Re: Performance issues

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(at)decina(dot)attlocal=# create table a(a_id serial primary key);
CREATE TABLE
decibel(at)decina(dot)attlocal=# create table b(a_id int);
CREATE TABLE
decibel(at)decina(dot)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(at)decina(dot)attlocal=# alter table b add primary key(a_id);
ALTER TABLE
decibel(at)decina(dot)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(at)decina(dot)attlocal=# alter table a drop constraint a_pkey;
ALTER TABLE
decibel(at)decina(dot)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