Re: Delete query takes exorbitant amount of time

From: Karim A Nassar
Subject: Re: Delete query takes exorbitant amount of time
Date: ,
Msg-id: Pine.SOL.4.21.0503290120550.5642-100000@coruscant.cet.nau.edu
(view: Whole thread, Raw)
In response to: Re: Delete query takes exorbitant amount of time  (Simon Riggs)
Responses: Re: Delete query takes exorbitant amount of time  (Bruno Wolff III)
List: pgsql-performance

Tree view

Delete query takes exorbitant amount of time  (Karim Nassar, )
 Re: Delete query takes exorbitant amount of time  (Tom Lane, )
  Re: Delete query takes exorbitant amount of time  (Mark Lewis, )
   Re: Delete query takes exorbitant amount of time  (Tom Lane, )
    Re: Delete query takes exorbitant amount of time  (Christopher Kings-Lynne, )
    Re: Delete query takes exorbitant amount of time  (Oleg Bartunov, )
    Re: Delete query takes exorbitant amount of time  (Mark Lewis, )
     Re: Delete query takes exorbitant amount of time  (Gaetano Mendola, )
   Re: Delete query takes exorbitant amount of time  (Christopher Kings-Lynne, )
  Re: Delete query takes exorbitant amount of time  (Karim Nassar, )
   Re: Delete query takes exorbitant amount of time  (Josh Berkus, )
    Re: Delete query takes exorbitant amount of time  (Karim Nassar, )
 Re: Delete query takes exorbitant amount of time  (Tom Lane, )
  Re: Delete query takes exorbitant amount of time  (Christopher Kings-Lynne, )
   Re: Delete query takes exorbitant amount of time  (Vivek Khera, )
   Re: Delete query takes exorbitant amount of time  (Tom Lane, )
    Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
     Re: Delete query takes exorbitant amount of time  (Tom Lane, )
      Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
       Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
        Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
         Re: Delete query takes exorbitant amount of time  (Tom Lane, )
          Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
           Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
            Re: Delete query takes exorbitant amount of time  (Tom Lane, )
             Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
         Re: Delete query takes exorbitant amount of time  (Christopher Kings-Lynne, )
     Re: Delete query takes exorbitant amount of time  (Karim Nassar, )
      Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
       Re: Delete query takes exorbitant amount of time  (Karim Nassar, )
        Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
         Re: Delete query takes exorbitant amount of time  (Karim Nassar, )
          Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
           Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
  Re: Delete query takes exorbitant amount of time  (Karim Nassar, )
 Re: Delete query takes exorbitant amount of time  (Tom Lane, )
 Re: Delete query takes exorbitant amount of time  (Josh Berkus, )
  Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
 Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
  Re: Delete query takes exorbitant amount of time  (Karim A Nassar, )
 Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
  Re: Delete query takes exorbitant amount of time  (Karim A Nassar, )
 Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
  Re: Delete query takes exorbitant amount of time  (Karim A Nassar, )
   Re: Delete query takes exorbitant amount of time  (Bruno Wolff III, )
 Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
  Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
   Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
    Re: Delete query takes exorbitant amount of time  (Tom Lane, )
     Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
      Re: Delete query takes exorbitant amount of time  (Tom Lane, )
       Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
        Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
     Re: Delete query takes exorbitant amount of time  (Stephan Szabo, )
   Re: Delete query takes exorbitant amount of time  (Tom Lane, )
   Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
  Re: Delete query takes exorbitant amount of time  (Tom Lane, )
   Re: Delete query takes exorbitant amount of time  (Simon Riggs, )
    Re: Delete query takes exorbitant amount of time  (Tom Lane, )
     Re: Delete query takes exorbitant amount of time  (Simon Riggs, )

> Each value has 1/13th of the table, which is too many rows per value to
> make an IndexScan an efficient way of deleting rows from the table.

But, the original question was that the delete that was taking a long time
was on a different table. I tried to delete 150 rows from a table with 750
rows, which is FK referenced from this large table. If I understand
correctly, Tom suggested that the length of time was due to a sequential
scan being done on the large table for each value being deleted from the
small one.

(I have no formal training in database administration nor database theory,
so please excuse me if I am being dumb.)

For this FK check, there only need be one referring id to invalidate the
delete. ISTM that for any delete with a FK reference, the index could
always be used to search for a single value in the referring table
(excepting very small tables). Why then must a sequential scan be
performed in this case, and/or in general?

--
Karim Nassar
Department of Computer Science
Box 15600, College of Engineering and Natural Sciences
Northern Arizona University,  Flagstaff, Arizona 86011
Office: (928) 523-5868 -=- Mobile: (928) 699-9221




pgsql-performance by date:

From: Dave Cramer
Date:
Subject: Re: How to improve db performance with $7K?
From: PFC
Date:
Subject: Re: Query Optimizer Failure / Possible Bug