Add a SHOW WARNINGS SUMMARY function.

Hi,

in [1], I presented an example of a statement that produce 50 million and twelve (50,000,012) warnings. All those cannot be reported by SHOW WARNINSG as the maximum value of max_error_count is 64K (as a side note, I agree that it does not make sense to buffer that many warnings). However, losing all information about those warnings is bad. A way to keep some information about those would be useful. This is the object of this feature request.

[1]: http://jfg-mysql.blogspot.com/2017/01/upgrading-mariadb-failed-because-ignoring-warnings.html

I suggest adding a "SHOW WARNINGS SUMMARY" statement. This would return a list of warning code, with their occurrence count, and with the associated generic warning message.

In the case of [1], the output of "SHOW WARNINGS SUMMARY" could be something like:

+---------+------+----------+--------------------------------------------------+

| Level   | Code | Count    | Message                                          |

+---------+------+----------+--------------------------------------------------+

| Warning | 1366 | 50000000 | Incorrect integer value: ? for column ? at row ? |

| Warning | ...  |        1 | ...                                              |

| Warning | ...  |      ... | ...                                              |

+---------+------+----------+--------------------------------------------------+

As of another feature request (MDEV-11834), "SHOW WARNINGS SUMMARY" should not report warnings that have been "hidden". So a "SHOW ALL WARNINGS SUMMARY" could also be implemented to allow to show the hidden warnings.

Many thanks,

JFG