Phil Currier wrote: > On 2/21/07, Bruce Momjian <[EMAIL PROTECTED]> wrote: > > 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. > > Well, as I understand it, that patch isn't really addressing the same > problem. Consider this table: > create table foo (a varchar(10), b int, c smallint, d int, e smallint, ....); > > There are two problems here: > > 1) On my machine, each int/smallint column pair takes up 8 bytes. 2 > of those 8 bytes are alignment padding wasted on the smallint field. > If we grouped all the smallint fields together within the tuple, that > space would not be lost.
Yes, good point. > 2) Each time you access any of the int/smallint fields, you have to > peek inside the varchar field to figure out its length. If we stored > the varchar field at the end of the tuple instead, the access times > for all the other fields would be measurably improved, by a factor > that greatly outweighs the small penalty imposed on the varchar field > itself. > > My understanding is that the varlena headers patch would potentially > reduce the size of the varchar header (which is definitely worthwhile > by itself), but it wouldn't help much for either of these problems. > Or am I misunderstanding what that patch does? > Agreed. -- Bruce Momjian <[EMAIL PROTECTED]> http://momjian.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
