Re: [COMMITTERS] pgsql: Add the "snapshot too old" feature

Kevin Grittner wrote:
> Add the "snapshot too old" feature
> 
> This feature is controlled by a new old_snapshot_threshold GUC.  A
> value of -1 disables the feature, and that is the default.  The
> value of 0 is just intended for testing.  Above that it is the
> number of minutes a snapshot can reach before pruning and vacuum
> are allowed to remove dead tuples which the snapshot would
> otherwise protect.  The xmin associated with a transaction ID does
> still protect dead tuples.  A connection which is using an "old"
> snapshot does not get an error unless it accesses a page modified
> recently enough that it might not be able to produce accurate
> results.

I think this formulation of TestForOldSnapshot as returning the Page it
checks is a bit strange; you seem to have done it that way only to be
able to write BufferGetPage in a reasonable manner.  I vote for changing
both those macros into inline functions instead, pursuant to
https://www.postgresql.org/message-id/20160409020835.GA727750%40alvherre.pgsql
and have TestForOldSnapshot return void.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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