Gianluca Alberici wrote:
I have ran into an interesting problem with 8.1 and i would like anybody to explain me if there's a problem with the planner or there's a problem with myself. In both cases a solution is welcome. The following query: SELECT sum(qty) FROM _abi_main_pof_r ampr inner join _abi_main_items ami on ampr.itemid=ami.id inner join _abi_main_pof_t ampt on ampr.poftid=ampt.poftid inner join _abi_main_contacts amc on ampt.contactid=amc.codest WHERE ampt.doctypeid in ('BCA','BSC') and amc.descr ilike '%SOCO%' and ampr.type=0; is explain analyze'd as followsAggregate (cost=1220.65..1220.66 rows=1 width=4) (actual time=95937.824..95937.824 rows=1 loops=1) -> Nested Loop (cost=0.00..1220.65 rows=1 width=4) (actual time=3695.250..95936.292 rows=1503 loops=1) -> Nested Loop (cost=0.00..1214.64 rows=1 width=8) (actual time=3695.229..95924.697 rows=1503 loops=1)Join Filter: ("inner".poftid = "outer".poftid)-> Nested Loop (cost=0.00..79.16 rows=1 width=4) (actual time=0.039..22.547 rows=2437 loops=1)Join Filter: ("inner".contactid = "outer".codest)-> Seq Scan on _abi_main_contacts amc (cost=0.00..1.29 rows=1 width=4) (actual time=0.029..0.034 rows=1 loops=1)Filter: ((descr)::text ~~* '%SOCO%'::text)-> Seq Scan on _abi_main_pof_t ampt (cost=0.00..77.53 rows=27 width=8) (actual time=0.006..15.820 rows=2702 loops=1) Filter: (((doctypeid)::text = 'BCA'::text) OR ((doctypeid)::text = 'BSC'::text)) -> Seq Scan on _abi_main_pof_r ampr (cost=0.00..1132.81 rows=214 width=12) (actual time=0.034..35.986 rows=8271 loops=2437)Filter: ("type" = 0)
The problem is in the estimate of that seq scan of _abi_main_pof_r table. The planner estimates that there's 214 rows with type=0, but in reality there's 8271. That misestimate throws off the rest of the plan.
...The same query, but with a condition change as ampr.type != 1 instead of ampr.type=0 SELECT sum(qty) FROM _abi_main_pof_r ampr inner join _abi_main_items ami on ampr.itemid=ami.id inner join _abi_main_pof_t ampt on ampr.poftid=ampt.poftid inner join _abi_main_contacts amc on ampt.contactid=amc.codest WHERE ampt.doctypeid in ('BCA','BSC') and amc.descr ilike '%SOCO%' and ampr.type != 1; is explain analyze'd as follows:Aggregate (cost=1446.13..1446.14 rows=1 width=4) (actual time=81.609..81.609 rows=1 loops=1) -> Nested Loop (cost=77.60..1446.12 rows=2 width=4) (actual time=22.597..80.944 rows=1503 loops=1) -> Nested Loop (cost=77.60..1434.12 rows=2 width=8) (actual time=22.577..72.785 rows=1503 loops=1)Join Filter: ("inner".contactid = "outer".codest)-> Seq Scan on _abi_main_contacts amc (cost=0.00..1.29 rows=1 width=4) (actual time=0.030..0.036 rows=1 loops=1)Filter: ((descr)::text ~~* '%SOCO%'::text)-> Hash Join (cost=77.60..1427.52 rows=425 width=12) (actual time=22.536..69.034 rows=8271 loops=1)Hash Cond: ("outer".poftid = "inner".poftid)-> Seq Scan on _abi_main_pof_r ampr (cost=0.00..1132.81 rows=42571 width=12) (actual time=0.035..37.045 rows=8271 loops=1)Filter: ("type" <> 1)
Here you can see that the planner is estimating that there's 42571 rows with type <> 1, which is also way off the reality in the other direction, but this time the plan it chooses happens to be better.
Run ANALYZE. The planner needs to have statistics on how many rows of each type there is to come up with the proper plan.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@xxxxxxxxxxxxxx so that your message can get through to the mailing list cleanly
