execinfra: reuse output row allocation in the helper

This commit makes it so that we reuse the output row in the
ProcOutputHelper if possible. Additionally, it removes a row alloc
object since it is not helpful and only leeds to wasteful allocations.
It also removes a redundant integer for the number of "internal
columns".

Release note: None

pkg/sql/execinfra/processorsbase.go

@@ -64,8 +64,6 @@ type DoesNotUseTxn interface {
 // ProcOutputHelper is a helper type that performs filtering and projection on
 // the output of a processor.
 type ProcOutputHelper struct {
-	numInternalCols int
-	RowAlloc        rowenc.EncDatumRowAlloc
 	// renderExprs has length > 0 if we have a rendering. Only one of renderExprs
 	// and outputCols can be set.
 	renderExprs []execinfrapb.ExprHelper
@@ -97,13 +95,18 @@ type ProcOutputHelper struct {
 
 // Reset resets this ProcOutputHelper, retaining allocated memory in its slices.
 func (h *ProcOutputHelper) Reset() {
-	// Deeply reset the render expressions. Note that we don't bother deeply
-	// resetting the types slice since the types are small objects.
+	// Deeply reset the render expressions and the output row. Note that we
+	// don't bother deeply resetting the types slice since the types are small
+	// objects.
 	for i := range h.renderExprs {
 		h.renderExprs[i] = execinfrapb.ExprHelper{}
 	}
+	for i := range h.outputRow {
+		h.outputRow[i] = rowenc.EncDatum{}
+	}
 	*h = ProcOutputHelper{
 		renderExprs: h.renderExprs[:0],
+		outputRow:   h.outputRow[:0],
 		OutputTypes: h.OutputTypes[:0],
 	}
 }
@@ -125,11 +128,10 @@ func (h *ProcOutputHelper) Init(
 	if post.Projection && len(post.RenderExprs) > 0 {
 		return errors.Errorf("post-processing has both projection and rendering: %s", post)
 	}
-	h.numInternalCols = len(coreOutputTypes)
 	if post.Projection {
 		for _, col := range post.OutputColumns {
-			if int(col) >= h.numInternalCols {
-				return errors.Errorf("invalid output column %d (only %d available)", col, h.numInternalCols)
+			if int(col) >= len(coreOutputTypes) {
+				return errors.Errorf("invalid output column %d (only %d available)", col, len(coreOutputTypes))
 			}
 		}
 		h.outputCols = post.OutputColumns
@@ -174,7 +176,14 @@ func (h *ProcOutputHelper) Init(
 	}
 	if h.outputCols != nil || len(h.renderExprs) > 0 {
 		// We're rendering or projecting, so allocate an output row.
-		h.outputRow = h.RowAlloc.AllocRow(len(h.OutputTypes))
+		if h.outputRow != nil && cap(h.outputRow) >= len(h.OutputTypes) {
+			// In some cases we might have no output columns, so nil outputRow
+			// would have sufficient width, yet nil row is a special value, so
+			// we can only reuse the old outputRow if it's non-nil.
+			h.outputRow = h.outputRow[:len(h.OutputTypes)]
+		} else {
+			h.outputRow = make(rowenc.EncDatumRow, len(h.OutputTypes))
+		}
 	}
 
 	h.offset = post.Offset

pkg/sql/rowexec/distinct.go

@@ -128,7 +128,7 @@ func newDistinct(
 		}); err != nil {
 		return nil, err
 	}
-	d.lastGroupKey = d.OutputHelper.RowAlloc.AllocRow(len(d.types))
+	d.lastGroupKey = make(rowenc.EncDatumRow, len(d.types))
 	d.haveLastGroupKey = false
 	// If we set up the arena when d is created, the pointer to the memAcc
 	// will be changed because the sortedDistinct case makes a copy of d.