From: Bruce Momjian <maillist@candle.pha.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Steven M. Wheeler <swheeler@sabre.com>
Cc: pgsql-sql@hub.org
Subject: Re: [SQL] Questions about vacuum analyze
Date: Mon, 11 Oct 1999 11:07:44 -0400 (EDT)
Message-ID: <199910111507.LAA29115@candle.pha.pa.us> (raw)
In-Reply-To: <7659.939652362@sss.pgh.pa.us>
> I didn't know much about it either until the comments in psort.c led me
> to read the relevant parts of Knuth. Tape rewind and disk seek times
Yes, that's what I did too.
> are not remotely comparable, especially when you're talking about N tape
> units versus one disk unit --- using more files (tapes) to minimize
> rewind time can be a big win on tapes, but put those same files on a
> disk and you're just spreading the seek time around differently. More
> to the point, though, tape-oriented algorithms tend to assume that tape
> space is free. Polyphase merge doesn't care in the least that it has
> several copies of any given record laying about on different tapes, only
> one of which is of interest anymore.
>
> I know how to get the sort space requirement down to 2x the actual data
> volume (just use a balanced merge instead of the "smarter" polyphase)
> but I am thinking about ways to get it down to data volume + a few
> percent with an extra layer of bookkeeping. The trick is to release
> and recycle space as soon as we have read in the tuples stored in it...
The thing I liked about the existing algorithm is that it did the
sorting without the page faulting/thrashing caused by many sort
algorithms. I can see better space reuse as being a big win because we
would get even less page faulting because the replaced page would
already be in memory.
--
Bruce Momjian | http://www.op.net/~candle
maillist@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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To: pgsql-sql@postgresql.org
Cc: maillist@candle.pha.pa.us, tgl@sss.pgh.pa.us, swheeler@sabre.com, pgsql-sql@hub.org
Subject: Re: [SQL] Questions about vacuum analyze
In-Reply-To: <199910111507.LAA29115@candle.pha.pa.us>
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