From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Martijn van Oosterhout <kleptog(at)cupid(dot)suninternet(dot)com>, Marten Feldtmann <marten(at)feki(dot)toppoint(dot)de>, Jesse Scott <jscott(at)bmi(dot)net>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Best way to "add" columns |
Date: | 2000-05-16 14:46:13 |
Message-ID: | 200005161446.KAA07441@candle.pha.pa.us |
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Lists: | pgsql-general |
> Martijn van Oosterhout <kleptog(at)cupid(dot)suninternet(dot)com> writes:
> > Marten Feldtmann wrote:
> >> The varable lengths columns should be at the end of the row, therefore
> >> it does not seem to be good to add an integer column after a varchar
> >> column.
>
> > 1. Is this true? Should variable length column really be at the end
> > of a row?
>
> There is some microscopic performance advantage if you put fixed-width
> columns before variable-width columns: columns that are at a fixed
> offset in the table records don't require scanning through earlier
> columns to access. But I'd be surprised if you could actually measure
> any difference, unless perhaps on tables with hundreds of columns.
>
> > 2. If so, surly postgres can reorder tham internally so that on disk
> > they are in the optimal format.
>
> There are notes in the source code indicating that the original
> Berkeley Postgres crew thought about this and decided it wasn't
> worth the trouble.
Yes, agreed. Can anyone measure the difference. It would show up
accessing columns past the variable length column.
--
Bruce Momjian | http://www.op.net/~candle
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
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