| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Peter Geoghegan <pg(at)bowt(dot)ie> |
| Cc: | Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Corey Huinker <corey(dot)huinker(at)gmail(dot)com> |
| Subject: | Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation) |
| Date: | 2018-01-18 18:27:39 |
| Message-ID: | CA+TgmoY0Eu3FjmjbD8Ji8BGGNEUvh3pPLztsXXSKPWesKZmDWQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Jan 18, 2018 at 1:14 PM, Peter Geoghegan <pg(at)bowt(dot)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
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