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Add parallel_leader_participation GUC.

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Sometimes, for testing, it's useful to have the leader do nothing but
read tuples from workers; and it's possible that could work out better
even in production.

Thomas Munro, reviewed by Amit Kapila and by me.  A few final tweaks
by me.

Discussion: http://postgr.es/m/CAEepm=2U++Lp3bNTv2Bv_kkr5NE2pOyHhxU=G0YTa4ZhSYhHiw@mail.gmail.com
This commit is contained in:
Robert Haas
2017-11-15 08:17:29 -05:00
parent 7518049980
commit e5253fdc4f
10 changed files with 205 additions and 9 deletions
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12
src/backend/optimizer/path/costsize.c
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@ -5137,7 +5137,6 @@ static double
get_parallel_divisor(Path *path)
{
double parallel_divisor = path->parallel_workers;
double leader_contribution;
/*
* Early experience with parallel query suggests that when there is only
@ -5150,9 +5149,14 @@ get_parallel_divisor(Path *path)
* its time servicing each worker, and the remainder executing the
* parallel plan.
*/
leader_contribution = 1.0 - (0.3 * path->parallel_workers);
if (leader_contribution > 0)
parallel_divisor += leader_contribution;
if (parallel_leader_participation)
{
double leader_contribution;
leader_contribution = 1.0 - (0.3 * path->parallel_workers);
if (leader_contribution > 0)
parallel_divisor += leader_contribution;
}
return parallel_divisor;
}