Bruce Momjian escribió: > Phil Currier wrote: > > On 2/21/07, Alvaro Herrera <[EMAIL PROTECTED]> wrote: > > > I'd expect the system being able to reoder the columns to the most > > > efficient order possible (performance-wise and padding-saving-wise), > > > automatically. When you create a table, sort the columns to the most > > > efficient order; ALTER TABLE ADD COLUMN just puts the new columns at the > > > end of the tuple; and anything that requires a rewrite of the table > > > (ALTER TABLE ... ALTER TYPE for example; would be cool to have CLUSTER > > > do it as well; and do it on TRUNCATE also) again recomputes the most > > > efficient order. > > > > That's exactly what I'm proposing. On table creation, the system > > chooses an efficient column order for you. The next time an ALTER > > TABLE operation forces a rewrite, the system would recompute the > > column storage order. I hadn't thought of having CLUSTER also redo > > the storage order, but that seems safe since it takes an exclusive > > lock on the table. I'm less sure about whether it's safe to do this > > during a TRUNCATE. > > Keep in mind we have a patch in process to reduce the varlena length and > reduce alignment requirements, so once that is in, reordering columns > will not be as important.
Yes, but the "cache offset" stuff is still significant, so there will be some benefit in putting all the fixed-length attributes at the start of the tuple, and varlena atts grouped at the end. -- Alvaro Herrera http://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc. ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq