Thread: Select count with offset returns nothing.

I am using a library which is emitting SQL like this  SELECT COUNT(*)
FROM batches LIMIT 15 OFFSET 15 the library fails because on postgres
this query returns nothing (not even zero as a result). Presumably it
returns some valid value on mysql and other databases.

Other than hacking the library is there anything I can do?

Oddly enough SELECT count(*) FROM batches LIMIT 15 returns the full
count of the table. Presumably the count has to be done on a subquery
right?

Tim Uckun <timuckun@gmail.com> writes:

> I am using a library which is emitting SQL like this  SELECT COUNT(*)
> FROM batches LIMIT 15 OFFSET 15 the library fails because on postgres
> this query returns nothing (not even zero as a result). Presumably it
> returns some valid value on mysql and other databases.
>
> Other than hacking the library is there anything I can do?

Using offset 15 there is totally bogus since the count aggregate is
going to return just 1 row.  Limit 15 is harmless but unnecessary and
misleading

The only way you're going to get the result is with offset 0 or offset
omitted.

Something like limit 15 offset 15 sounds like a paginator feature
trying to show page 2 of pages having 15 rows each.

> Oddly enough SELECT count(*) FROM batches LIMIT 15 returns the full
> count of the table. Presumably the count has to be done on a subquery

I do not find this odd :-)

HTH

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--
Jerry Sievers
Postgres DBA/Development Consulting
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On Fri, Aug 5, 2011 at 4:51 AM, Tim Uckun <timuckun@gmail.com> wrote:
> I am using a library which is emitting SQL like this  SELECT COUNT(*)
> FROM batches LIMIT 15 OFFSET 15 the library fails because on postgres
> this query returns nothing (not even zero as a result). Presumably it
> returns some valid value on mysql and other databases.
>
> Other than hacking the library is there anything I can do?
>
> Oddly enough SELECT count(*) FROM batches LIMIT 15 returns the full
> count of the table. Presumably the count has to be done on a subquery
> right?

Since select count(*) will return one row, you're limit / offset are
in the wrong place.  You want something like:

select count(*) from (select * from  batches ordery by something limit
15 offset 15);

Yea I figured it would need a subquery. I filed a ticket with the
library.  Hopefully they will fix it.

Tim Uckun <timuckun@gmail.com> writes:
> I am using a library which is emitting SQL like this  SELECT COUNT(*)
> FROM batches LIMIT 15 OFFSET 15 the library fails because on postgres
> this query returns nothing (not even zero as a result). Presumably it
> returns some valid value on mysql and other databases.

FWIW, I just tried it on mysql 5.5.2, and got results similar to
Postgres: the limit/offset are considered to apply to the single
aggregated row, not the aggregate's input rows as that library
seems to be expecting.  I wonder what database they were using
for testing ...

            regards, tom lane