Best way to get distinct count from a query joining two tables (multiple join possibilities)

问题

I have 2 tables, table Actions & table Users. Actions -> Users is many-one association.

Table Actions (has thousands of rows)

• id
• uuid
• name
• type
• created_by
• org_id

Table Users (has a max of hundred rows)

• id
• username
• org_id
• org_name

I am trying to get the best join query to obtain a count with a WHERE clause. I need the count of distinct created_bys from table Actions with an org_name in Table Users that contains 'myorg'. Also, (Actions.created_by = Users.username)

I currently have the below queries (producing expected results) and wonder which is better and if it can be optimized further?

Query 1:

select count(distinct Actions.created_by)
from Actions join
     Users
     on Users.org_id = Actions.org_id 
where Users.org_name like '%myorg%';

Query 2:

select count(distinct Users.username)
from Users join
     Actions
     on Actions.created_by = Users.username 
where Users.org_name like '%myorg%';

回答1:

The fastest approach is to modify the second query:

select count(*)
from Users u
where exists (select 1
              from Actions a
              where a.created_by = u.username 
             )
where u.org_name like '%myorg%';

Then the best index is on actions(created_by).

来源：https://stackoverflow.com/questions/61037164/best-way-to-get-distinct-count-from-a-query-joining-two-tables-multiple-join-po