The data files contain only one active version of every record. Different versions of that same record are maintained in a version store.
Kind regards/met vriendelijke groet, Serge Fonville http://www.sergefonville.nl Convince Microsoft! They need to add TRUNCATE PARTITION in SQL Server https://connect.microsoft.com/SQLServer/feedback/details/417926/truncate-partition-of-partitioned-table 2012/11/29 Igor Neyman <iney...@perceptron.com> > > From: Mike Christensen [mailto:m...@kitchenpc.com] > Sent: Thursday, November 29, 2012 1:35 PM > To: stho...@optionshouse.com > Cc: Jeff Janes; Schnabel, Robert D.; pgsql-general@postgresql.org > Subject: Re: When does CLUSTER occur? > > > On Thu, Nov 29, 2012 at 10:28 AM, Shaun Thomas <stho...@optionshouse.com> > wrote: > On 11/29/2012 12:20 PM, Jeff Janes wrote: > It would maintain an imperfect clustering, but still much better than > current behavior. > > I thought about that, too. The "imperfect clustering" made me erase > everything I'd written. If the clustering is imperfect, it's not really > clustering. It would mean less random reads to restart the seek chain > though, so it would be a perceptible gain. But it's still not real > clustering until the order is maintained indefinitely. > > So far as I know, that simply can't be done with MVCC. Especially on an > insert-only table that's clustered on a column unrelated to insert order. > > > How is this implemented in MS SQL then? Obviously, MS SQL supports real > clustering and has MVCC.. > > -- > > The way MVCC implemented in Postgres (not allowing "in place" updates) is > obviously complicates things in regards to clustering. > > As for MS SQL, here is how it implements clustering: > The index on which the table is clustered actually becomes a "physical > holder" (for the lack of better term) of the table, meaning the complete > table tuples are stored in the leaf blocks of the index it is clustered on. > There is no other place in the database where they are stored physically. > > Regards, > Igor Neyman > > > -- > Sent via pgsql-general mailing list (pgsql-general@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general >
