standard table KEY_COLUMN_USAGE should only show keys where
a user has some privileges on every column of the key

standard table TABLE_CONSTRAINTS should show tables where
a user has any non-SELECT privilege on the table or on any column
of the table

standard table REFERENTIAL_CONSTRAINTS is defined in terms of
TABLE_CONSTRAINTS, so the same rule applies. If the user
has no rights to see the REFERENCED_TABLE_NAME value, it should be NULL

SHOW INDEX (and STATISTICS table) is non-standard, but it seems
reasonable to use the same logic as for KEY_COLUMN_USAGE.