Merge #62386

62386: colbuilder: minor optimization of removing redundant allocations r=yuzefovich a=yuzefovich

At the end of `NewColOperator` call we have to enforce that the result
column type schema is as expected. We do so by comparing the actual and
the expected types and planning casts if needed with a simple project op
to remove the leftovers. Previously, we would always allocate a uint32
slice for the projection and a types slice in `addProjection`,
regardless of the fact whether projection is redundant or not (note that
in `NewSimpleProjectOp` we would end up returning the original
operator). This is unnecessary in most cases, so we now delay the
allocation of the projection slice until we find the first column that
needs a cast.

Release note: None

Co-authored-by: Yahor Yuzefovich <[email protected]>

pkg/sql/colexec/colbuilder/execplan.go

@@ -1323,7 +1323,9 @@ func NewColOperator(
 	if len(args.Spec.ResultTypes) != len(r.ColumnTypes) {
 		return r, errors.AssertionFailedf("unexpectedly different number of columns are output: expected %v, actual %v", args.Spec.ResultTypes, r.ColumnTypes)
 	}
-	projection := make([]uint32, len(args.Spec.ResultTypes))
+	// projection is lazily allocated when the first column that needs an
+	// explicit cast is found. It'll remain nil if projection isn't necessary.
+	var projection []uint32
 	for i := range args.Spec.ResultTypes {
 		expected, actual := args.Spec.ResultTypes[i], r.ColumnTypes[i]
 		if !actual.Identical(expected) {
@@ -1351,12 +1353,23 @@ func NewColOperator(
 				}
 			}
 			r.ColumnTypes = resultTypes
+			if projection == nil {
+				// This is the first column that needs an explicit cast, so we
+				// need to actually allocate the slice and set all previous
+				// columns to be used as is.
+				projection = make([]uint32, len(args.Spec.ResultTypes))
+				for j := 0; j < i; j++ {
+					projection[j] = uint32(j)
+				}
+			}
 			projection[i] = uint32(castedIdx)
-		} else {
+		} else if projection != nil {
 			projection[i] = uint32(i)
 		}
 	}
-	r.Op, r.ColumnTypes = addProjection(r.Op, r.ColumnTypes, projection)
+	if projection != nil {
+		r.Op, r.ColumnTypes = addProjection(r.Op, r.ColumnTypes, projection)
+	}
 	if args.TestingKnobs.PlanInvariantsCheckers {
 		// Plan an invariants checker if it isn't already the root of the tree.
 		if _, isInvariantsChecker := r.Op.(*colexec.InvariantsChecker); !isInvariantsChecker {