you typically
deleting?

regards, tom lane