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Mysql大表查询注意事项

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.在mysql console连接数据库时：
加入-q选项，mysql -h hostname -u root -p -q

2.在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).

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