mysql - 为什么在 mysql 中显示查询配置文件时有 25 个“检查权限”状态

Question

在 SET PROFILING=1 之后，我运行了一个多连接查询（多个左连接，每个左连接内部都是来自多个相同表的子查询）。运行两次此查询后，请参阅下面的每个状态所花费的时间：

mysql> SHOW PROFILE SOURCE FOR QUERY 6;
+--------------------------------+----------+-----------------------+--------------+-------------+
| Status                         | Duration | Source_function       | Source_file  | Source_line |
+--------------------------------+----------+-----------------------+--------------+-------------+
| starting                       | 0.000046 | NULL                  | NULL         |        NULL |
| Waiting for query cache lock   | 0.000010 | try_lock              | sql_cache.cc |         454 |
| checking query cache for query | 0.000035 | send_result_to_client | sql_cache.cc |        1561 |
| checking privileges on cached  | 0.000009 | send_result_to_client | sql_cache.cc |        1652 |
| checking permissions           | 0.000012 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000009 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000008 | check_access          | sql_parse.cc |        4751 |
| checking permissions           | 0.000009 | check_access          | sql_parse.cc |        4751 |
| sending cached result to clien | 0.000094 | send_result_to_client | sql_cache.cc |        1749 |
| logging slow query             | 0.000009 | log_slow_statement    | sql_parse.cc |        1461 |
| cleaning up                    | 0.000010 | dispatch_command      | sql_parse.cc |        1417 |
+--------------------------------+----------+-----------------------+--------------+-------------+
31 rows in set (0.00 sec)

我想知道为什么我有这么多“检查权限”。查询结果没有问题，查询的基本结构是：

             select field1, field2, etc
                 from A
                    inner join B on (A.id = B.bid)
                    inner join C on (...)
                     ...
                    left join
                        (select some fields
                             from A
                                 inner join B on (A.id = B.bid)
                                 inner join C on (...)
                                  ...
                             where ...
                             group by ...) as tbl_new1 on (...)
                    left join
                        (select some fields
                             from A
                                 inner join B on (A.id = B.bid)
                                 inner join C on (...)
                                  ...
                             where ...
                             group by ...) as tbl_new2 on (...)
                     left join
                      (select some fields
                             from A
                                 inner join B on (A.id = B.bid)
                                 inner join C on (...)
                                  ...
                             where ...
                             group by ...) as tbl_new3 on (...)

                 where ...
                GROUP BY ... ;

谢谢。