On 12/17/2011 11:24 AM, Filip Rembiałkowski wrote:
Normally there is no chance it could work,
because (a) the planner does not know all possible values of a column,
and (b) btree indexes cannot search on "not equal" operator.
Is there an index type that can check "not equal"?
This specific column has a limited number of possible values - it is
essentially an enumerated list.
Roxanne
BTW I've just made a case where - logically - it could work, but it
still does not:
create table nums ( num int4 not null, check(num=1 or num=2) );
insert into nums select case when random()<=0.99 then 1 else 2 end
from generate_series(1,1000000);
create index nums_idx on nums(num);
analyze nums;
set constraint_exclusion to 'on';
explain select * from nums where num<>1;
--planner could estimate selectivity as 1%, and use index with "=2"
filter basing on check constraint?
2011/12/17 Roxanne Reid-Bennett<rox@xxxxxxxxxxx>:
I have a query that used<> against an indexed column. In this
case I can use the reverse and use in or = and get the performance
I need... but "in general"... will the planner ever use an index when the
related column is compared using<>?
I feel like the answer is no, but wanted to ask.
Roxanne
Postgres Version 8.4.9 PostGIS version 1.5.2
Context for question:
I have the following query:
select *
from op_region opr, yield_segment_info ysi, data_location dl
where opr.op_region_id in
(select distinct op_region_id
from yield_point
where yield> 0
and area> 0
and ST_GeometryType(location)<> 'ST_Point'
)
and ysi.op_region_id = opr.op_region_id
and dl.data_set_id = opr.data_set_id
Yield_Point has 161,575,599 records
where yield>0 and area> 0 has 161,263,193 records,
where ST_GeometryType(location)<> 'ST_Point' has just 231 records
yield_segment_info has 165,929 records
op_region has 566,212 records
data_location has 394,763
All of these have a high volume of insert/delete's.
The tables have recently been vacuum full'd and the indexes reindexed.
[they are under the management of the autovacuum, but we forced a cleanup on
the chance that things had degraded...]
If I run an explain analyze:
"Nested Loop
(cost=5068203.00..5068230.31 rows=3 width=225308)
(actual time=192571.730..193625.728 rows=236 loops=1)"
"->Nested Loop
(cost=5068203.00..5068219.66 rows=1 width=57329)
(actual time=192522.573..192786.698 rows=230 loops=1)"
" ->Nested Loop
(cost=5068203.00..5068211.36 rows=1 width=57268)
(actual time=192509.822..192638.446 rows=230 loops=1)"
" ->HashAggregate
(cost=5068203.00..5068203.01 rows=1 width=4)
(actual time=192471.507..192471.682 rows=230 loops=1)"
" ->Seq Scan on yield_point
(cost=0.00..5068203.00 rows=1 width=4)
(actual time=602.174..192471.177 rows=230 loops=1)"
" Filter: ((yield> 0::double precision) AND
(area> 0::double precision) AND
(st_geometrytype(location)<> 'ST_Point'::text))"
" ->Index Scan using op_region_pkey on op_region opr
(cost=0.00..8.33 rows=1 width=57264)
(actual time=0.723..0.723 rows=1 loops=230)"
" Index Cond: (opr.op_region_id = yield_point.op_region_id)"
" ->Index Scan using yield_segment_info_key on yield_segment_info ysi
(cost=0.00..8.29 rows=1 width=65)
(actual time=0.643..0.643 rows=1 loops=230)"
" Index Cond: (ysi.op_region_id = opr.op_region_id)"
"->Index Scan using data_location_data_set_idx on data_location dl
(cost=0.00..10.61 rows=3 width=167979)
(actual time=3.611..3.646 rows=1 loops=230)"
"Index Cond: (dl.data_set_id = opr.data_set_id)"
"Total runtime: 193625.955 ms"
yield_point has the following indexes:
btree on ST_GeometryType(location)
gist on location
btree on op_region_id
I've also tried an index on
((yield> 0::double precision) AND (area> 0::double precision) AND
(st_geometrytype(location)<> 'ST_Point'::text))
... it still goes for the sequential scan.
But if I change it to st_geometrytype(location) = 'ST_Polygon' or
even in ('ST_Polygon','ST_MultiPolygon')
the planner uses the index.
Roxanne
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