Re: [PATCH] Simplify EXISTS subqueries containing LIMIT

On Wed, Oct 22, 2014 at 1:37 PM, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> I've had a bit of a look at this and here's a couple of things:

Thanks. Sorry I didn't back to you earlier, I almost forgot about the review.

> /*
> + * LIMIT clause can be removed if it's a positive constant or ALL, to
> + * prevent it from being an optimization barrier. It's a common meme to put
> + * LIMIT 1 within EXISTS subqueries.
> + */
>
> I think this comment may be better explained along the lines of:
>
> "A subquery which has a LIMIT clause with a positive value is effectively a
> no-op in this scenario. With EXISTS we only care about the first row anyway,
> so any positive limit value will have no behavioral change to the query, so
> we'll simply remove the LIMIT clause. If we're unable to prove that the
> LIMIT value is a positive number then we'd better not touch it."

That's a bit redundant. Here's what I wrote instead, does this sound better?

* A subquery that has a LIMIT clause with a positive value or NULL causes
* no behavioral change to the query. With EXISTS we only care about the
* first row anyway, so we'll simply remove the LIMIT clause. If the LIMIT
* value does not reduce to a constant that's positive or NULL then do not
* touch it.

> + /* Checking for negative values is done later; 0 is just silly */
> + if (! limit->constisnull && DatumGetInt64(limit->constvalue) <= 0)
>
> I'm a bit confused by the comment here. You're saying that we'll check for
> negatives later... but you're checking for <= 0 on the next line... Did I
> miss something or is this a mistake?

I meant that the case when a negative value raises an error is checked
later in the execution pass. But you're right, this code has nothing
to do with that so I've removed the mention about negative values. It
now says:

/* If it's not NULL and not positive, keep it as a regular subquery */

> I guess here you're just testing to ensure that you've not broken this and
> that the new code does not accidentally remove the LIMIT clause. I think it
> also might be worth adding some tests to ensure that co-related EXISTS
> clauses with a positive LIMIT value get properly changed into a SEMI JOIN
> instead of remaining as a subplan. And also make sure that a LIMIT 0 or
> LIMIT -1 gets left as a subplan. I'd say such a test would supersede the
> above test, and I think it's also the case you were talking about optimising
> anyway?

Sure, this is a planner-only change so it makes sense to test EXPLAIN output.

The dilemma I always have is: wouldn't this cause failures due to plan
instability, if for example autoanalyze gets around to this table
earlier or later and nudges it from a hash join to merge etc? Or is
this not a problem?

Patch attached with all above changes.

Regards,
Marti