opt: add more tracing events around planning

Add a few more tracing events around the use of the optimizer.

Release note: None

pkg/sql/plan_opt.go

@@ -405,6 +405,7 @@ func (opc *optPlanningCtx) buildReusableMemo(ctx context.Context) (_ *memo.Memo,
  // operators that can be constant folded, then fully optimize it now - it
  // can be reused without further changes to build the execution tree.
  if !f.Memo().HasPlaceholders() && !f.FoldingControl().PreventedStableFold() {
+ opc.log(ctx, "optimizing (no placeholders)")
  if _, err := opc.optimizer.Optimize(); err != nil {
  return nil, err
  }
@@ -499,6 +500,8 @@ func (opc *optPlanningCtx) buildExecMemo(ctx context.Context) (_ *memo.Memo, _ e
  }
  opc.flags.Set(planFlagOptCacheMiss)
  opc.log(ctx, "query cache miss")
+ } else {
+ opc.log(ctx, "not using query cache")
  }
 
  // We are executing a statement for which there is no reusable memo