sql: initial batch sizes too large with no row estimate hint

[erikgrinaker]

While reviewing #62282, kv95 benchmarks were shown to regress somewhat. This still holds after landing on master, with e.g. kv95/enc=false/nodes=1/cpu=32 giving 66304 ops/s before the patch and 58862 ops/s after the patch. When reverting to using 1 as the minimum capacity instead of estimatedRowCount in the ResetMaybeReallocate call the performance goes back to the baseline:

cockroach/pkg/sql/colfetcher/cfetcher.go

Line 329 in 5041b22

rf.typs, rf.machine.batch, estimatedRowCount, rf.memoryLimit,

While debugging, it was found that estimatedRowCount would often be 0, which would cause ResetMaybeReallocate to allocate a batch of coldata.BatchSize() (i.e. 1024) every time, instead of 1 which was the old behavior. This probably explains the performance drop.

cockroach/pkg/sql/colmem/allocator.go

Lines 168 to 170 in 5041b22

	} else if minCapacity == 0 || minCapacity > coldata.BatchSize() {
	minCapacity = coldata.BatchSize()
	}