Re: slow update on 1M rows (worse with indexes) - Mailing list pgsql-performance

how about saying:

lock table versions_9d in EXCLUSIVE mode;
UPDATE versions_9d SET flag=2;
commit;

Ismo

On Thu, 22 Feb 2007, Gabriel Biberian wrote:

> Hello,
>
> I experience significant performance issues with postgresql and updates.
> I have a table which contains ~1M rows.
> Layout:
> TOTO=# \d versions_9d;
>             Table «public.versions_9d»
>  Colonne   |          Type          | Modificateurs
> ------------+------------------------+---------------
> hash       | character(32)          |
> date       | integer                | default 0
> diff       | integer                | default 0
> flag       | integer                | default 0
> size       | bigint                 | default 0
> zip_size   | bigint                 | default 0
> jds        | integer                | default 0
> scanned    | integer                | default 0
> dead       | integer                | default 0
>
> Test case:
> Create a new DB and load a dump of the above database with 976009 rows, then i
> perform updates on the whole table.  I recorded the time taken for each full
> update and the amount of extra disk space used.  Each consecutive update of
> the table is slower than the previous
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=2"
> UPDATE 976009
> real    0m41.542s
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=3"
> UPDATE 976009
> real    0m45.140s (+480M)
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=4"
> UPDATE 976009
> real    1m10.554s (+240M)
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=5"
> UPDATE 976009
> real    1m24.065s (+127M)
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=6"
> UPDATE 976009
> real    1m17.758s (+288M)
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=7"
> UPDATE 976009
> real    1m26.777s (+288M)
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=8"
> UPDATE 976009
> real    1m39.151s (+289M)
>
> Then i tried adding an index to the table on the column date (int) that stores
> unix timestamps.
> TOTO=# CREATE INDEX versions_index ON versions_9d (date);
> (-60M) disk space goes down on index creation
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=9"
> UPDATE 976009
> real    3m8.219s (+328M)
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=8"
> UPDATE 976009
> real    6m24.716s (+326M)
> beebox@evobrik01:~$ time psql TOTO -c "UPDATE versions_9d SET flag=10"
> UPDATE 976009
> real    8m25.274s (+321M)
>
> As a sanity check, i loaded mysql5 and tried the same database and updates.
> With mysql, the update always lasts ~8s.
> The conclusions I have come to is that update==insert+delete which seems very
> heavy when index are present (and heavy disk wise on big tables). Is there a
> switch i can flip to optimise this?
>
> Thanks in advance,
> Gabriel Biberian
>
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