Improve Scan error messages

pgtype/pgtype.go

@@ -582,6 +582,7 @@ func (scanPlanAnyTextToBytes) Scan(src []byte, dst any) error {
 }
 
 type scanPlanFail struct {
+	m          *Map
 	oid        uint32
 	formatCode int16
 }
@@ -597,7 +598,14 @@ func (plan *scanPlanFail) Scan(src []byte, dst any) error {
 		format = fmt.Sprintf("unknown %d", plan.formatCode)
 	}
 
-	return fmt.Errorf("cannot scan OID %v in %v format into %T", plan.oid, format, dst)
+	var dataTypeName string
+	if t, ok := plan.m.oidToType[plan.oid]; ok {
+		dataTypeName = t.Name
+	} else {
+		dataTypeName = "unknown type"
+	}
+
+	return fmt.Errorf("cannot scan %s (OID %d) in %v format into %T", dataTypeName, plan.oid, format, dst)
 }
 
 // TryWrapScanPlanFunc is a function that tries to create a wrapper plan for target. If successful it returns a plan
@@ -1201,7 +1209,7 @@ func (m *Map) planScan(oid uint32, formatCode int16, target any) ScanPlan {
 		return &scanPlanSQLScanner{formatCode: formatCode}
 	}
 
-	return &scanPlanFail{oid: oid, formatCode: formatCode}
+	return &scanPlanFail{m: m, oid: oid, formatCode: formatCode}
 }
 
 func (m *Map) Scan(oid uint32, formatCode int16, src []byte, dst any) error {

query_test.go

@@ -274,8 +274,8 @@ func TestRowsScanDoesNotAllowScanningBinaryFormatValuesIntoString(t *testing.T)
 	var s string
 
 	err := conn.QueryRow(context.Background(), "select point(1,2)").Scan(&s)
-	if err == nil || !(strings.Contains(err.Error(), "cannot scan OID 600 in binary format into *string")) {
-		t.Fatalf("Expected Scan to fail to encode binary value into string but: %v", err)
+	if err == nil || !(strings.Contains(err.Error(), "cannot scan point (OID 600) in binary format into *string")) {
+		t.Fatalf("Expected Scan to fail to scan binary value into string but: %v", err)
 	}
 
 	ensureConnValid(t, conn)
@@ -397,7 +397,7 @@ func TestConnQueryReadWrongTypeError(t *testing.T) {
 		t.Fatal("Expected Rows to have an error after an improper read but it didn't")
 	}
 
-	if rows.Err().Error() != "can't scan into dest[0]: cannot scan OID 23 in binary format into *time.Time" {
+	if rows.Err().Error() != "can't scan into dest[0]: cannot scan int4 (OID 23) in binary format into *time.Time" {
 		t.Fatalf("Expected different Rows.Err(): %v", rows.Err())
 	}
 
@@ -983,7 +983,7 @@ func TestQueryRowErrors(t *testing.T) {
 	}{
 		{"select $1::badtype", []any{"Jack"}, []any{&actual.i16}, `type "badtype" does not exist`},
 		{"SYNTAX ERROR", []any{}, []any{&actual.i16}, "SQLSTATE 42601"},
-		{"select $1::text", []any{"Jack"}, []any{&actual.i16}, "cannot scan OID 25 in text format into *int16"},
+		{"select $1::text", []any{"Jack"}, []any{&actual.i16}, "cannot scan text (OID 25) in text format into *int16"},
 		{"select $1::point", []any{int(705)}, []any{&actual.s}, "unable to encode 705 into binary format for point (OID 600)"},
 	}
 

rows_test.go

@@ -79,7 +79,7 @@ func TestForEachRowScanError(t *testing.T) {
 			actualResults = append(actualResults, []any{a, b})
 			return nil
 		})
-		require.EqualError(t, err, "can't scan into dest[0]: cannot scan OID 25 in text format into *int")
+		require.EqualError(t, err, "can't scan into dest[0]: cannot scan text (OID 25) in text format into *int")
 		require.Equal(t, pgconn.CommandTag{}, ct)
 	})
 }