Re: rows estimate in explain analyze for the BRIN index

Emre Hasegeli <emre@hasegeli.com> writes:
>> I don’t see how to solve this problem without changing explain analyze output to accommodate for “unknown” value. I
don’tthink “0” is a non-confusing representation of “unknown” for most people, and from the practical standpoint, a
“besteffort” estimate is better than 0 (i.e. I will be able to estimate how efficient BRIN index is for my tables in
termsof the number of tuples retrieved/thrown away)
 

We do already have a nearby precedent for returning zero when we don't
have an accurate answer: that's what BitmapAnd and BitmapOr plan nodes
do.  (This is documented btw, at the bottom of section 14.1.)

> The number of retrieved and thrown away rows are already available on
> the upper part of the plan.  Bitmap Index Scan should provide the rows
> that matched the index.

It doesn't have that information.

> Another alternative would be just returning
> the number of matching pages (by not multiplying with 10).  It might
> be better understood.

No, it would not, at least not unless we found a way to explicitly mark
the output as being blocks not rows (which would doubtless break a lot of
existing client-side code).  Zero is fairly clearly an impossible value,
whereas anything that's not zero is going to be taken at face value by
many users.

On balance I think likely the best thing to do is return zero, and
document that behavior.
        regards, tom lane