Re: Bitmap index scans use of filters on available columns

Hi,

On 11/07/2015 02:18 AM, Robert Haas wrote:
> On Fri, Nov 6, 2015 at 7:11 PM, Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>>> I think LEAKPROOF is probably fine for this. How would the new thing
>>> be different?
>>
>> I don't think so - AFAIK "leakproof" is about not revealing information
>> about arguments, nothing more and nothing less. It does not say whether it's
>> safe to evaluate on indexes, and I don't think we should extend the meaning
>> in this direction.
>
> That seems like a non-answer answer.

I'm not claiming I have an answer, really. My knowledge of leakproof
stuff is a bit shallow. Also, I had a few beers earlier today, which
does not really improve the depth of knowledge on any topic except
politics and football (aka soccer). So you may be perfectly right.

> Clearly, if a function can leak information about it's arguments,
> for example by throwing an error, we can't call it on tuples that
> might not even be visible, or the behavior of the query might be
> change. So any function that is not leakproof is also not safe for
> this purpose.
>
> Now that doesn't rule out the possibility that the functions for
> which this optimization is safe are a subset of the leakproof
> functions - but offhand I don't see why that should be the case. The
> index tuple is just a tuple, and the values it contains are just
> datums, no more or less than in a heap tuple. There could be a reason
> for concern here, but saying that it might not be "safe to evaluate
> on indexes" just begs the question: WHY wouldn't that be safe?

Ah. For some reason I thought the "division by zero" example is one of
the non-safe cases, but now I see int4div is not marked as leakproof, so
we couldn't push it to index anyway.

I've however also noticed that all the 'like' procedures are marked as
not leak proof, which is a bit unfortunate because that's the example
from Jeff's e-mail that started this thread.

regards

--
Tomas Vondra http://www.2ndQuadrant.com/
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