Not HOT enough

Some time ago, I changed GetOldestXmin() to ignore procs in other
databases resulting in a potentially later xmin.

GetSnapshotData() was not touched when that happened, even though the
comments say "...This is the same computation done by
GetOldestXmin(true, true)." The transam/README file says it stronger
"GetSnapshotData also performs an oldest-xmin calculation (which had
better
match GetOldestXmin's)". Doh.

As a result, VACUUM ignores procs in other databases, whereas HOT does
not. That means we aren't cleaning up as much as we could do when
running multiple databases. If its OK for VACUUM, then it must be OK
for HOT cleanup also.

Attached patch ignores procs in other databases during
GetSnapshotData() when IsMVCCSnapshot(), using similar coding to
GetOldestXmin().

Any doubters?

I suggest this is backpatched a few releases.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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