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.


Andres Freund

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