mysql - When is JOIN executed in this case? -

let's select data table "posts":

id   |  bigint(20)   |  pk text |  varchar(400) 

and table "likes":

user_id  |  bigint(20) post_id  |  bigint(20) 

those 2 fields joined primary key

let's select on table "posts" 500'000 records simply

select id, text posts limit 0,20 

and need know posts have (there can 1 per user/post, see primary-key definition). do

select id, text posts left join likes on posts.id = likes.post_id limit 0,20 

will query join on 20 records or on 500'000 of table "posts"? no column of "likes" used in where/group clause in effective query.

output of

explain  select id posts left join likes on posts.id = likes.post_id limit 0 , 20  id  select_type table   type    possible_keys   key key_len ref rows    1   simple  posts   index   null    primary 16  null    58  using index 1   simple  likes   ref primary primary 8   legendaily.posts.id 1   using index 

thanks