Re: Simple machine-killing query!

Victor Ciurus <vikcious(at)gmail(dot)com> writes:
> What I am requested to do is to keep all records from 'BIGMA' that do
> not apear in 'DIRTY'
> So far I have tried solving this by going for:

> [explain] select * from BIGMA where string not in (select * from DIRTY);
> QUERY PLAN
> ------------------------------------------------------------------------
> Seq Scan on bigma (cost=0.00..24582291.25 rows=500 width=145)
> Filter: (NOT (subplan))
> SubPlan
> -> Seq Scan on dirty (cost=0.00..42904.63 rows=2503963 width=82)
> (4 rows)

If you are using PG 7.4, you can get reasonable performance out of this
approach, but you need to jack sort_mem up to the point where the whole
DIRTY table will fit into sort_mem (so that you get a hashed-subplan
plan and not a plain subplan). If you find yourself setting sort_mem to
more than say half of your machine's available RAM, you should probably
forget that idea.

> [explain] select * from bigma,dirty where bigma.email!=dirty.email;

This of course does not give the right answer at all.

A trick that people sometimes use is an outer join:

select * from bigma left join dirty on (bigma.email=dirty.email)
where dirty.email is null;

Understanding why this works is left as an exercise for the reader
... but it does work, and pretty well too. If you're using pre-7.4
PG then this is about the only effective solution AFAIR.

regards, tom lane