Why doesn't the field table in the Drupal database use entity_id as a primary key? I'm working on trying to optimize a View SQL query and I noticed that the query being executed is using a join on a the entity_id field of a field table. I noticed that for some reason unbeknownst to me the entity_id field is not a primary key. The delta field is a primary key. 

Can anyone enlighten me as to why this is the case? Can I safely tag entity_id as a primary key without incurring a wrath of unintended consequences? Thank you for your help.
The primary key for a field table is compound, consisting of:


entity_type
deleted
entity_id
language
delta


It's probably just the way your GUI is displaying the data that's confusing. This is Navicat's representation, for example:



If not, then something fishy has happened. Since the entity_id is supposed to be part of the compound key, if it isn't, it should be safe to add it to the rest. Take a backup first just in case.