opt, sql: reshape the result row in an UPSERT mutation

This change reshapes the result row in an upsert to have NULL values
when values for the column weren't specified.

Release note: None

pkg/sql/rowcontainer/datum_row_container.go

@@ -195,9 +195,7 @@ func (c *RowContainer) allocChunks(ctx context.Context, numChunks int) error {
 func (c *RowContainer) rowSize(row tree.Datums) int64 {
 	rsz := c.fixedColsSize
 	for _, i := range c.varSizedColumns {
-		if i < len(row) && row[i] != nil {
-			rsz += int64(row[i].Size())
-		}
+		rsz += int64(row[i].Size())
 	}
 	return rsz
 }

pkg/sql/tablewriter_upsert_opt.go

@@ -207,18 +207,18 @@ func (tu *optTableUpserter) updateConflictingRow(
 		return nil
 	}
 
-	// We now need a row that has the shape of the result row.
+	// We now need a row that has the shape of the result row with
+	// the appropriate return columns. Make sure all the fetch columns
+	// are present.
+	tu.resultRow = tu.makeResultFromRow(fetchRow, tu.ru.FetchColIDtoRowIndex)
+
+	// Make sure all the updated columns are present.
 	for colID, returnIndex := range tu.colIDToReturnIndex {
 		// If an update value for a given column exists, use that; else use the
 		// existing value of that column if it has been fetched.
 		rowIndex, ok := tu.ru.UpdateColIDtoRowIndex[colID]
 		if ok {
 			tu.resultRow[returnIndex] = updateValues[rowIndex]
-		} else {
-			rowIndex, ok = tu.ru.FetchColIDtoRowIndex[colID]
-			if ok {
-				tu.resultRow[returnIndex] = fetchRow[rowIndex]
-			}
 		}
 	}