Re: Improve output of BitmapAnd EXPLAIN ANALYZE

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I don't like Tom's proposal of trying to fake up a value here when
> EXPLAIN ANALYZE is in use. Reporting "exact" and "lossy" values for
> BitmapAnd would be a fine enhancement, but artificially trying to
> flatten that back into a row count is going to be confusing, not
> helpful. (Just last week I saw a case where the fact that many pages
> were being lossified caused a performance problem ... so treating
> lossy pages as if they don't exist would have led to a lot of
> head-scratching, because under Tom's proposal the row count would have
> been way off.)

It would very often be the case that the value I suggested would be exact,
so this complaint seems off-base to me.

If we were willing to add an additional output line, we could also report
the number of lossy pages in the result bitmap, and people would then
know not to trust the reported rowcount as gospel. But it's still useful
to have it. I'm envisioning output like

-> BitmapOr (cost=... rows=2000 width=0) (actual time=... rows=1942 loops=1)

in the no-lossy-pages case, otherwise

-> BitmapOr (cost=... rows=4000 width=0) (actual time=... rows=3945 loops=1)
Lossy Bitmap: exact entries=2469, lossy pages=123

There's nothing misleading about that, IMO. (Exercise for the reader:
what rows/page estimate did I assume?)

regards, tom lane