Re: Performance problem on delete from for 10k rows. May - Mailing list pgsql-performance

Hi All,

I rerun the example with the debug info turned on in postgresl. As you
can see all dependent tables (that as foreign key on table IC) are
emptied before the DELETE FROM IC statement is issued.  For what I
understand the performance problem seem to came from those selects that
point back to IC ( LOG:  statement: SELECT 1 FROM ONLY "public"."ic" x
WHERE "icnum" = $1 FOR UPDATE OF x).  There are 6 of them.  I don't know
where they are comming from.  But if I want to delete the content of the
table (~10k) it may be long to those 6 selects for each deleted rows.
Why are those selects are there ?  Are those select really run on each
row deleted?

I'm running version 7.4.5 on cygwin.  I ran the same delete from
pgAdminIII and I got 945562ms for all the deletes within the same
transaction  .. (so I was wrong saying it took less time in
PgAdminIII... sorry about this).

Do you have any idea why those 6 selects are there?

Maybe I can drop indexes before deleting the content of the table.  I
didn't planned to because tables are quite small and it's more
complicated in my environment.  And tell me if I'm wrong but if I drop
indexed do I have to reload all my stored procedure (to reset the
planner related info)??? Remember having read that somewhere.. (was it
in the Postgresql General Bit newletter ...anyway)

Thanks for your help I really appréciate it :-)

/David

LOG:  duration: 144.000 ms
LOG:  statement: DELETE FROM YN
LOG:  duration: 30.000 ms
LOG:  statement: DELETE FROM YO
LOG:  statement: SELECT 1 FROM ONLY "public"."yo" x WHERE "yotype" = $1
AND "yonum" = $2 FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."yn" x WHERE "ynyotype" =
$1 AND "ynyonum" = $2 FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."yo" x WHERE "yotype" = $1
AND "yonum" = $2 FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."yr" x WHERE "yryotype" =
$1 AND "yryonum" = $2 FOR UPDATE OF x
LOG:  duration: 83.000 ms
LOG:  connection received: host=127.0.0.1 port=2196
LOG:  connection authorized: user=admin database=webCatalog
LOG:  statement: set datestyle to 'ISO'; select version(), case when
pg_encoding_to_char(1) = 'SQL_ASCII' then 'UNKNOWN' else
getdatabaseencoding() end;
LOG:  duration: 2.000 ms
LOG:  statement: set client_encoding = 'UNICODE'
LOG:  duration: 0.000 ms
LOG:  statement: DELETE FROM IY
LOG:  duration: 71.000 ms
LOG:  statement: DELETE FROM IA
LOG:  duration: 17.000 ms
LOG:  statement: DELETE FROM IQ
LOG:  duration: 384.000 ms
LOG:  statement: DELETE FROM IC
LOG:  statement: SELECT 1 FROM ONLY "public"."ic" x WHERE "icnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."iq" x WHERE "iqicnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."ic" x WHERE "icnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."ia" x WHERE "iaicnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."ic" x WHERE "icnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."iy" x WHERE "iyicnumo" =
$1 FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."ic" x WHERE "icnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."iy" x WHERE "iyicnumr" =
$1 FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."ic" x WHERE "icnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."il" x WHERE "ilicnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."ic" x WHERE "icnum" = $1
FOR UPDATE OF x
LOG:  statement: SELECT 1 FROM ONLY "public"."bd" x WHERE "bdicnum" = $1
FOR UPDATE OF x
LOG:  duration: 656807.000 msMichael Fuhr wrote:





-----------------------
DELETE FROM BM;
DELETE FROM BD;
DELETE FROM BO;
DELETE FROM IL;
DELETE FROM YR;
DELETE FROM YN;
DELETE FROM YO;
DELETE FROM IY;
DELETE FROM IA;
DELETE FROM IQ;
DELETE FROM IC;

Michael Fuhr wrote:

>On Tue, Mar 15, 2005 at 04:24:17PM -0500, David Gagnon wrote:
>
>
>
>> Il get this strange problem when deleting rows from a Java program.
>>Sometime (For what I noticed it's not all the time) the server take
>>almost forever to delete rows from table.
>>
>>
>
>Do other tables have foreign key references to the table you're
>deleting from?  If so, are there indexes on the foreign key columns?
>
>Do you have triggers or rules on the table?
>
>Have you queried pg_locks during the long-lasting deletes to see
>if the deleting transaction is waiting for a lock on something?
>
>
>
>>I rememeber having tried to delete the content of my table (IC) from
>>PgAdminIII and I took couples of seconds!!! Not minutes.
>>
>>
>
>How many records did you delete in this case?  If there are foreign
>key references, how many records were in the referencing tables?
>How repeatable is the disparity in delete time?  A single test case
>might have been done under different conditions, so it might not
>mean much.  No offense intended, but "I remember" doesn't carry as
>much weight as a documented example.
>
>
>