多线程执行 MySQL 查询的时候会遇到 Packet sequence number wrong 错误。

Traceback (most recent call last):
  File "/tmp/db.py", line 167, in _execute
    rowcount = cur.execute(sql, args)
  File "/usr/local/lib/python2.7/site-packages/pymysql/cursors.py", line 163, in execute
    result = self._query(query)
  File "/usr/local/lib/python2.7/site-packages/pymysql/cursors.py", line 321, in _query
    conn.query(q)
  File "/usr/local/lib/python2.7/site-packages/pymysql/connections.py", line 505, in query
    self._affected_rows = self._read_query_result(unbuffered=unbuffered)
  File "/usr/local/lib/python2.7/site-packages/pymysql/connections.py", line 724, in _read_query_result
    result.read()
  File "/usr/local/lib/python2.7/site-packages/pymysql/connections.py", line 1076, in read
    self._read_result_packet(first_packet)
  File "/usr/local/lib/python2.7/site-packages/pymysql/connections.py", line 1146, in _read_result_packet
    self._read_rowdata_packet()
  File "/usr/local/lib/python2.7/site-packages/pymysql/connections.py", line 1180, in _read_rowdata_packet
    packet = self.connection._read_packet()
  File "/usr/local/lib/python2.7/site-packages/pymysql/connections.py", line 660, in _read_packet
    % (packet_number, self._next_seq_id))
InternalError: Packet sequence number wrong - got 51 expected 178

多个查询共用连接，并发查询的时候，现在的 MySQL 网络传输机制无法判断这个返回属于哪个查询。
所以每个线程应该使用不同的连接，或者在 execute 之前加一个锁。