mysql并行查詢問題

業(yè)務(wù)表數(shù)據(jù)量過大分表設(shè)計4張，然后需求是將4張表全量數(shù)據(jù)導(dǎo)出。

測試機(jī)器4核，做法是開啟4個線程同時查詢，最終結(jié)果匯總。

如果一個線程查詢一張表耗時 2s，那么要實(shí)現(xiàn)的是 4個線程跑完也是耗時 2s（利用cpu多核真正并行），但現(xiàn)實(shí)是耗時 2 * 4 約等于 8s多。

真的困擾多時的一個問題，還望大佬相助，跪謝！

附上mysql參數(shù)：

+-----------------------------------------+---------------------------+
| Variable_name                           | Value                     |
+-----------------------------------------+---------------------------+
| innodb_purge_threads                    | 0                         |
| innodb_read_io_threads                  | 4                         |
| innodb_thread_concurrency               | 10                        |
| innodb_thread_sleep_delay               | 10000                     |
| innodb_write_io_threads                 | 4                         |
| max_delayed_threads                     | 20                        |
| max_insert_delayed_threads              | 20                        |
| myisam_repair_threads                   | 1                         |
| performance_schema_max_thread_classes   | 50                        |
| performance_schema_max_thread_instances | 1000                      |
| pseudo_thread_id                        | 81                        |
| thread_cache_size                       | 8                         |
| thread_concurrency                      | 10                        |
| thread_handling                         | one-thread-per-connection |
| thread_stack                            | 262144                    |
+-----------------------------------------+---------------------------+

mysql 版本是 5.5.28