Re: [WIP PATCH] Index scan offset optimisation using visibility map

Michail Nikolaev <michail(dot)nikolaev(at)gmail(dot)com> writes:
> [ offset_index_only_v4.patch ]

I started to go through this patch with the intention of committing it,
but the more I looked at it the less happy I got. What you've done to
IndexNext() is a complete disaster from a modularity standpoint: it now
knows all about the interactions between index_getnext, index_getnext_tid,
and index_fetch_heap. Or that is, it knows too much and still not enough,
because it's flat out wrong for the case that xs_continue_hot is set.
You can't call index_getnext_tid when that's still true from last time.

I'm not sure about a nicer way to refactor that, but I'd suggest that
maybe you need an additional function in indexam.c that hides all this
knowledge about the internal behavior of an IndexScanDesc.

The PredicateLockPage call also troubles me quite a bit, not only from
a modularity standpoint but because that implies a somewhat-user-visible
behavioral change when this optimization activates. People who are using
serializable mode are not going to find it to be an improvement if their
queries fail and need retries a lot more often than they did before.
I don't know if that problem is bad enough that we should disable skipping
when serializable mode is active, but it's something to think about.

You haven't documented the behavior required for tuple-skipping in any
meaningful fashion, particularly not the expectation that the child plan
node will still return tuples that just need not contain any valid
content. Also, this particular implementation of that:

+ ExecClearTuple(slot);
+ slot->tts_isempty = false;
+ slot->tts_nvalid = 0;

is a gross hack and probably wrong. You could use ExecStoreAllNullTuple,
perhaps.

I'm disturbed by the fact that you've not taught the planner about the
potential cost saving from this, so that it won't have any particular
reason to pick a regular indexscan over some other plan type when this
optimization applies. Maybe there's no practical way to do that, or maybe
it wouldn't really matter in practice; I've not looked into it. But not
doing anything feels like a hack.

Setting this back to Waiting on Author.

regards, tom lane