when doing set_field_to_new_field (from switch_to_nullable_trigger_fields())
make sure that the field we're about to change actually belongs
to the right table (otherwise we cannot dereference new_field[]
array as the wrong table might have more fields than
new_field[] has elements)
Case: table with a NOT NULL field, BEFORE UPDATE trigger,
and UPDATE with a subquery that uses GROUP BY on that
NOT NULL field, and needs a temporary table for it.
Because of the BEFORE trigger, the field becomes nullable
temporarily. But its Item_field (used in GROUP BY) doesn't.
When working with the temptable some code looked at
item->maybe_null, some - at field->null_ptr.
The fix: make Item_field nullable when its field is.
This triggers an assert. The group key size is calculated
before the item is made nullable, so the group key doesn't
have a null byte. The fix: make fields/items nullable
before the group key size is calculated.
Don't compare "field == table->next_number_field" because the field
can be special nullable field copy created by the trigger.
Compare field_index values instead.
NOT NULL constraint must be checked *after* the BEFORE triggers.
That is for INSERT and UPDATE statements even NOT NULL fields
must be able to store a NULL temporarily at least while
BEFORE INSERT/UPDATE triggers are running.
