Re: Catalog/Metadata consistency during changeset extraction from wal

Excerpts from Kevin Grittner's message of lun jun 25 14:50:54 -0400 2012:

> One fine point regarding before and after images -- if a value
> doesn't change in an UPDATE, there's no reason to include it in both
> the BEFORE and AFTER tuple images, as long as we have the null
> column bitmaps -- or some other way of distinguishing unchanged from
> NULL.  (This could be especially important when the unchanged column
> was a 50 MB bytea.)

Yeah, probably the best is to have the whole thing in BEFORE, and just
send AFTER values for those columns that changed, and include the
'replace' bool array (probably packed as a bitmap), so that the update
can be trivially constructed at the other end just like in
heap_modify_tuple.

--
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
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