coldata,sql: remove some todos

This commit removes several TODOs that I have prototyped addressing and
decided to abandon the prototypes, namely:
- checking whether `coldata.BatchSize()` atomic has influence on
performance (the benchmarks and TPCH queries showed that the impact is
negligible if any)
- tuning default batch size (I did that a while ago, and the best
batch size according tpchvec/bench was 1280, barely better than current
1024 which is a lot nicer number)
- pooling allocations of `execFactory` objects (this showed some
improvement on one workload and a regression on another).

Release justification: non-production code changes.

Release note: None

pkg/col/coldata/batch.go

@@ -72,15 +72,16 @@ type Batch interface {
 
 var _ Batch = &MemBatch{}
 
-// TODO(jordan): tune.
+// defaultBatchSize is the size of batches that is used in the non-test setting.
+// Initially, 1024 was picked based on MonetDB/X100 paper and was later
+// confirmed to be very good using tpchvec/bench benchmark on TPC-H queries
+// (the best number according to that benchmark was 1280, but it was negligibly
+// better, so we decided to keep 1024 as it is a power of 2).
 const defaultBatchSize = 1024
 
 var batchSize int64 = defaultBatchSize
 
 // BatchSize is the maximum number of tuples that fit in a column batch.
-// TODO(yuzefovich): we are treating this method almost as if it were a
-// constant while it performs an atomic operation. Think through whether it has
-// a noticeable performance hit.
 func BatchSize() int {
 	return int(atomic.LoadInt64(&batchSize))
 }

pkg/sql/plan_opt.go

@@ -198,9 +198,6 @@ func (p *planner) makeOptimizerPlan(ctx context.Context) error {
 	}
 
 	// Build the plan tree.
-	// TODO(yuzefovich): we're creating a new exec.Factory for every query, but
-	// we probably could pool those allocations using sync.Pool. Investigate
-	// this.
 	if mode := p.SessionData().ExperimentalDistSQLPlanningMode; mode != sessiondata.ExperimentalDistSQLPlanningOff {
 		planningMode := distSQLDefaultPlanning
 		// If this transaction has modified or created any types, it is not safe to