Re: Out of memory error on select

On Tue, 2005-04-05 at 16:04, Werner Bohl wrote:
> I have a fairly large table (21M) records. One field of type varchar(16)
> has some duplicate values, which I'm trying to identify.
> Executing select dup_field from dup_table group by dup_field having
> count(*) > 1 errs with Out of Memory error. Server has 4GB memory, the
> backend-process errs after 3.7GB consumed. Is there any work-around that
> I may use to get this duplicates?
>
> Explain output:
> "HashAggregate (cost=881509.02..881510.02 rows=200 width=20)"
> " Filter: (count(*) > 1)"
> " -> Seq Scan on lssi_base (cost=0.00..872950.68 rows=1711668
> width=20)"
>
> Why is the hash eating so much memory? A fast calc of the memory
> occupied by this data is less than 512MB.

Have you run analyze across this table? It looks like either you
haven't or the query planner is making a mistake about how many rows it
expects to get from this.

HashAggregate chews through memory pretty fast, and it best used for
smaller sets, so it's usually a mistake when the planner picks it for
large ones.