[clickhouse-jdbc] describe SELECT queries to retrieve metadata

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## Latest 
 
+### New Features
+- Describe non-executed SELECT queries in prepared statements to provide metadata (https://github.com/ClickHouse/clickhouse-java/issues/1430)
+
 ## 0.6.1
 
 ### New Features

clickhouse-jdbc/src/main/java/com/clickhouse/jdbc/ClickHousePreparedStatement.java

@@ -109,7 +109,17 @@ default void setClob(int parameterIndex, Clob x) throws SQLException {
     @Override
     default ResultSetMetaData getMetaData() throws SQLException {
         ResultSet currentResult = getResultSet();
-        return currentResult != null ? currentResult.getMetaData() : null;
+        if (currentResult != null) {
+            return currentResult.getMetaData();
+        } else if (getLargeUpdateCount() != -1L) {
+            return null; // Update query
+        }
+
+        return describeQueryResult();
+    }
+
+    default ResultSetMetaData describeQueryResult() throws SQLException {
+        return null;
     }
 
     @Override

clickhouse-jdbc/src/main/java/com/clickhouse/jdbc/internal/SqlBasedPreparedStatement.java

@@ -5,7 +5,9 @@
 import java.sql.Date;
 import java.sql.ParameterMetaData;
 import java.sql.ResultSet;
+import java.sql.ResultSetMetaData;
 import java.sql.SQLException;
+import java.sql.Statement;
 import java.sql.Time;
 import java.sql.Timestamp;
 import java.time.LocalDate;
@@ -35,7 +37,9 @@
 import com.clickhouse.data.value.ClickHouseStringValue;
 import com.clickhouse.logging.Logger;
 import com.clickhouse.logging.LoggerFactory;
+import com.clickhouse.jdbc.ClickHouseConnection;
 import com.clickhouse.jdbc.ClickHousePreparedStatement;
+import com.clickhouse.jdbc.ClickHouseResultSetMetaData;
 import com.clickhouse.jdbc.JdbcParameterizedQuery;
 import com.clickhouse.jdbc.SqlExceptionUtils;
 import com.clickhouse.jdbc.parser.ClickHouseSqlStatement;
@@ -239,6 +243,42 @@ protected int getMaxParameterIndex() {
         return templates.length;
     }
 
+    @Override
+    public ResultSetMetaData describeQueryResult() throws SQLException {
+        // No metadata unless query has been recognized as SELECT
+        if (!parsedStmt.isRecognized() || !parsedStmt.isQuery()) {
+            return null;
+        }
+
+        final String[] vals;
+        if (batch.isEmpty()) {
+            vals = new String[values.length];
+            System.arraycopy(this.values, 0, vals, 0, values.length);
+        } else {
+            vals = batch.get(0);
+        }
+        for (int i = 0; i < values.length; i++) {
+            if (vals[i] == null) {
+                vals[i] = ClickHouseValues.NULL_EXPR;
+            }
+        }
+
+        StringBuilder sb = new StringBuilder("desc (");
+        preparedQuery.apply(sb, vals);
+        sb.append(')');
+
+        List<ClickHouseColumn> columns = new LinkedList<>();
+        ClickHouseConnection conn = getConnection();
+        try (Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery(sb.toString())) {
+            while (rs.next()) {
+                columns.add(ClickHouseColumn.of(rs.getString(1), rs.getString(2)));
+            }
+        }
+
+        return ClickHouseResultSetMetaData.of(conn.getJdbcConfig(), conn.getCurrentDatabase(), "",
+            Collections.unmodifiableList(new ArrayList<>(columns)), mapper, conn.getTypeMap());
+    }
+
     @Override
     public ResultSet executeQuery() throws SQLException {
         ensureParams();

clickhouse-jdbc/src/test/java/com/clickhouse/jdbc/ClickHousePreparedStatementTest.java

@@ -4,6 +4,7 @@
 import java.io.File;
 import java.io.IOException;
 import java.math.BigDecimal;
+import java.math.BigInteger;
 import java.net.Inet4Address;
 import java.net.Inet6Address;
 import java.net.MalformedURLException;
@@ -16,6 +17,7 @@
 import java.sql.ParameterMetaData;
 import java.sql.PreparedStatement;
 import java.sql.ResultSet;
+import java.sql.ResultSetMetaData;
 import java.sql.SQLException;
 import java.sql.Statement;
 import java.sql.Timestamp;
@@ -2006,6 +2008,80 @@ public void testInsertWithSettings() throws SQLException {
         }
     }
 
