Horton

Re: why not parallel seq scan for slow functions

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndquadrant.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Jeff Janes <jeff.janes@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2017-08-21T09:45:46Z
Lists: pgsql-hackers 
On 21 August 2017 at 10:08, Amit Kapila <amit.kapila16@gmail.com> wrote:

> Thoughts?

This seems like a very basic problem for parallel queries.

The problem seems to be that we are calculating the cost of the plan
rather than the speed of the plan.

Clearly, a parallel task has a higher overall cost but a lower time to
complete if resources are available.

We have the choice of 1) adding a new optimizable quantity, or of 2)
treating cost = speed, so we actually reduce the cost of a parallel
plan rather than increasing it so it is more likely to be picked.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Commits

  1. Rewrite the code that applies scan/join targets to paths.

  2. Teach create_projection_plan to omit projection where possible.

  3. Make the upper part of the planner work by generating and comparing Paths.