On 5/9/06, PFC <lists@xxxxxxxxxx> wrote:
> You might consider just selecting your primary key or a set of
> primary keys to involved relations in your search query. If you
> currently use "select *" this can make your result set very large.
>
> Copying all the result set to the temp. costs you additional IO
> that you propably dont need.
It is a bit of a catch : I need this information, because the purpose of
the query is to retrieve these objects. I can first store the ids, then
retrieve the objects, but it's one more query.
> Also you might try:
> SELECT * FROM somewhere JOIN result USING (id)
> Instead of:
> SELECT * FROM somewhere WHERE id IN (SELECT id FROM result)
Yes you're right in this case ; however the query to retrieve the owners
needs to eliminate duplicates, which IN() does.
Well, you can either
SELECT * FROM somewhere JOIN (SELECT id FROM result GROUP BY id) AS
a USING (id);
or even, for large number of ids:
CREATE TEMPORARY TABLE result_ids AS SELECT id FROM RESULT GROUP BY id;
SELECT * FROM somewhere JOIN result_ids USING (id);
> On the other hand if your search query runs in 10ms it seems to be fast
> enough for you to run it multiple times. Theres propably no point in
> optimizing anything in such case.
I don't think so :
- 10 ms is a mean time, sometimes it can take much more time, sometimes
it's faster.
- Repeating the query might yield different results if records were added
or deleted in the meantime.
You may SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
though locking might bite you. :)
- Complex search queries have imprecise rowcount estimates ; hence the
joins that I would add to them will get suboptimal plans.
Using a temp table is really the cleanest solution now ; but it's too
slow so I reverted to generating big IN() clauses in the application.
A thought, haven't checked it though, but...
You might want to use PL to store values, say PLperl, or even C, say:
create or replace function perl_store(name text, val int) returns void
as $$ my $name = shift; push @{$foo{$name}}, shift; return $$ LANGUAGE
plperl;
select perl_store('someids', id) from something group by id;
(you may need to warp it inside count())
Then use it:
create or replace function perl_retr(name text) returns setof int as
$$ my $name = shift; return $foo{$name} $$ LANGUAGE plperl;
select * from someother join perl_retr('someids') AS a(id) using (id);
All is in the memory. Of course, you need to do some cleanup, test it,
etc, etc, etc. :)
Should work faster than a in-application solution :)
Regards,
Dawid
