Re: Poor performance for delete query

On 24 April 2014 23:25, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Jonatan Evald Buus <jonatan(dot)buus(at)cellpointmobile(dot)com> writes:
> > On 24 April 2014 22:29, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> I'd really have to bet that you forgot to index one of the referencing
> >> tables.
>
> > *That was our first thought, so we went through the child tables to check
> > but apparently we missed some. (please see below for the difference in
> the
> > explain analyze output)*
>
> I'm confused. Your second EXPLAIN ANALYZE looks like you fixed the
> problem. Are you still thinking there's an issue?
>

I believe we improved it at least, whether it's permanently fixed remaines
to be seen once transaction volume increases again.

>
> > Why is "order2transaction_fk" being triggered twice? Is that because
> > there're two affected rows?
>
> No, I'd have expected a delete of multiple rows to show as calls=N,
> not N separate entries.
>
> Maybe there are recursive queries buried under here somewhere?
> That is, are you expecting any of the cascaded deletes to cascade further?
> I don't recall exactly what EXPLAIN is likely to do with such cases.
>

Deleting from the TopTable (Transaction), I'd expect the following effects:
- 0 affected rows in Address using
*address2transaction_fk*- 0 affected rows in Certificate using
*certificate2transaction_fk*- 0 affected rows in Note using
*note2transaction_fk*
- 1 - N affected rows in Order using *order2transaction_fk*

A deletion in "Order" would also trigger an ON DELETE CASCADE to
Certificate using *certificate2order_fk*, which affects 0 rows.

This doesn't explain the extra trigger of "order2transaction_fk".
Any guidelines as to how we may investigate this further would be greatly
appreciated.

> regards, tom lane
>

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