Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Sat, Jun 4, 2016 at 4:21 PM, Kevin Grittner <kgrittn@gmail.com> wrote:
> On Fri, Jun 3, 2016 at 4:24 PM, Kevin Grittner <kgrittn@gmail.com> wrote:
>> Consequently, causing the index to be ignored in planning when
>> using the old index
>
> That last line should have read "using an old snapshot"
>
>> is not a nice optimization, but necessary for
>> correctness.  We already have logic to do this for other cases
>> (like HOT updates), so it is a matter of tying in to that existing
>> code correctly.  This won't be all that novel.
>
> Just to demonstrate that, the minimal patch to fix behavior in this
> area would be:
>
> diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
> index 31a1438..6c379da 100644
> --- a/src/backend/catalog/index.c
> +++ b/src/backend/catalog/index.c
> @@ -2268,6 +2268,9 @@ IndexBuildHeapRangeScan(Relation heapRelation,
>         Assert(numblocks == InvalidBlockNumber);
>     }
>
> +   if (old_snapshot_threshold >= 0)
> +       indexInfo->ii_BrokenHotChain = true;
> +
>     reltuples = 0;
>
>     /*
>
> Of course, ii_BrokenHotChain should be renamed to something like
> ii_UnsafeForOldSnapshots, and some comments need to be updated; but
> the above is the substance of it.

I don't know why we'd want to rename it like that...

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company