Re: Exposing the Xact commit order to the user

On Sun, 2010-05-23 at 16:21 -0400, Jan Wieck wrote:

> In some systems (data warehousing, replication), the order of commits is
> important, since that is the order in which changes have become visible.
> This information could theoretically be extracted from the WAL, but
> scanning the entire WAL just to extract this tidbit of information would
> be excruciatingly painful.

I think it would be quite simple to read WAL. WALSender reads the WAL
file after its been flushed, so it would be simple for it to read a blob
of WAL and then extract the commit order from it.

Overall though, it would be easier and more efficient to *add* info to
WAL and then do all this processing *after* WAL has been transported
elsewhere. Extracting info with DDL triggers, normal triggers, commit
order and everything else seems like too much work to me. Every other
RDBMS has moved away from trigger-based replication and we should give
that serious consideration also.

--
Simon Riggs www.2ndQuadrant.com