Re: Locking concurrency: select for update vs update

Streamsoft - Mirek Szajowski <m.szajowski@xxxxxxxxxxxxx> · Tue, 7 Jun 2016 15:26:27 +0200



    Thanks 

    
    after your description I found select name from
      phone_number_type  WHERE id_phone_number_type=4 for NO KEY
      update (Postgresql 9.3 )

    
    W dniu 2016-06-07 o 15:24, Tom Lane
      pisze:

    
      Streamsoft - Mirek Szajowski <m.szajowski@xxxxxxxxxxxxx> writes:

      
        Why I can't execute 'select for update' but I can update?

      
      In recent PG versions, the lock held due to having inserted an FK
dependent row effectively only locks the key fields of the parent row.
UPDATE can tell whether you're trying to change the row's key fields,
and it will proceed if you aren't.  SELECT FOR UPDATE has to lock the
whole row (since it must assume you might be intending to change any
fields of the row); so it blocks until the FK lock goes away.

			regards, tom lane