Horton

Re: Parallel tuplesort (for parallel B-Tree index creation)

Peter Geoghegan <pg@heroku.com>

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From: Peter Geoghegan <pg@heroku.com>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>, Corey Huinker <corey.huinker@gmail.com>
Date: 2016-09-06T22:47:54Z
Lists: pgsql-hackers 
On Tue, Sep 6, 2016 at 2:46 PM, Peter Geoghegan <pg@heroku.com> wrote:
> Feel free to make a counter-proposal for a cap. I'm not attached to
> 500. I'm mostly worried about blatant waste with very large workMem
> sizings. Tens of thousands of tapes is just crazy. The amount of data
> that you need to have as input is very large when workMem is big
> enough for this new cap to be enforced.

If tuplesort callers passed a hint about the number of tuples that
would ultimately be sorted, and (for the sake of argument) it was
magically 100% accurate, then theoretically we could just allocate the
right number of tapes up-front. That discussion is a big can of worms,
though. There are of course obvious disadvantages that come with a
localized cost model, even if you're prepared to add some "slop" to
the allocation size or whatever.

-- 
Peter Geoghegan

Commits

  1. Support parallel btree index builds.

  2. Report an ERROR if a parallel worker fails to start properly.

  3. Transfer state pertaining to pending REINDEX operations to workers.

  4. Add a barrier primitive for synchronizing backends.

  5. Allow DML commands that create tables to use parallel query.

  6. Refactor GetOldestXmin() to use flags

  7. Fix regression in parallel planning against inheritance tables.

  8. Don't create "holes" in BufFiles, in the new logtape code.

  9. Simplify the code for logical tape read buffers.

  10. Fix excessive memory consumption in the new sort pre-reading code.

  11. Implement binary heap replace-top operation in a smarter way.

  12. Cosmetic code cleanup in commands/extension.c.

  13. Speed up planner's scanning for parallel-query hazards.

  14. Read from the same worker repeatedly until it returns no tuple.

  15. Improve tuplesort.c to support variable merge order. The original coding