Konstantinos Krikellas wrote:
Hi,
I am using PostgreSQL for benchmarking and I study the following query:
SELECT *
FROM k10_1, k10_2, k10_3, k10_4, k10_5
WHERE k10_1.times4 = k10_2.times4
AND k10_2.times4 = k10_3.times4
AND k10_3.times4 = k10_4.times4
AND k10_4.times4 = k10_5.times4
Each table has 10000 tuples of 72 bytes each (constant). The field
That's 72 bytes plus about 24 for each row header. Plus 4 bytes for the
text length, and you can't assume each character is only one byte if
you're using UTF-8 or similar.
times4 in every table is valued in [0,2500), each value appearing
exactly four times but in random order. It is easy to extract that the
result has exactly 2,560,000 tuples or approximate size 185 MB. The
database has been 'VACUUM FULL'-ed and is static.
When I execute this query with EXPLAIN ANALYSE, the query is executed in
10-20 sec and consumes only 8Mb of memory, depending to the machine (I
have tried it on P4-2.0GHz, P4-2.2GHz and Athlon 4200++ 64x2, all with 2
Gb RAM and Linux OS, Ubuntu Edgy or Fedora 6). However, when I execute
exactly the same query normally and direct the output to /dev/null,
PostgreSQL consumes all the available memory (RAM and swap), and the
query cannot be executed, as I receive the message 'Out of memory'. The
same thing happened to all the machines. I have tried to adjust working
memory and shared buffers but it still performed in the same way.
Not happening here (8.2.x, output redirected using "\o /dev/null") - are
you sure it's not psql (or whatever client) that's using up your memory,
as it tries to build the entire result set before sending it to
/dev/null? Don't forget, you've got 5 copies of the columns so that
would be ~ 700MB.
If it is the backend, you'll need to give some of the tuning parameters
you're using, since it works here on my much smaller dev server (1GB RAM
and plenty of other stuff using it).
--
Richard Huxton
Archonet Ltd
