>> offers = Offer.objects.all()
>> for term in ('ThemePark', 'London'):
>> offers = offers.extra(where=["""
>> EXISTS (
>> SELECT 0
>> FROM app_offer_term ot
>> ON o.id = ot.offer_id
>> INNER JOIN app_term t
>> ON ot.term_id = t.id
>> WHERE app_offer.id = ot.offer_id
>> AND t.term = %s
>> )"""], params=[term])
>>
>
> please, could someone explain to me:
> - what does "SELECT 0" do?
"SELECT 0" just returns *something*. The EXISTS clause only
cares if any *rows* were returned, not what their content was.
It could just as easily have been "SELECT *" or "SELECT
'frobniculator'" or anything else. However, if the database
doesn't optimize the call, anything more than one column and
anything requiring a lookup into a table (such as a column value)
is extra effort/cycles. Thus, "SELECT 0" is a suggestion to the
database that if it has to bring something back, a single
non-calculated common constant will suffice. A minor
optimization, and I find it helpful as a reminder to myself that
I don't really care about the contents of the the SELECT
statement, just that *something* came back.
> - and "ON o.id = ot.offer_id"? Is it needed?
Hmm...looks like a bogus copy&paste on my part; a remnant of my
testing. With that line, it's bad SQL. It should just be
...
FROM app_offer_term ot
INNER JOIN app_term t
ON ot.term_id = t.id
...
-tim
PS: you sent your reply directly to me, rather than to the
mailing list. In this case, I could answer your question, but in
general, you may want to reply to the list so that others can
help too as it's pretty easy to hit the limits of my answers. :)
I've replied CC'ing the ML so that in case others have the same
questions you did, they can benefit from the answers too.
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