Re: old_snapshot_threshold's interaction with hash index

On Tue, May 3, 2016 at 10:45 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Mon, May 2, 2016 at 04:02:35PM -0500, Kevin Grittner wrote:
>> On Sun, May 1, 2016 at 1:43 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> > On Sun, May 1, 2016 at 12:05 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> >>
>> >> Currently we do the test for old snapshot (TestForOldSnapshot) for hash
>> >> indexes while scanning them. Does this test makes any sense for hash
>> >> indexes considering LSN on hash index will always be zero (as hash indexes
>> >> are not WAL-logged)? It seems to me that PageLSN check in
>> >> TestForOldSnapshot() will always return false which means that the error
>> >> "snapshot too old" won't be generated for hash indexes.
>> >>
>> >> Am I missing something here, if not, then I think we need a way to
>> >> prohibit pruning for hash indexes based on old_snapshot_threshold?
>> >
>> > What I mean to say here is prohibit pruning the relation which has hash
>> > index based on old_snapshot_threshold.
>>
>> Good spot; added to the open issues page.
>
> Uh, I have no idea how this would be fixed if the PageLSN is zero. Do
> you?

Yes, I see three ways, the most obvious of which is what Amit
suggested -- don't do early vacuum on a table which has a hash index.

--
Kevin Grittner
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