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

On Fri, Jun 3, 2016 at 4:24 PM, Kevin Grittner <kgrittn(at)gmail(dot)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.

Attached is what I have so far. I'm still looking at what other
comments might need to be adjusted, but thought I should put this
much out for comment now.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company