Horton

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

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Rushabh Lathia <rushabh.lathia@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>, Corey Huinker <corey.huinker@gmail.com>
Date: 2018-01-18T18:27:39Z
Lists: pgsql-hackers 
On Thu, Jan 18, 2018 at 1:14 PM, Peter Geoghegan <pg@bowt.ie> wrote:
> That seems pretty far fetched.

I don't think it is, and there are plenty of other examples.  All you
need is a query plan that involves significant CPU work both below the
Gather node and above the Gather node.  It's not difficult to find
plans like that; there are TPC-H queries that generate plans like
that.

> But even if it wasn't, my position
> would not change. This could happen only because the planner
> determined that it was the cheapest plan when
> parallel_leader_participation happened to be off. But clearly a
> "degenerate parallel CREATE INDEX" will never be faster than a serial
> CREATE INDEX, and there is a simple way to always avoid one. So why
> not do so?

That's an excellent argument for making parallel CREATE INDEX ignore
parallel_leader_participation entirely.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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