Message-ID: <e43828bbf4c1dc81ce8a38e9b769fd7bda45d5b4.camel@j-davis.com>
Subject: Re: Default setting for enable_hashagg_disk
From: Jeff Davis <pgsql@j-davis.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Justin Pryzby <pryzby@telsasoft.com>,
 David Rowley <dgrowleyml@gmail.com>, Robert Haas <robertmhaas@gmail.com>,
 Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra
 <tomas.vondra@2ndquadrant.com>,  pgsql-hackers@postgresql.org
Date: Thu, 25 Jun 2020 14:28:02 -0700
In-Reply-To: <20200625195607.GA16766@momjian.us>
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On Thu, 2020-06-25 at 15:56 -0400, Bruce Momjian wrote:
> It is my understanding that spill of sorts is mostly read
> sequentially,
> while hash reads are random.  Is that right?  Is that not being
> costed
> properly?

I don't think there's a major problem with the cost model, but it could
probably use some tweaking.

Hash writes are random. The hash reads should be mostly sequential (for
large partitions it will be 128-block extents, or 1MB). The cost model
assumes 50% sequential and 50% random.

Sorts are written sequentially and read randomly, but there's
prefetching to keep the reads from being too random. The cost model
assumes 75% sequential and 25% random.

Overall, the IO pattern is better for Sort, but not dramatically so.
Tomas Vondra did some nice analysis here:


https://www.postgresql.org/message-id/20200525021045.dilgcsmgiu4l5jpa@development

That resulted in getting the prealloc and projection patches in.

Regards,
	Jeff Davis