Hi,
On July 10, 2019 1:25:49 PM PDT, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>Andres Freund <andres@anarazel.de> writes:
>> On 2019-07-10 16:02:50 -0400, Tom Lane wrote:
>>> Should we try to transpose some of this logic to below the AM API,
>>> and if so, what should that look like?
>
>> I wonder if we could push this down into a separate type of
>visibility
>> (or maybe redefine NonVacuumable)? So that the scan wouldn't ever
>> return them in the first place? It's a bit annoying to have a
>visibility
>> type that's perhaps best called "what selfuncs.c needs", but still
>seams
>> cleaner than having a separate callback? And the explanation for why
>> that's possibly needed also seems to better belong inside the AMs.
>
>SnapshotNonVacuumable is already basically "what selfuncs.c needs" ---
>nothing else is using it. So I wouldn't hesitate to redefine it if
>that'd fix the problem. But it doesn't sound like a promising
>approach.
>The problem here isn't whether the heap tuple is live or dead, it's
>whether it can be trusted to match the index entry.
Well, if we defined i the snapshot semantics as "return rows that are not vacuumable and safe for selfuncs.c
considerations"it seems like a visibility concern if you squint a tiny bit. It's a mostly checking for certain kinds of
deadrows, after all.
> Also, snapshot
>checking isn't really below the AM API anyway is it?
It is, imo. Scans callbacks are expected to only return visible rows. And if a tuple in a slot needs to be checked for
visibilitylater, that goes through a callback. Don't see how you could otherwise make different non-pgheap storages
work.
>I wonder if we'd be better off to switch over to using data directly
>from the index entry, rather than trying to recover it from the heap.
>We'd then need to make sure that the logic exploits btree's "killed
>index entry" ability, so that dead extremal values are ignored as
>much as possible. But that'd get us out of the broken-HOT-chain
>problem.
Hm, that doesn't sound like a trivial change. Not at my computer for 20min, so can't check rn, but does the index entry
alwayshave all the information we need?
Andres
Andres
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