On Sat, Sep 25, 2004 at 11:14:53AM +0100, Simon Riggs wrote: > >Jim C. Nasby > > Fair questions. MVCC has been tightly locked into Postgres/SQL for the whole > of its history. There is much written on this and you should search some > more - references are in the manual.
Well, not quite it's whole history: MVCC showed up in 6.5. Vacuum's been there since before SQL. Actually, is a bit of a historical accident. My understanding of the squence of events is that Hewlett-Packard donated an early WORM optical drive to the Stonebraker lab. Since it's write once, it had the beavior that you could only append to files. Someone thought it might be useful for auditing, etc., so they wrote the first storage mananger for postgres to accommodate that drive. The other storage manager at the time was for battery-backed, persistent RAM. So, all this append-only writing leads to files with lots of dead tuples, so the vacuum command was added to reclaim space. I think on the WORM drive, this was supposed to mark blocks 'invisible' in some sense. I don't know if the WORM drive ever actually got used with postgres. Ross -- Ross Reedstrom, Ph.D. [EMAIL PROTECTED] Research Scientist phone: 713-348-6166 The Connexions Project http://cnx.rice.edu fax: 713-348-3665 Rice University MS-375, Houston, TX 77005 GPG Key fingerprint = F023 82C8 9B0E 2CC6 0D8E F888 D3AE 810E 88F0 BEDE ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
