Thanks Alex I test your solution and is realy more faster.
Nested Loop (cost=1743.31..2044.58 rows=50 width=908) (actual time= 101.695..106.178 rows=50 loops=1) -> HashAggregate (cost=1743.31..1743.31 rows=50 width=108) (actual time= 101.509..101.567 rows=50 loops=1) -> Subquery Scan "IN_subquery" (cost=1741.60..1743.19 rows=50 width=108) (actual time=101.327..101.456 rows=50 loops=1) -> Limit (cost=1741.60..1742.69 rows=50 width=108) (actual time=101.313..101.383 rows=50 loops=1) -> Seq Scan on overview (cost=0.00..3283.07rows=150807 width=108) (actual time= 0.036..72.249 rows=80050 loops=1) -> Index Scan using i_documentcontent_id on t_documentcontent (cost= 0.00..6.01 rows=1 width=908) (actual time=0.083..0.085 rows=1 loops=50) Index Cond: ((t_documentcontent._id)::text = ("outer"._id)::text) Total runtime: 106.323 ms I now need to see what trigers i need to add, and test the insertions. Thanks again On Jan 14, 2008 5:54 AM, Alex Turner <[EMAIL PROTECTED]> wrote: > Here is a table I threw together to demonstrate the approximate speed of a > materialized view in this case: > > trend=# explain analyze select property_id from overview order by > property_id limit 50 offset 50000; > QUERY > PLAN > > ---------------------------------------------------------------------------------------------------------------------------- > Limit (cost=19112.75..19112.88 rows=50 width=8) (actual time= > 446.048..446.125 rows=50 loops=1) > -> Sort (cost=18987.75..19400.49 rows=165094 width=8) (actual time= > 384.788..424.433 rows=50050 loops=1) > Sort Key: property_id > -> Seq Scan on overview (cost=0.00..2501.94 rows=165094 > width=8) (actual time= 0.012..88.691 rows=173409 loops=1) > Total runtime: 447.578 ms > (5 rows) > > trend=# select count(*) from overview; > count > -------- > 173409 > (1 row) > > trend=# > > It's not great - but it's better than 47 seconds (The machine I'm running > it on is far from big iron, so these results should be fairly typical for > any modern x86 box - also this materialized view is almost certainly in RAM, > and therefore IO speed is irrelevant). > > Tom lane has already suggested another approach, whereby you order your > results, then select the next 10 from the set where the id is greater than > the greatest of the last one: > > select id from overview order by id limit 50; > > x = get row['id'] // for row 1 > do something > x=get row['id'] // for row 2 > do something > ... > x=get row['id'] // for row 50 > > select id from overview where id>x order by id limit 50. > > The order by is relevant and infact imperative, because you must order > your results somehow, otherwise your pagination will produce different > results each time you try it as database updates will affect the order the > rows come back by default without an order by clause. > > Let me say that again to be clear: The order rows come back if you don't > specify an order by can change! so pulling rows without an order by is a > REALLY bad idea. This will break your pagination if a database update > happens between someone viewing a page and hitting next to view the next > page. > > Alex > > > On Jan 13, 2008 11:43 PM, Alex Turner <[EMAIL PROTECTED]> wrote: > > > If you have to access the data this way (with no where clause at all - > > which sometimes you do) then I have already provided a solution that will > > work reasonably well. If you create what is essentially a materialized view > > of just the id field, the sequence scan will return much fewer pages than > > when you do it on the main table. Then you join it to the indexed main > > table, and page in just the rows you need. Voila - much faster result. Of > > course we haven't really talked about how that will affect insert speed and > > delete speed if you trigger then up, but you haven't really talked about any > > requirements there. > > > > Alex > > > > > > On Jan 13, 2008 11:27 PM, pepone. onrez <[EMAIL PROTECTED] > wrote: > > > > > Sorry Alex i forget mention that i have setscan of in my last test. > > > > > > now I have set seqscan on and indexscan on and added order by _id > > > > > > The table has an index in the _id field > > > > > > CREATE INDEX i_documentcontent_document > > > ON t_documentcontent > > > USING btree > > > (_document); > > > > > > The database was rencently vacum analyze , but not vacun full > > > > > > here is the explain of 2 diferent queries , when i put a large OFFSET > > > > > > EXPLAIN ANALYZE SELECT > > > t_documentcontent._id AS _id > > > FROM t_documentcontent ORDER BY _id LIMIT 50 OFFSET 50000 > > > > > > "Limit (cost=137068.24..137068.36 rows=50 width=58) (actual time= > > > 41119.702..41119.792 rows=50 loops=1)" > > > " -> Sort (cost=136943.24..137320.26 rows=150807 width=58) (actual > > > time=41064.802..41100.424 rows=50050 loops=1)" > > > " Sort Key: _id" > > > " -> Seq Scan on t_documentcontent (cost= > > > 0.00..110772.07rows=150807 width=58) (actual time= > > > 106.679..33267.194 rows=150807 loops=1)" > > > "Total runtime: 41120.015 ms" > > > > > > EXPLAIN ANALYZE SELECT > > > t_documentcontent._id AS _id > > > FROM t_documentcontent ORDER BY _id LIMIT 50 OFFSET 10000 > > > > > > "Limit (cost=39839.37..40038.56 rows=50 width=58) (actual time= > > > 1172.969..1194.228 rows=50 loops=1)" > > > " -> Index Scan using i_documentcontent_id on t_documentcontent > > > (cost=0.00..600805.54 rows=150807 width=58) (actual time= > > > 0.077..1189.688 rows=10050 loops=1)" > > > "Total runtime: 1194.316 ms" > > > > > > Tom > > > i using uuid for the _id field that is the primary key add a WHERE > > > id > ? don 't apply > > > the cursor aproach is also not suitable for same of my queries > > > > > > I use this query for paginate contents of a filesysstem with lots of > > > documents avoid offset is not posible always > > > > > > >