I’ve got a table

CREATE TABLE IF NOT EXISTS metadata (
    user_id   int not null,
    entry    VARCHAR(4096) not null
);

I am trying to add a PRIMARY KEY, limiting its size on entry column to 16 bytes.
I don’t want to index full 2730 (B-TREE limit) characters on the entry field due to storage overhead.

ALTER TABLE metadata ADD PRIMARY KEY (user_id, (entry::varchar(16)));

unfortunately such syntax isn’t allowed

SQL Error [42601]: ERROR: syntax error at or near "("

nor is the:

ALTER TABLE metadata ADD PRIMARY KEY (user_id, entry(16)) -- That would probably work on MySQL

It seems that I can create a normal index just fine

create index some_index on metadata(user_id, (entry::varchar(16)));

Is there any way to limit column size in a PRIMARY INDEX?