coldata: bugfix to coldata.Copy with NULL and SelOnDest

pkg/col/coldata/vec_test.go

@@ -359,6 +359,55 @@ func TestCopyNulls(t *testing.T) {
 	}
 }
 
+func TestCopySelOnDestDoesNotUnsetOldNulls(t *testing.T) {
+	const typ = coltypes.Int64
+
+	// Set up the destination vector. It is all nulls except for a single
+	// non-null at index 0.
+	dst := NewMemColumn(typ, BatchSize)
+	dstInts := dst.Int64()
+	for i := range dstInts {
+		dstInts[i] = 1
+	}
+	dst.Nulls().SetNulls()
+	dst.Nulls().UnsetNull(0)
+
+	// Set up the source vector with two nulls.
+	src := NewMemColumn(typ, BatchSize)
+	srcInts := src.Int64()
+	for i := range srcInts {
+		srcInts[i] = 2
+	}
+	src.Nulls().SetNull(0)
+	src.Nulls().SetNull(3)
+
+	// Using a small selection vector and SelOnDest, perform a copy and verify
+	// that nulls in between the selected tuples weren't unset.
+	copyArgs := CopySliceArgs{
+		SelOnDest: true,
+		SliceArgs: SliceArgs{
+			ColType:     typ,
+			Src:         src,
+			SrcStartIdx: 1,
+			SrcEndIdx:   3,
+			Sel:         []uint16{0, 1, 3},
+		},
+	}
+
+	dst.Copy(copyArgs)
+
+	// 0 was not null in dest and null in source, but it wasn't selected. Not null.
+	require.False(t, dst.Nulls().NullAt(0))
+	// 1 was null in dest and not null in source: it becomes not null.
+	require.False(t, dst.Nulls().NullAt(1))
+	// 2 wasn't included in the selection vector: it stays null.
+	require.True(t, dst.Nulls().NullAt(2))
+	// 3 was null in dest and null in source: it stays null.
+	require.True(t, dst.Nulls().NullAt(3))
+	// 4 wasn't included: it stays null.
+	require.True(t, dst.Nulls().NullAt(4))
+}
+
 func BenchmarkAppend(b *testing.B) {
 	const typ = coltypes.Int64
 

pkg/col/coldata/vec_tmpl.go

@@ -93,6 +93,7 @@ func _COPY_WITH_SEL(
 			} else {
 				v := execgen.UNSAFEGET(fromCol, int(selIdx))
 				// {{if .SelOnDest}}
+				m.nulls.UnsetNull64(uint64(selIdx))
 				execgen.SET(toCol, int(selIdx), v)
 				// {{else}}
 				execgen.SET(toCol, i+int(args.DestIdx), v)
@@ -118,7 +119,14 @@ func _COPY_WITH_SEL(
 // */}}
 
 func (m *memColumn) Copy(args CopySliceArgs) {
-	m.Nulls().UnsetNullRange(args.DestIdx, args.DestIdx+(args.SrcEndIdx-args.SrcStartIdx))
+	if !args.SelOnDest {
+		// We're about to overwrite this entire range, so unset all the nulls.
+		m.Nulls().UnsetNullRange(args.DestIdx, args.DestIdx+(args.SrcEndIdx-args.SrcStartIdx))
+	}
+	// } else {
+	// SelOnDest indicates that we're applying the input selection vector as a lens
+	// into the output vector as well. We'll set the non-nulls by hand below.
+	// }
 
 	switch args.ColType {
 	// {{range .}}