聊聊jdbc statement的fetchSize

序

本文主要研究一下jdbc statement的fetchSize

fetchSize

這裏以postgres jdbc driver爲例，主要是由於postgres的jdbc driver有公開源碼，並且命名比較規範。以前看oracle jdbc，因爲沒有源碼，反編譯出來一大堆var1，var2等的變量命名，很是晦澀。

默認狀況下pgjdbc driver會一次性拉取全部結果集，也就是在executeQuery的時候。對於大數據量的查詢來講，很是容易形成OOM。這種場景就須要設置fetchSize，執行query的時候先返回第一批數據，以後next完一批數據以後再去拉取下一批。

可是這個有幾個要求：

• 數據庫必須使用V3協議，即pg7.4+
• connection的autoCommit必須爲false，由於開啓autoCommit的話，查詢完成cursor會被關閉，那麼下次就不能再fetch了。另外ResultSet必須是ResultSet.TYPE_FORWARD_ONLY類型，這個是默認的。也就是說沒法向後滾動。
• 查詢語句必須是單條，不能是用分號組成的多條查詢

實例代碼

@Test
    public void testReadTimeout() throws SQLException {
        Connection connection = dataSource.getConnection();
        //https://jdbc.postgresql.org/documentation/head/query.html
        connection.setAutoCommit(false); //NOTE 爲了設置fetchSize,必須設置爲false

        String sql = "select * from demo_table";
        PreparedStatement pstmt;
        try {
            pstmt = (PreparedStatement)connection.prepareStatement(sql);
            pstmt.setFetchSize(50); 
            System.out.println("ps.getQueryTimeout():" + pstmt.getQueryTimeout());
            System.out.println("ps.getFetchSize():" + pstmt.getFetchSize());
            System.out.println("ps.getFetchDirection():" + pstmt.getFetchDirection());
            System.out.println("ps.getMaxFieldSize():" + pstmt.getMaxFieldSize());

            ResultSet rs = pstmt.executeQuery(); 
            //NOTE 這裏返回了就表明statement執行完成,默認返回fetchSize的數據
            int col = rs.getMetaData().getColumnCount();
            System.out.println("============================");
            while (rs.next()) { 
                for (int i = 1; i <= col; i++) {
                    System.out.print(rs.getObject(i));
                }
                System.out.println("");
            }
            System.out.println("============================");
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            //close resources
        }
    }

源碼解析

postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/jdbc/PgPreparedStatement.java

/*
   * A Prepared SQL query is executed and its ResultSet is returned
   *
   * @return a ResultSet that contains the data produced by the * query - never null
   *
   * @exception SQLException if a database access error occurs
   */
  public java.sql.ResultSet executeQuery() throws SQLException {
    if (!executeWithFlags(0)) {
      throw new PSQLException(GT.tr("No results were returned by the query."), PSQLState.NO_DATA);
    }

    if (result.getNext() != null) {
      throw new PSQLException(GT.tr("Multiple ResultSets were returned by the query."),
          PSQLState.TOO_MANY_RESULTS);
    }

    return result.getResultSet();
  }

executeQuery首先調用executeWithFlags方法，源碼裏頭直接寫在if裏頭的，這個不是推薦的方式，由於放在if比較容易忽略。

• executeWithFlags

public boolean executeWithFlags(int flags) throws SQLException {
    try {
      checkClosed();

      if (connection.getPreferQueryMode() == PreferQueryMode.SIMPLE) {
        flags |= QueryExecutor.QUERY_EXECUTE_AS_SIMPLE;
      }

      execute(preparedQuery, preparedParameters, flags);

      return (result != null && result.getResultSet() != null);
    } finally {
      defaultTimeZone = null;
    }
  }

protected final void execute(CachedQuery cachedQuery, ParameterList queryParameters, int flags)
      throws SQLException {
    try {
      executeInternal(cachedQuery, queryParameters, flags);
    } catch (SQLException e) {
      // Don't retry composite queries as it might get partially executed
      if (cachedQuery.query.getSubqueries() != null
          || !connection.getQueryExecutor().willHealOnRetry(e)) {
        throw e;
      }
      cachedQuery.query.close();
      // Execute the query one more time
      executeInternal(cachedQuery, queryParameters, flags);
    }
  }

