sql/analyzer: add rule to avoid unnecessary casts

sql/analyzer/optimization_rules.go

@@ -245,6 +245,25 @@ func moveJoinConditionsToFilter(ctx *sql.Context, a *Analyzer, n sql.Node) (sql.
 	})
 }
 
+func removeUnnecessaryConverts(ctx *sql.Context, a *Analyzer, n sql.Node) (sql.Node, error) {
+	span, _ := ctx.Span("remove_unnecessary_converts")
+	defer span.Finish()
+
+	if !n.Resolved() {
+		return n, nil
+	}
+
+	a.Log("removing unnecessary converts, node of type: %T", n)
+
+	return n.TransformExpressionsUp(func(e sql.Expression) (sql.Expression, error) {
+		if c, ok := e.(*expression.Convert); ok && c.Child.Type() == c.Type() {
+			return c.Child, nil
+		}
+
+		return e, nil
+	})
+}
+
 // containsSources checks that all `needle` sources are contained inside `haystack`.
 func containsSources(haystack, needle []string) bool {
 	for _, s := range needle {

sql/analyzer/optimization_rules_test.go

@@ -396,3 +396,50 @@ func TestEvalFilter(t *testing.T) {
 		})
 	}
 }
+
+func TestRemoveUnnecessaryConverts(t *testing.T) {
+	testCases := []struct {
+		name      string
+		childExpr sql.Expression
+		castType  string
+		expected  sql.Expression
+	}{
+		{
+			"unnecessary cast",
+			expression.NewLiteral([]byte{}, sql.Blob),
+			"binary",
+			expression.NewLiteral([]byte{}, sql.Blob),
+		},
+		{
+			"necessary cast",
+			expression.NewLiteral("foo", sql.Text),
+			"signed",
+			expression.NewConvert(
+				expression.NewLiteral("foo", sql.Text),
+				"signed",
+			),
+		},
+	}
+
+	for _, tt := range testCases {
+		t.Run(tt.name, func(t *testing.T) {
+			require := require.New(t)
+
+			node := plan.NewProject([]sql.Expression{
+				expression.NewConvert(tt.childExpr, tt.castType),
+			},
+				plan.NewResolvedTable(mem.NewTable("foo", nil)),
+			)
+
+			result, err := removeUnnecessaryConverts(
+				sql.NewEmptyContext(),
+				NewDefault(nil),
+				node,
+			)
+			require.NoError(err)
+
+			resultExpr := result.(*plan.Project).Projections[0]
+			require.Equal(tt.expected, resultExpr)
+		})
+	}
+}

sql/analyzer/rules.go

@@ -32,6 +32,7 @@ var OnceBeforeDefault = []Rule{
 // OnceAfterDefault contains the rules to be applied just once after the
 // DefaultRules.
 var OnceAfterDefault = []Rule{
+	{"remove_unnecessary_converts", removeUnnecessaryConverts},
 	{"assign_catalog", assignCatalog},
 	{"pushdown", pushdown},
 	{"erase_projection", eraseProjection},