On 02/19/2015 03:31 PM, Kevin Grittner wrote:
What about having the long running snapshots declare their working
set, and then only take them into account for global xmin for
relations that are in the working set? Like a SET TRANSACTION WORKING
SET command. This way the error is deterministic, vacuum on the high
churn tables doesn't have to wait for the old transaction delay to
expire and we avoid a hard to tune GUC (what do I need to set
old_snapshot_threshold to, to reduce bloat while not having "normal"
transactions abort).
Let me make sure I understand your suggestion. You are suggesting
that within a transaction you can declare a list of tables which
should get the "snapshot too old" error (or something like it) if a
page is read which was modified after the snapshot was taken?
Snapshots within that transaction would not constrain the effective
global xmin for purposes of vacuuming those particular tables?
I thought it meant that the declared tables would only be vacuumed
conservatively, so the transaction would expect not to see "snapshot too
old" from them.
cheers
andrew
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