customfield_params is used to add plugins params.
Eg if you have 1000 childs in one of my plugin, you need from 10000 to 20000 chars., but this is not a big problem, this are parameters
But customfield_params is not searchable(or should never be) so it's not important, it can be Mysql long text field, this not change something, because mysql do not save it in the table directly, but only a reference to the text and most time the text is never put into the memory.
If you use Mysql text field, then mysql have to search for the pointer and look in this, and only after, return the value, it's why text should not be used for search if possible.
and i was asking about difference in search seed between:
1. 10.000 letters text in one custom field value or
2. 4 custom fields with 2500 letters texts
No one search for this text(or i never had a user had this request), but for the other custom_value and this is important.
The query check for the
customfield_id eg.for customfield color, can be
10. and after search for the value :
RED, but more this field is big and more the sql engine need time to find it.
So if you have a shop with 1000 customfields, you dont really see the diference, but if you have 100000 customfields then yes the result can be 10X slower or more using TEXT and 2 or 3 time slower in case of big varchar.
If varchar was only 64 chars, then this can be faster too, so currenty, it's a compromise of the two(size + speed)