這裏又調用execute方法，在調用executeInternal

executeInternal

postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/jdbc/PgPreparedStatement.java

private void executeInternal(CachedQuery cachedQuery, ParameterList queryParameters, int flags)
      throws SQLException {
    closeForNextExecution();

    // Enable cursor-based resultset if possible.
    if (fetchSize > 0 && !wantsScrollableResultSet() && !connection.getAutoCommit()
        && !wantsHoldableResultSet()) {
      flags |= QueryExecutor.QUERY_FORWARD_CURSOR;
    }

    if (wantsGeneratedKeysOnce || wantsGeneratedKeysAlways) {
      flags |= QueryExecutor.QUERY_BOTH_ROWS_AND_STATUS;

      // If the no results flag is set (from executeUpdate)
      // clear it so we get the generated keys results.
      //
      if ((flags & QueryExecutor.QUERY_NO_RESULTS) != 0) {
        flags &= ~(QueryExecutor.QUERY_NO_RESULTS);
      }
    }

    if (isOneShotQuery(cachedQuery)) {
      flags |= QueryExecutor.QUERY_ONESHOT;
    }
    // Only use named statements after we hit the threshold. Note that only
    // named statements can be transferred in binary format.

    if (connection.getAutoCommit()) {
      flags |= QueryExecutor.QUERY_SUPPRESS_BEGIN;
    }

    // updateable result sets do not yet support binary updates
    if (concurrency != ResultSet.CONCUR_READ_ONLY) {
      flags |= QueryExecutor.QUERY_NO_BINARY_TRANSFER;
    }

    Query queryToExecute = cachedQuery.query;

    if (queryToExecute.isEmpty()) {
      flags |= QueryExecutor.QUERY_SUPPRESS_BEGIN;
    }

    if (!queryToExecute.isStatementDescribed() && forceBinaryTransfers
        && (flags & QueryExecutor.QUERY_EXECUTE_AS_SIMPLE) == 0) {
      // Simple 'Q' execution does not need to know parameter types
      // When binaryTransfer is forced, then we need to know resulting parameter and column types,
      // thus sending a describe request.
      int flags2 = flags | QueryExecutor.QUERY_DESCRIBE_ONLY;
      StatementResultHandler handler2 = new StatementResultHandler();
      connection.getQueryExecutor().execute(queryToExecute, queryParameters, handler2, 0, 0,
          flags2);
      ResultWrapper result2 = handler2.getResults();
      if (result2 != null) {
        result2.getResultSet().close();
      }
    }

    StatementResultHandler handler = new StatementResultHandler();
    result = null;
    try {
      startTimer();
      connection.getQueryExecutor().execute(queryToExecute, queryParameters, handler, maxrows,
          fetchSize, flags);
    } finally {
      killTimerTask();
    }
    result = firstUnclosedResult = handler.getResults();

    if (wantsGeneratedKeysOnce || wantsGeneratedKeysAlways) {
      generatedKeys = result;
      result = result.getNext();

      if (wantsGeneratedKeysOnce) {
        wantsGeneratedKeysOnce = false;
      }
    }

  }

主要看這段

connection.getQueryExecutor().execute(queryToExecute, queryParameters, handler, maxrows,
          fetchSize, flags);

經過把fetchSize傳遞進去，拉取指定大小的result

最後調用sendExecute以及processResults方法來拉取數據
postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/core/v3/QueryExecutorImpl.java

private void sendExecute(SimpleQuery query, Portal portal, int limit) throws IOException {
    //
    // Send Execute.
    //

    if (logger.logDebug()) {
      logger.debug(" FE=> Execute(portal=" + portal + ",limit=" + limit + ")");
    }

    byte[] encodedPortalName = (portal == null ? null : portal.getEncodedPortalName());
    int encodedSize = (encodedPortalName == null ? 0 : encodedPortalName.length);

