MySQL 查詢大表注意事項

在執行查詢時，Mysql預設把結果全部load到記憶體後再返回（這種模式可理解為Oracle的ALL_ROWS最佳化模式），如果表資料量太大的話，會導致記憶體溢位。

1、mysql console連線資料庫時：

  加入-q選項，mysql -h hostname -u root -p -q

2、jdbc連線資料庫時：

在連線串中加入useCursorFetch=true

在建立的語句中，加入setFetchSize，如

stmt = conn.createStatement(java.sql.ResultSet.TYPE_FORWARD_ONLY,java.sql.ResultSet.CONCUR_READ_ONLY);

stmt.setFetchSize(Integer.MIN_VALUE);

注意：

The Integer.MIN_VALUE is used by the MySQL driver as a signal to switch to streaming result set mode. It is not used as a value.

See the documentation, under "Resultset".In summary:

By default, ResultSets are completely

retrieved and stored in memory. You can tell the driver to stream the

results back one row at a time by setting

stmt.setFetchSize(Integer.MIN_VALUE); (in combination with a

forward-only, read-only result set).