From: | Andres Freund <andres(at)anarazel(dot)de> |
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To: | Craig Ringer <craig(at)2ndquadrant(dot)com> |
Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Timeline following for logical slots |
Date: | 2016-04-04 06:30:33 |
Message-ID: | 20160404063033.GC2431@awork2.anarazel.de |
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Lists: | pgsql-hackers |
On 2016-04-04 14:24:52 +0800, Craig Ringer wrote:
> I don't feel like I've grasped this properly yet. I think it's referring to
> the pg_logical/snapshots/ serialization, the use of which allows us to
> avoid doing extra work in SnapBuildFindSnapshot(...), but doesn't seem to
> be crucial for correct function. After all, decoding still restarts at the
> restart_lsn and feeds relevant xact info into the snapshot builder,
> accumulates invalidation information, etc.
restart_lsn is set to the last known point where a) all changes for
ongoing transactions are available b) we can re-build visiblity
information when we start reading from there.
As we essentially can only start determining visibility information
whenever processing a xl_running_xacts record. Building visibility
information means that there has to be a xl_running_xacts to start
from. To build full visibility information we also have to wait till we
have seen all in-progress transactions finish. So we dump visibility
information every now and then, so we can re-use the information we'd
already assembled.
Greetings,
Andres Freund
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