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 > > ---------------------------(end of broadcast)--------------------------- > TIP 7: You can help support the PostgreSQL project by donating at > > http://www.postgresql.org/about/donate >