+    @Test(groups = "integration")
+    public void testGetMetadataTypes() throws SQLException {
+        try (Connection conn = newConnection(new Properties());
+            PreparedStatement ps = conn.prepareStatement("select ? a, ? b")) {
+            ResultSetMetaData md = ps.getMetaData();
+            Assert.assertEquals(md.getColumnCount(), 2);
+            Assert.assertEquals(md.getColumnName(1), "a");
+            Assert.assertEquals(md.getColumnTypeName(1), "Nullable(Nothing)");
+            Assert.assertEquals(md.getColumnName(2), "b");
+            Assert.assertEquals(md.getColumnTypeName(2), "Nullable(Nothing)");
+
+            ps.setString(1, "x");
+            md = ps.getMetaData();
+            Assert.assertEquals(md.getColumnCount(), 2);
+            Assert.assertEquals(md.getColumnName(1), "a");
+            Assert.assertEquals(md.getColumnTypeName(1), "String");
+            Assert.assertEquals(md.getColumnName(2), "b");
+            Assert.assertEquals(md.getColumnTypeName(2), "Nullable(Nothing)");
+
+            ps.setObject(2, new BigInteger("12345"));
+            md = ps.getMetaData();
+            Assert.assertEquals(md.getColumnCount(), 2);
+            Assert.assertEquals(md.getColumnName(1), "a");
+            Assert.assertEquals(md.getColumnTypeName(1), "String");
+            Assert.assertEquals(md.getColumnName(2), "b");
+            Assert.assertEquals(md.getColumnTypeName(2), "UInt16");
+
+            ps.addBatch();
+            ps.setInt(1, 2);
+            md = ps.getMetaData();
+            Assert.assertEquals(md.getColumnCount(), 2);
+            Assert.assertEquals(md.getColumnName(1), "a");
+            Assert.assertEquals(md.getColumnTypeName(1), "String");
+            Assert.assertEquals(md.getColumnName(2), "b");
+            Assert.assertEquals(md.getColumnTypeName(2), "UInt16");
+
+            ps.clearBatch();
+            ps.clearParameters();
+            md = ps.getMetaData();
+            Assert.assertEquals(md.getColumnCount(), 2);
+            Assert.assertEquals(md.getColumnName(1), "a");
+            Assert.assertEquals(md.getColumnTypeName(1), "Nullable(Nothing)");
+            Assert.assertEquals(md.getColumnName(2), "b");
+            Assert.assertEquals(md.getColumnTypeName(2), "Nullable(Nothing)");
+        }
+    }
+
+    @Test(groups = "integration")
+    public void testGetMetadataStatements() throws SQLException {
+        try (Connection conn = newConnection(new Properties());
+            PreparedStatement createPs = conn.prepareStatement("create table test_get_metadata_statements (col String) Engine=Log");
+            PreparedStatement selectPs = conn.prepareStatement("select 'Hello, World!'");
+            PreparedStatement insertPs = conn.prepareStatement(
+                "insert into test_get_metadata_statements select 'Hello, World!'");
+            PreparedStatement updatePs = conn.prepareStatement(
+                "update test_get_metadata_statements set col = 'Bye, World!'");
+            PreparedStatement grantPs = conn.prepareStatement("grant select on * to default");
+            PreparedStatement commitPS = conn.prepareStatement("commit");) {
+
+            // Only select shall have valid metadata
+            ResultSetMetaData selectMetaData = selectPs.getMetaData();
+            Assert.assertNotNull(selectMetaData);
+            Assert.assertEquals(selectMetaData.getColumnCount(), 1);
+            Assert.assertEquals(selectMetaData.getColumnTypeName(1), "String");
+
+            // The rest shall return null
+            Assert.assertNull(createPs.getMetaData());
+            Assert.assertNull(insertPs.getMetaData());
+            Assert.assertNull(updatePs.getMetaData());
+            Assert.assertNull(grantPs.getMetaData());
+            Assert.assertNull(commitPS.getMetaData());
+        }
+    }
+
     @Test(groups = "integration")
     public void testGetParameterMetaData() throws SQLException {
         try (Connection conn = newConnection(new Properties());