    // Total size = 4 (size field) + 1 + N (source portal) + 4 (max rows)
    pgStream.sendChar('E'); // Execute
    pgStream.sendInteger4(4 + 1 + encodedSize + 4); // message size
    if (encodedPortalName != null) {
      pgStream.send(encodedPortalName); // portal name
    }
    pgStream.sendChar(0); // portal name terminator
    pgStream.sendInteger4(limit); // row limit

    pendingExecuteQueue.add(new ExecuteRequest(query, portal, false));
  }

protected void processResults(ResultHandler handler, int flags) throws IOException {
    boolean noResults = (flags & QueryExecutor.QUERY_NO_RESULTS) != 0;
    boolean bothRowsAndStatus = (flags & QueryExecutor.QUERY_BOTH_ROWS_AND_STATUS) != 0;

    List<byte[][]> tuples = null;

    int c;
    boolean endQuery = false;

    // At the end of a command execution we have the CommandComplete
    // message to tell us we're done, but with a describeOnly command
    // we have no real flag to let us know we're done. We've got to
    // look for the next RowDescription or NoData message and return
    // from there.
    boolean doneAfterRowDescNoData = false;

    while (!endQuery) {
      c = pgStream.receiveChar();
      switch (c) {
        case 'A': // Asynchronous Notify
          receiveAsyncNotify();
          break;

        case '1': // Parse Complete (response to Parse)
          pgStream.receiveInteger4(); // len, discarded

          SimpleQuery parsedQuery = pendingParseQueue.removeFirst();
          String parsedStatementName = parsedQuery.getStatementName();
          //...
      }
  }
}

next

postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/jdbc/PgResultSet.java

public boolean next() throws SQLException {
    checkClosed();

    if (onInsertRow) {
      throw new PSQLException(GT.tr("Can''t use relative move methods while on the insert row."),
          PSQLState.INVALID_CURSOR_STATE);
    }

    if (current_row + 1 >= rows.size()) {
      if (cursor == null || (maxRows > 0 && row_offset + rows.size() >= maxRows)) {
        current_row = rows.size();
        this_row = null;
        rowBuffer = null;
        return false; // End of the resultset.
      }

      // Ask for some more data.
      row_offset += rows.size(); // We are discarding some data.

      int fetchRows = fetchSize;
      if (maxRows != 0) {
        if (fetchRows == 0 || row_offset + fetchRows > maxRows) {
          // Fetch would exceed maxRows, limit it.
          fetchRows = maxRows - row_offset;
        }
      }

      // Execute the fetch and update this resultset.
      connection.getQueryExecutor().fetch(cursor, new CursorResultHandler(), fetchRows);

      current_row = 0;

      // Test the new rows array.
      if (rows.isEmpty()) {
        this_row = null;
        rowBuffer = null;
        return false;
      }
    } else {
      current_row++;
    }

    initRowBuffer();
    return true;
  }

next方法能夠看到，首先判斷current_row + 1是否小於rows.size()，小於的話，那就current_row++；不然表示這一批fetchSize的數據被消費完了，須要判斷是否結束或者拉取下一批數據，以後更新current_row

connection.getQueryExecutor().fetch(cursor, new CursorResultHandler(), fetchRows);

這個方法拉取fetchRows條數的下一批數據

• initRowBuffer

private void initRowBuffer() {
    this_row = rows.get(current_row);
    // We only need a copy of the current row if we're going to
    // modify it via an updatable resultset.
    if (resultsetconcurrency == ResultSet.CONCUR_UPDATABLE) {
      rowBuffer = new byte[this_row.length][];
      System.arraycopy(this_row, 0, rowBuffer, 0, this_row.length);
    } else {
      rowBuffer = null;
    }
  }

這就是next移動以後，把要消費的這行數據放到rowBuffer裏頭。

小結

對於查詢數據量大的場景下，很是有必要設置fetchSize，不然全量拉取很容易OOM，可是使用fetchSize的時候，要求數據可以在遍歷resultSet的時候及時處理，而不是收集完全部數據返回回去再去處理。

doc

• pgjdbc-REL9.4.1212
• jdbc-postgresql-query
• 正確使用MySQL JDBC setFetchSize()方法解決JDBC處理大結果集 java.lang.OutOfMemoryError: Java heap space