Simon Riggs <simon@2ndQuadrant.com> writes:
> Doing that only makes sense when we're running a SELECT. Setting the
> all visible bit immediately prior to an UPDATE that clears it again is
> pointless effort, generating extra work for no reason.
On the other hand, the HOT prune operation itself is worthless when
we're running a SELECT. The only reason we do it that way is that we
have to prune before the query starts to use the page, else pruning
might invalidate pointers-to-tuples that are being held within the
query plan tree.
Maybe it's time to look at what it'd take for the low-level scan
operations to know whether they're scanning the target relation of
an UPDATE query, so that we could skip pruning altogether except
when a HOT update could conceivably ensue. I think this was discussed
back when HOT went in, but nobody wanted to make the patch more invasive
than it had to be.
regards, tom lane