sql/mysql: improve foreign-keys inspection query

[a8m]

EXPLAIN before is described here: #1727 (comment)

EXPLAIN after:

+----+-------------------+------------+------------+--------+------------------------+------------+---------+-----------------------+------+----------+----------------------------------------------+
| id | select_type       | table      | partitions | type   | possible_keys          | key        | key_len | ref                   | rows | filtered | Extra                                        |
+----+-------------------+------------+------------+--------+------------------------+------------+---------+-----------------------+------+----------+----------------------------------------------+
|  1 | PRIMARY           | cat        | NULL       | index  | PRIMARY                | name       | 194     | NULL                  |    1 |   100.00 | Using index; Using temporary; Using filesort |
|  1 | PRIMARY           | cat        | NULL       | index  | PRIMARY                | name       | 194     | NULL                  |    1 |   100.00 | Using index; Using join buffer (hash join)   |
|  1 | PRIMARY           | sch        | NULL       | ref    | PRIMARY,catalog_id     | catalog_id | 8       | mysql.cat.id          |    5 |   100.00 | Using where; Using index                     |
|  1 | PRIMARY           | tbl        | NULL       | ref    | schema_id              | schema_id  | 8       | mysql.sch.id          |   76 |   100.00 | Rematerialize (<derived3>)                   |
|  1 | PRIMARY           | <derived3> | NULL       | ALL    | NULL                   | NULL       | NULL    | NULL                  |    2 |   100.00 | NULL                                         |
|  1 | PRIMARY           | fk         | NULL       | ALL    | schema_id,table_id     | NULL       | NULL    | NULL                  |  840 |   100.00 | Using where; Using join buffer (hash join)   |
|  1 | PRIMARY           | sch        | NULL       | eq_ref | PRIMARY,catalog_id     | PRIMARY    | 8       | mysql.fk.schema_id    |    1 |   100.00 | Using where                                  |
|  1 | PRIMARY           | tbl        | NULL       | eq_ref | PRIMARY                | PRIMARY    | 8       | mysql.fk.table_id     |    1 |   100.00 | Using where                                  |
|  1 | PRIMARY           | col        | NULL       | eq_ref | PRIMARY                | PRIMARY    | 8       | constraints.column_id |    1 |   100.00 | Using where                                  |
|  3 | DEPENDENT DERIVED | NULL       | NULL       | NULL   | NULL                   | NULL       | NULL    | NULL                  | NULL |     NULL | Impossible WHERE                             |
|  4 | DEPENDENT UNION   | fk         | NULL       | ref    | PRIMARY,table_id       | table_id   | 8       | mysql.tbl.id          |    1 |   100.00 | NULL                                         |
|  4 | DEPENDENT UNION   | fkcu       | NULL       | ref    | PRIMARY,foreign_key_id | PRIMARY    | 8       | mysql.fk.id           |    1 |   100.00 | NULL                                         |
+----+-------------------+------------+------------+--------+------------------------+------------+---------+-----------------------+------+----------+----------------------------------------------+

[giautm]

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