Tom Lane wrote:
> Paul Tillotson <pntil@shentel.net> writes:
>> Does postgres actually do multiple concurrent sorts within a single
>> backend?
>
> Certainly. Consider for example a merge join with each input being
> sorted by an explicit sort step. DISTINCT, ORDER BY, UNION, and
> related operators require their own sort steps in the current
> implementation. It's not difficult to invent queries that require
> arbitrarily large numbers of sort steps.
Tom, in Bruce's document on performance tuning, the page titled
"Multiple CPUs" states:
"POSTGRESQL uses a multi-process model, meaning each database connection
has its own Unix process...POSTGRESQL does not use multi-threading to
allow a single process to use multiple CPUs."
I took this to mean that PostgreSQL was not multi-threaded at all, and
that each connection was serviced by a single, non-threaded process.
Have I interpreted this incorrectly? Are you saying that the backend
process actually is multi-threaded? In the example you site, multiple
sorts could be accomplished serially in a non-threaded process.
--
Guy Rouillier
