Re: UPDATEDs slowing SELECTs in a fully cached database

From: Kevin Grittner
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
Date: ,
Msg-id: 4E1C158E020000250003F22A@gw.wicourts.gov
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In response to: Re: UPDATEDs slowing SELECTs in a fully cached database  (lars)
Responses: Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure)
List: pgsql-performance

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UPDATEDs slowing SELECTs in a fully cached database  (lars, )
 Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
  Re: UPDATEDs slowing SELECTs in a fully cached database  (Craig Ringer, )
   Re: UPDATEDs slowing SELECTs in a fully cached database  (lars hofhansl, )
    Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
     Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
      Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
       Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure, )
        Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
         Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure, )
        Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
         Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
          Re: UPDATEDs slowing SELECTs in a fully cached database  (Ivan Voras, )
           Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
            Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
             Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
              Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
              Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
               Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
                Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
               Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
                Re: UPDATEDs slowing SELECTs in a fully cached database  (Tom Lane, )
                 Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
                  Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure, )
                  Re: UPDATEDs slowing SELECTs in a fully cached database  (Jeff Janes, )
                 Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
                  Re: UPDATEDs slowing SELECTs in a fully cached database  (Tom Lane, )
                   Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
                    Re: UPDATEDs slowing SELECTs in a fully cached database  (Robert Klemme, )
                    Re: UPDATEDs slowing SELECTs in a fully cached database  (Tom Lane, )
                     Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
                Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
                 Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
                  Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
                   Re: UPDATEDs slowing SELECTs in a fully cached database  (Jeff Janes, )
                    Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
                     Re: UPDATEDs slowing SELECTs in a fully cached database  (Jeff Janes, )
                      Re: UPDATEDs slowing SELECTs in a fully cached database  (Greg Smith, )
              Re: UPDATEDs slowing SELECTs in a fully cached database  (Jeff Janes, )
               Re: UPDATEDs slowing SELECTs in a fully cached database  (Tom Lane, )
             Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure, )
              Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
               Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure, )
                Re: UPDATEDs slowing SELECTs in a fully cached database  (Pavan Deolasee, )
                 Re: UPDATEDs slowing SELECTs in a fully cached database  (lars hofhansl, )
                  Re: UPDATEDs slowing SELECTs in a fully cached database  (Jeff Janes, )
            Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
 Re: UPDATEDs slowing SELECTs in a fully cached database  ("", )
  Re: UPDATEDs slowing SELECTs in a fully cached database  (Robert Klemme, )
   Re: UPDATEDs slowing SELECTs in a fully cached database  ("", )
   Re: UPDATEDs slowing SELECTs in a fully cached database  (lars, )
    Re: UPDATEDs slowing SELECTs in a fully cached database  ("Kevin Grittner", )
     Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure, )
 Re: UPDATEDs slowing SELECTs in a fully cached database  (Ivan Voras, )
  Re: UPDATEDs slowing SELECTs in a fully cached database  (Merlin Moncure, )
   Re: UPDATEDs slowing SELECTs in a fully cached database  (Ivan Voras, )
 Re: UPDATEDs slowing SELECTs in a fully cached database  (Lars, )

lars <> wrote:

> I am not trying to optimize this particular use case, but rather
> to understand what Postgres is doing, and why SELECT queries are
> affected negatively (sometimes severely) by concurrent (or even
> preceding) UPDATEs at all when the database resides in the cache
> completely.

I think we're stuck in terms of trying to guess what is causing the
behavior you are seeing.  Things which would help get it "unstuck":

(1)  More information about your load process.  Looking at the code,
I could sort of see a possible path to this behavior if the load
process involves any adjustments beyond straight INSERTs or COPYs
in.

(2)  You could poke around with a profiler, a debugger, and/or
the contrib/pageinspect module to sort things out.

(3)  You could post a reproducible test case -- where you start with
CREATE TABLE and populate with something like the generate_series()
function and go through a clearly described set of steps to see the
behavior.   With the someone else could do the profiling, debugging,
and/or page inspection.

From what you've said, it seems like either you're omitting some
detail as irrelevant which is actually significant, or you've found
a bug we should hunt down and fix.  I really don't know which it is.

-Kevin


pgsql-performance by date:

From: lars
Date:
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
From: "Kevin Grittner"
Date:
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database