在执行查询时，Mysql默认把结果全部load到内存后再返回（这种模式可理解为Oracle的ALL_ROWS优化模式），如果表数据量太大的话，会导致内存溢出。
1、mysql console连接数据库时：
加入-q选项，mysql -h hostname -u root -p -q
2、jdbc连接数据库时：
在连接串中加入useCursorFetch=true
在创建的语句中，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).
