dynamic searchable fields, best practice?

I have a Lexicon model, and I want user to be able to create dynamic
feature to every lexicon.

And I have a complicate search interface that let user search on every
single feature (including the dynamic ones) belonged to Lexicon model.

I could have used a serialized text field to save all the dynamic
information if they are not for searching.

In case I want to let user search on all fields, I have created a
DynamicField Model to hold all dynamically created features.

But imagine I have 1,000,000,000 lexicon, and if one create a dynamic
feature for every lexicon, this will result creating 1,000,000,000
rows in DynamicField model.

So the sql search function will become quite inefficient while a lot
of dynamic features created.

Is there a better solution for this situation?

Which way should I take?

searching for a better db design for dynamic fields

try to tuning mysql(add cache fields, add index ...) with current db