Re: Cursors with Large, Ordered Result Sets

On 27 Mar 2003, Rupert wrote:

> Thanks for the quick reply and sorry for the slow response.
>
> Yes, this is very similar to what we are currently doing and it seems
> to be working rather well - much to my surprise. However, I still have
> the same questions regarding the actual steps being taken by the DBMS
> to order a massive result set. Doesn't it need to have the entire
> result in memory before it can return the first records? If so, and

Whether or not you need to sort depends on the query.

For example:

select * from foo order by a;

could choose to use an index on foo.a to do the ordering rather than a
sort. It'll choose based on what it thinks will be the best plan.

> the result is larger than the RAM, does it dump it to disk and then
> sort?

It's based on the sort_mem GUC parameter and will go to disk if necessary.