The bitmap heap scan skip fetch optimization skips fetching the heap
block when a page is set all-visible in the visibility map and no
columns from the table are needed to satisfy the query.
2b73a8cd33 and c3953226a0 changed the control flow of bitmap heap scan
to use the read stream API. The read stream API returns buffers
containing blocks to the user. To make this work with the skip fetch
optimization, we keep a count of the empty tuples we need to emit for
all the blocks skipped and only emit the empty tuples after processing
the next block fetched from the heap or at the end of the scan.
It's incorrect to recheck NULL tuples, so we must set `recheck` to false
before yielding control back to BitmapHeapNext(). This was done before
emitting any remaining empty tuples at the end of the scan but not for
empty tuples emitted during the scan. This meant that if a page fetched
from the heap did require recheck and set `recheck` to true and then we
emitted empty tuples for subsequent blocks, we would get wrong results.
Fix this by always setting `recheck` to false before emitting empty
tuples.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Tested-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/496f7acd-881c-4df3-9bd3-8f8534dfec26%40gmail.com
indexscan always occurs in one call, and the results are returned in a
TIDBitmap instead of a limited-size array of TIDs. This should improve
speed a little by reducing AM entry/exit overhead, and it is necessary
infrastructure if we are ever to support bitmap indexes.
In an only slightly related change, add support for TIDBitmaps to preserve
(somewhat lossily) the knowledge that particular TIDs reported by an index
need to have their quals rechecked when the heap is visited. This facility
is not really used yet; we'll need to extend the forced-recheck feature to
plain indexscans before it's useful, and that hasn't been coded yet.
The intent is to use it to clean up 8.3's horrid @@@ kluge for text search
with weighted queries. There might be other uses in future, but that one
alone is sufficient reason.
Heikki Linnakangas, with some adjustments by me.
