planner: fix gen-col mistakenly resolved after duplicate expression index substitution (#67692) (#68132)

ref #67552

Author: ti-chi-bot

pkg/expression/column.go

@@ -696,11 +696,22 @@ func (col *Column) ResolveIndicesByVirtualExpr(ctx EvalContext, schema *Schema)
 }
 
 func (col *Column) resolveIndicesByVirtualExpr(ctx EvalContext, schema *Schema) bool {
+	fallbackIdx := -1
 	for i, c := range schema.Columns {
-		if c.EqualByExprAndID(ctx, col) {
+		if c.EqualColumn(col) {
 			col.Index = i
 			return true
 		}
+		// Different expression indexes may create hidden generated columns with the same
+		// virtual expression. Prefer the exact column ID when it exists; only fall back
+		// to expression equality when the selected schema has no exact column match.
+		if fallbackIdx == -1 && c.EqualByExprAndID(ctx, col) {
+			fallbackIdx = i
+		}
+	}
+	if fallbackIdx != -1 {
+		col.Index = fallbackIdx
+		return true
 	}
 	return false
 }

pkg/expression/column_test.go

@@ -94,6 +94,24 @@ func TestColumn(t *testing.T) {
 	require.Zero(t, timeVal.Compare(tm))
 	require.False(t, isNull)
 	require.NoError(t, err)
+
+	t.Run("resolve virtual expression prefers exact column ID", func(t *testing.T) {
+		strTp := types.NewFieldType(mysql.TypeVarchar)
+		baseCol := &Column{UniqueID: 10, RetType: strTp}
+		virtualExpr := NewFunctionInternal(ctx, ast.Lower, strTp, baseCol)
+		exprOnlyMatchedCol := &Column{UniqueID: 12, RetType: strTp, VirtualExpr: virtualExpr.Clone()}
+		exactMatchedCol := &Column{UniqueID: 11, RetType: strTp, VirtualExpr: virtualExpr.Clone()}
+		targetCol := &Column{UniqueID: exactMatchedCol.UniqueID, RetType: strTp, VirtualExpr: virtualExpr.Clone()}
+
+		resolvedExpr, ok := targetCol.ResolveIndicesByVirtualExpr(ctx.GetEvalCtx(), NewSchema(exprOnlyMatchedCol, exactMatchedCol))
+		require.True(t, ok)
+		require.Equal(t, 1, resolvedExpr.(*Column).Index)
+
+		ambiguousTargetCol := &Column{UniqueID: 13, RetType: strTp, VirtualExpr: virtualExpr.Clone()}
+		resolvedExpr, ok = ambiguousTargetCol.ResolveIndicesByVirtualExpr(ctx.GetEvalCtx(), NewSchema(exprOnlyMatchedCol, exactMatchedCol))
+		require.True(t, ok)
+		require.Equal(t, 0, resolvedExpr.(*Column).Index)
+	})
 }
 
 func TestColumnHashCode(t *testing.T) {