MySQL magic, or why does DELETE take forever?

Question

MySQL

There is a table with 100000 records. The table looks like this:

increment	data [text]	dataId[varchar45]	userId[int11]

We make a request to delete fields with a nested select from another table.

DELETE 
  FROM userdata 
  WHERE dataId in (
    SELECT concat('ufi',dbIncrement) 
    FROM ufields 
    WHERE podvedId='val' 
      and formId='val' 
      and parentContainerId='val' 
      and (descriptor_x='val' or descriptor_y='val')
  )
// очень долго выполняется, сервер падает

Separately , requests are executed quickly, but in a single request, execution puts the server.
We tried to index the fields on which the selection takes place, but this did not affect the execution time.
What could be the reason for this behaviour? Why does the simplest DELETE, which is to remove two fields, put the server?

Answer 1

DELETE `ud`.*
  FROM `userdata` AS `ud`
  JOIN `ufields` AS `uf` 
    ON `ud`.`dataId` = CONCAT('ufi', `uf`.`dbIncrement`) 
    AND `uf`.`podvedId` = 'val' 
    AND `uf`.`formId` = 'val' 
    AND `uf`.`parentContainerId` = 'val' 
    AND (`uf`.`descriptor_y` = 'val' OR `uf`.`descriptor_y` = 'val')

Answer 2

Push result in the temporary table and make selection from it. Where it will no longer be necessary to perform concat.