使用WHERE EXISTS条件时的性能问题

Question

I have two tables that hold GBs of data.

When I perform a query with "WHERE EXISTS ...", on either table, the whole MySQL goes down.

Example of the query:

DELETE FROM `records`  
where exists (
    select * 
    from `measurements` 
    where `file_id` = 17
    and measurements.id = records.measurement_id
    ) 

I am not sure on where to start debugging or how this can be solved.

Example of a query which works fine on the same server running on a different database, but takes forever on the main DB

select * 
from `params` 
where exists (
    select * 
    from `records` 
    where `params`.`record_id` = `records`.`id` and exists (
        select * 
        from `measurements` 
        where `records`.`measurement_id` = `measurements`.`id`
            and `file_id` = 17"
    )
)

Answer 1

You simply perform DELETE on entire table if subquery returns any row:

DELETE FROM `records`;

I believe you want correlated subquery:

DELETE FROM `records`  
where exists (
  select * 
  from `measurements` 
  where `file_id` = 17
   and `measurements`.col_name = `records`.col_name 
) ;

EDIT:

You are correct about correlation part. The query still taking DB down

You should check metrics (how many % of intital table you are removing). If more than 20-30% I would simply use CTAs and recreate table.

Second thing: you need to check FK ( ON DELETE CASCADE )

Answer 2

I had to rewrite query using LEFT JOINs to perform deletes. The reason why "exists" took too long is because "exists" has to check each record against the query.