Horton

Re: Default setting for enable_hashagg_disk

Jeff Davis <pgsql@j-davis.com>

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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: 2020-06-25T21:28:02Z
Lists: pgsql-hackers, pgsql-docs 
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

Commits

  1. Add hash_mem_multiplier GUC.

  2. HashAgg: use better cardinality estimate for recursive spilling.

  3. Remove hashagg_avoid_disk_plan GUC.

  4. Doc fixup for hashagg_avoid_disk_plan GUC.

  5. Rework HashAgg GUCs.

  6. Disk-based Hash Aggregation.

  7. Implement partition-wise grouping/aggregation.

  8. Defer creation of partially-grouped relation until it's needed.