sql: default to batch size 1 in allocator

In #62282, the estimated row count was passed into the scan batch
allocator to avoid growing the batch from 1. However, this also changed
the default batch size from 1 to 1024 when no row count estimate was
available, giving significant overhead when fetching small result sets.
On `kv95/enc=false/nodes=1/cpu=32` this reduced performance from 66304
ops/s to 58862 ops/s (median of 5 runs), since these are single-row
reads without estimates.

This patch reverts the default batch size to 1 when no row count
estimate is available. This fully fixes the `kv95` performance
regression. YCSB/E takes a small hit going from 1895 ops/s to 1786
ops/s, but this only seems to happen because it takes a while for the
statistics to update: sometime within the first minute of the test
(after the 1-minute ramp-up period), throughput abruptly changes from
~700 ops/s to ~1800 ops/s, so using a 2-minute ramp-up period in
roachtest would mostly eliminate any difference.

Release note: None

pkg/sql/colmem/allocator.go

@@ -156,16 +156,18 @@ func (a *Allocator) NewMemBatchNoCols(typs []*types.T, capacity int) coldata.Bat
 // NOTE: if the reallocation occurs, then the memory under the old batch is
 // released, so it is expected that the caller will lose the references to the
 // old batch.
-// Note: the method assumes that minCapacity is at least 1 and will "truncate"
-// minCapacity if it is larger than coldata.BatchSize().
+// Note: the method assumes that minCapacity is at least 0 and will clamp
+// minCapacity to be between 1 and coldata.BatchSize() inclusive.
 // TODO(yuzefovich): change the contract so that maxBatchMemSize takes priority
 // over minCapacity.
 func (a *Allocator) ResetMaybeReallocate(
 	typs []*types.T, oldBatch coldata.Batch, minCapacity int, maxBatchMemSize int64,
 ) (newBatch coldata.Batch, reallocated bool) {
 	if minCapacity < 0 {
 		colexecerror.InternalError(errors.AssertionFailedf("invalid minCapacity %d", minCapacity))
-	} else if minCapacity == 0 || minCapacity > coldata.BatchSize() {
+	} else if minCapacity == 0 {
+		minCapacity = 1
+	} else if minCapacity > coldata.BatchSize() {
 		minCapacity = coldata.BatchSize()
 	}
 	reallocated = true