On 3/16/15 3:59 PM, Tomas Vondra wrote:
On 16.3.2015 20:43, Jim Nasby wrote:
On 3/13/15 7:12 PM, Tomas Vondra wrote:
(4) I suspect many of the relations referenced in the views are not
actually needed in the query, i.e. the join is performed but
then it's just discarded because those columns are not used.
Try to simplify the views as much has possible - remove all the
tables that are not really necessary to run the query. If two
queries need different tables, maybe defining two views is
a better approach.
A better alternative with multi-purpose views is to use an outer
join instead of an inner join. With an outer join if you ultimately
don't refer to any of the columns in a particular table Postgres will
remove the table from the query completely.
Really? Because a quick test suggests otherwise:
db=# create table test_a (id int);
CREATE TABLE
db=# create table test_b (id int);
CREATE TABLE
db=# explain select test_a.* from test_a left join test_b using (id);
QUERY PLAN
----------------------------------------------------------------------
Merge Left Join (cost=359.57..860.00 rows=32512 width=4)
Merge Cond: (test_a.id = test_b.id)
-> Sort (cost=179.78..186.16 rows=2550 width=4)
Sort Key: test_a.id
-> Seq Scan on test_a (cost=0.00..35.50 rows=2550 width=4)
-> Sort (cost=179.78..186.16 rows=2550 width=4)
Sort Key: test_b.id
-> Seq Scan on test_b (cost=0.00..35.50 rows=2550 width=4)
(8 rows)
Also, how would that work with duplicate rows in the referenced table?
Right, I neglected to mention that the omitted table must also be unique
on the join key:
decibel@decina.attlocal=# create table a(a_id serial primary key);
CREATE TABLE
decibel@decina.attlocal=# create table b(a_id int);
CREATE TABLE
decibel@decina.attlocal=# explain analyze select a.* from a left join b
using(a_id);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
Hash Right Join (cost=67.38..137.94 rows=2550 width=4) (actual
time=0.035..0.035 rows=0 loops=1)
Hash Cond: (b.a_id = a.a_id)
-> Seq Scan on b (cost=0.00..35.50 rows=2550 width=4) (never executed)
-> Hash (cost=35.50..35.50 rows=2550 width=4) (actual
time=0.002..0.002 rows=0 loops=1)
Buckets: 4096 Batches: 1 Memory Usage: 32kB
-> Seq Scan on a (cost=0.00..35.50 rows=2550 width=4)
(actual time=0.001..0.001 rows=0 loops=1)
Planning time: 0.380 ms
Execution time: 0.086 ms
(8 rows)
decibel@decina.attlocal=# alter table b add primary key(a_id);
ALTER TABLE
decibel@decina.attlocal=# explain analyze select a.* from a left join b
using(a_id);
QUERY PLAN
-----------------------------------------------------------------------------------------------
Seq Scan on a (cost=0.00..35.50 rows=2550 width=4) (actual
time=0.001..0.001 rows=0 loops=1)
Planning time: 0.247 ms
Execution time: 0.029 ms
(3 rows)
decibel@decina.attlocal=# alter table a drop constraint a_pkey;
ALTER TABLE
decibel@decina.attlocal=# explain analyze select a.* from a left join b
using(a_id);
QUERY PLAN
-----------------------------------------------------------------------------------------------
Seq Scan on a (cost=0.00..35.50 rows=2550 width=4) (actual
time=0.001..0.001 rows=0 loops=1)
Planning time: 0.098 ms
Execution time: 0.011 ms
(3 rows)
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
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