distsql: pool physical plans

[jordanlewis]

First 2 commits are part of #30607

name                           old time/op    new time/op    delta
FlowSetup/distribute=false-24    16.3µs ± 2%    15.9µs ± 2%  -1.96%
(p=0.001 n=8+9)

name                           old alloc/op   new alloc/op   delta
FlowSetup/distribute=false-24    25.6kB ± 0%    25.1kB ± 0%  -2.29%
(p=0.000 n=10+10)

name                           old allocs/op  new allocs/op  delta
FlowSetup/distribute=false-24       246 ± 0%       243 ± 0%  -1.22%
(p=0.000 n=10+10)

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

This change is 

[RaduBerinde]

Reviewable status: complete! 0 of 0 LGTMs obtained

---

pkg/sql/distsql_physical_planner.go, line 1098 at r2 (raw file):

	nPartitions := len(spanPartitions)
	if cap(p.ResultRouters) >= nPartitions {
		p.ResultRouters = p.ResultRouters[:nPartitions]

We may be relying on these slices starting out as zero. In this case we are resurrecting the last state. (btw I'd go back and check your other similar changes with this in mind).

I believe that go 1.11 recognizes statements like append(s, make([]T, n)...) and avoids the allocation. So these could all become x = append(x[:0], make([]T, n)...), assuming you're not looking to backport.

[jordanlewis]

Yeah I thought I was fastidious about checking for assumptions like that - I must have missed some. Also we de-upgraded from Go 1.11, or else I'd use your syntax.

[jordanlewis]

Yeah I thought I was fastidious about checking for assumptions like that - I must have missed some. Also we de-upgraded from Go 1.11, or else I'd use your syntax.

[asubiotto]

Reviewed 7 of 7 files at r1, 4 of 4 files at r2.
Reviewable status: complete! 0 of 0 LGTMs obtained

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pkg/sql/distsql_physical_planner.go, line 1153 at r2 (raw file):

		nTypes = len(n.desc.Columns)
	}
	types := p.ResultTypes[:0]

nit: This reslicing is unnecessary I think.

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pkg/sql/distsql_physical_planner.go, line 2031 at r2 (raw file):

}

var physicalPlanPool = sync.Pool{

I would stick these next to PhysicalPlan.Release