fix(jdbc): derive column type name from decltype (#76)

libsql-jdbc/src/main/java/com/github/magnusp/libsql/jdbc/LibsqlResultSetMetaData.java

@@ -120,8 +120,18 @@ public int isNullable(int column) throws SQLException {
 
     @Override
     public boolean isSigned(int column) throws SQLException {
-        String typeName = getColumnTypeName(column);
-        return "NUMERIC".equals(typeName) || "INTEGER".equals(typeName) || "REAL".equals(typeName);
+        if (declType(column) != null) {
+            return switch (getColumnTypeName(column)) {
+                case "NUMERIC", "INTEGER", "REAL" -> true;
+                default -> false;
+            };
+        }
+        // An expression column has no declared type, so signedness follows the
+        // storage class of the current value, as xerial does.
+        return switch (storageType(runtimeValue(column))) {
+            case INTEGER, FLOAT, NULL -> true;
+            case TEXT, BLOB -> false;
+        };
     }
 
     @Override
@@ -286,13 +296,7 @@ public String getColumnTypeName(int column) throws SQLException {
             matcher.find();
             return matcher.group(1).toUpperCase(Locale.ENGLISH);
         }
-        return switch (sampleType(column)) {
-            case INTEGER -> "INTEGER";
-            case FLOAT -> "FLOAT";
-            case BLOB -> "BLOB";
-            case TEXT -> "TEXT";
-            default -> "NUMERIC";
-        };
+        return "NUMERIC";
     }
 
     @Override
@@ -374,10 +378,6 @@ private Value sampleValue(int column) {
         return null;
     }
 
-    private SampleType sampleType(int column) {
-        return storageType(sampleValue(column));
-    }
-
     private static SampleType storageType(Value value) {
         if (value instanceof Value.Integer) {
             return SampleType.INTEGER;

libsql-jdbc/src/test/java/com/github/magnusp/libsql/jdbc/LibsqlResultSetMetaDataTest.java

@@ -269,7 +269,7 @@ void matchesCastOnlyInLowerCase() throws SQLException {
                 "SELECT CAST(1 AS BOOLEAN), cast(1 as BOOLEAN)",
                 null);
 
-        assertThat(metadata.getColumnTypeName(1)).as("uppercase CAST").isEqualTo("INTEGER");
+        assertThat(metadata.getColumnTypeName(1)).as("uppercase CAST").isEqualTo("NUMERIC");
         assertThat(metadata.getColumnType(1)).as("uppercase CAST").isEqualTo(Types.INTEGER);
         assertThat(metadata.isSigned(1)).as("uppercase CAST").isTrue();
 
@@ -378,6 +378,55 @@ void reportsCurrentRowClassNameForMixedTypeColumn() throws SQLException {
                 .isEqualTo(Object.class.getName());
     }
 
+    @Test
+    void reportsUnknownTypeNameForExpressionColumnAtEveryCursorPosition() throws SQLException {
+        StmtResult result = new StmtResult(
+                List.of(new Col("x", null)), List.of(new Row(List.of(Value.of(1L)))), 0, null);
+        LibsqlResultSet resultSet = new LibsqlResultSet(null, result);
+        ResultSetMetaData metadata = resultSet.getMetaData();
+
+        assertThat(metadata.getColumnTypeName(1))
+                .as("before the first row")
+                .isEqualTo("NUMERIC");
+
+        assertThat(resultSet.next()).isTrue();
+        assertThat(metadata.getColumnTypeName(1)).as("on a row").isEqualTo("NUMERIC");
+
+        assertThat(resultSet.next()).isFalse();
+        assertThat(metadata.getColumnTypeName(1))
+                .as("after the last row")
+                .isEqualTo("NUMERIC");
+    }
+
+    @Test
+    void reportsUnknownTypeNameForMixedTypeColumn() throws SQLException {
+        StmtResult result = new StmtResult(
+                List.of(new Col("x", null)),
+                List.of(new Row(List.of(Value.of(1L))), new Row(List.of(Value.of("a")))),
+                0,
+                null);
+        LibsqlResultSet resultSet = new LibsqlResultSet(null, result);
+        ResultSetMetaData metadata = resultSet.getMetaData();
+
+        assertThat(resultSet.next()).isTrue();
+        assertThat(metadata.getColumnTypeName(1))
+                .as("on the integer row")
+                .isEqualTo("NUMERIC");
+        assertThat(metadata.isSigned(1)).as("on the integer row").isTrue();
+
+        assertThat(resultSet.next()).isTrue();
+        assertThat(metadata.getColumnTypeName(1))
+                .as("on the text row")
+                .isEqualTo("NUMERIC");
+        assertThat(metadata.isSigned(1)).as("on the text row").isFalse();
+
+        assertThat(resultSet.next()).isFalse();
+        assertThat(metadata.getColumnTypeName(1))
+                .as("after the last row")
+                .isEqualTo("NUMERIC");
+        assertThat(metadata.isSigned(1)).as("after the last row").isTrue();
+    }
+
     @Test
     void reportsClassNameForCurrentRowOutsideIntRange() throws SQLException {
         StmtResult result = new StmtResult(