R: R: [PERFORM] Query plan on identical tables differs . Why ?
-Messaggio originale- Da: Tom Lane [mailto:[EMAIL PROTECTED] Inviato: giovedì 13 maggio 2004 17.01 A: Fabio Panizzutti Cc: 'Shridhar Daithankar'; [EMAIL PROTECTED] Oggetto: Re: R: [PERFORM] Query plan on identical tables differs . Why ? Fabio Panizzutti [EMAIL PROTECTED] writes: I don't understand why the planner chose a different query plan on identical tables with same indexes . Different data statistics; not to mention different table sizes (the cost equations are not linear). Have you ANALYZEd (or VACUUM ANALYZEd) both tables recently? If the stats are up to date but still not doing the right thing, you might try increasing the statistics target for the larger table's tag_id column. See ALTER TABLE SET STATISTICS. regards, tom lane All tables are vacumed and analyzed . I try so set statistics to 1000 to tag_id columns with ALTER TABLE SET STATISTIC, revacuum analyze , but the planner choose the same query plan . I'm trying now to change the indexes . Thanks ---(end of broadcast)--- TIP 8: explain analyze is your friend
R: R: [PERFORM] Query plan on identical tables differs . Why ?
-Messaggio originale- Da: Stephan Szabo [mailto:[EMAIL PROTECTED] Inviato: giovedì 13 maggio 2004 17.17 A: Fabio Panizzutti Cc: 'Shridhar Daithankar'; [EMAIL PROTECTED] Oggetto: Re: R: [PERFORM] Query plan on identical tables differs . Why ? On Thu, 13 May 2004, Fabio Panizzutti wrote: I don't understand why the planner chose a different query plan on identical tables with same indexes . Because it's more than table structure that affects the choice made by the planner. In addition the statistics about the values that are there as well as the estimated size of the table have effects. One way to see is to see what it thinks is best is to remove the indexes it is using and see what plan it gives then, how long it takes and the estimated costs for those plans. In other suggestions, I think having a (tag_id, data_tag) index rather than (data_tag, tag_id) may be a win for queries like this. Also, unless you're doing many select queries by only the first field of the composite index and you're not doing very many insert/update/deletes, you may want to drop the other index on just that field. Thanks for your attention , i change the indexes on the tables as you suggested : storico=# \d storico_misure_short Table tenore.storico_misure_short Column |Type | Modifiers -+-+--- data_tag| timestamp without time zone | not null tag_id | integer | not null unita_misura| character varying(6)| not null valore_tag | numeric(20,3) | not null qualita | integer | not null numero_campioni | numeric(5,0)| frequenza_campionamento | numeric(3,0)| Indexes: storico_misure_short_idx primary key, btree (tag_id, data_tag) storico_misure_short_data_tag_idx2 btree (data_tag) storico=# \d storico_misure Table tenore.storico_misure Column |Type | Modifiers -+-+--- data_tag| timestamp without time zone | not null tag_id | integer | not null unita_misura| character varying(6)| not null valore_tag | numeric(20,3) | not null qualita | integer | not null numero_campioni | numeric(5,0)| frequenza_campionamento | numeric(3,0)| Indexes: storico_misure_idx primary key, btree (tag_id, data_tag) storico_misure_data_tag_idx2 btree (data_tag) And now performance are similar and the planner works correctly : storico=# \d storico_misure_short Table tenore.storico_misure_short Column |Type | Modifiers -+-+--- data_tag| timestamp without time zone | not null tag_id | integer | not null unita_misura| character varying(6)| not null valore_tag | numeric(20,3) | not null qualita | integer | not null numero_campioni | numeric(5,0)| frequenza_campionamento | numeric(3,0)| Indexes: storico_misure_short_idx primary key, btree (tag_id, data_tag) storico_misure_short_data_tag_idx2 btree (data_tag) storico=# \d storico_misure Table tenore.storico_misure Column |Type | Modifiers -+-+--- data_tag| timestamp without time zone | not null tag_id | integer | not null unita_misura| character varying(6)| not null valore_tag | numeric(20,3) | not null qualita | integer | not null numero_campioni | numeric(5,0)| frequenza_campionamento | numeric(3,0)| Indexes: storico_misure_idx primary key, btree (tag_id, data_tag) storico_misure_data_tag_idx2 btree (data_tag) storico=# explain analyze select tag_id,valore_tag,data_tag from storico_misure_short where (data_tag'2004-05-03' and data_tag '2004-05-12') and tag_id=37423 ; QUERY PLAN -- Index Scan using storico_misure_short_idx on storico_misure_short (cost=0.00..2104.47 rows=584 width=20) (actual time=0.232..39.932 rows=864 loops=1) Index Cond: ((tag_id = 37423) AND (data_tag '2004-05-03 00:00:00
R: R: [PERFORM] Query plan on identical tables differs . Why ?
Index Scan using pk_storico_misure_2 on storico_misure (cost=0.00..1984.64 rows=658 width=21) (actual time=723.441..1858.107 rows=835 loops=1) Index Cond: ((data_tag '2004-05-03 00:00:00'::timestamp without time zone) AND (data_tag '2004-05-12 00:00:00'::timestamp without time zone) AND (tag_id = 37423)) Either most of the time is spent skipping index tuples in the data_tag range 2004-05-03 to 2004-05-12 which have tag_id 37423, or getting those 835 rows causes a lot of disk seeks. If the former is true, an index on (tag_id, data_tag) will help. Is true , i recreate the indexes making an index on (tag_id, data_tag) and works fine . In your first message you wrote: fsync = false Do this only if you don't care for your data. I set it to false , for performance tests .I've a stored procedure that make about 2000 insert in 2 tables and 2000 delete in another and with fsync false perfomrmance are 2.000 -3.000 ms (stable) with fsync 3.000 ms to 15.000 ms . I trust in my hardware an O.S so fsync setting is a big dubt for my production enviroment . Thanks a lot Bye ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
Re: R: R: [PERFORM] Query plan on identical tables differs . Why ?
I trust in my hardware an O.S so fsync setting is a big dubt for my production enviroment . Then you are making a big mistake, loving your hardware more than your data... Chris ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
R: R: R: [PERFORM] Query plan on identical tables differs . Why ?
-Messaggio originale- Da: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Per conto di Christopher Kings-Lynne Inviato: venerdì 14 maggio 2004 11.55 A: Fabio Panizzutti Cc: 'Manfred Koizar'; [EMAIL PROTECTED] Oggetto: Re: R: R: [PERFORM] Query plan on identical tables differs . Why ? I trust in my hardware an O.S so fsync setting is a big dubt for my production enviroment . Then you are making a big mistake, loving your hardware more than your data... Chris I'm testing for better performance in insert/delete so i turn off fsync , i don't love hardware more than data , so i'll set fsync on in the production enviroment . Thanks a lot Best regards Fabio ---(end of broadcast)--- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
