Re: How to optimize WHERE column_a IS NOT NULL OR column_b = 'value' - Mailing list pgsql-sql

Hi

 

2014-08-11 18:01 GMT+02:00 Andreas Joseph Krogh <andreas@visena.com>:

Hi folks,

 

I have the following schema (simplified for this example).

 

create table folder(id integer primary key, name varchar not null);

 

create table document(id serial primary key, name varchar not null, owner_id integer not null, folder_id integer references folder(id));

create index document_owner_idx ON document(owner_id);

create index document_folder_idx ON document(folder_id);

 

insert into folder(id, name) values(1, 'Folder A');

insert into folder(id, name) values(2, 'Folder B');

insert into document(name, owner_id, folder_id) values('Document A',  1, 1);

insert into document(name, owner_id, folder_id) values('Document B',  1, NULL);

insert into document(name, owner_id, folder_id) values('Document C',  2, 2);

insert into document(name, owner_id, folder_id) values('Document D',  2, NULL);

 

select f.id, f.name, doc.id, doc.owner_id, doc.name

FROM document doc left outer join folder f ON doc.folder_id = f.id

WHERE doc.folder_id is not null OR doc.owner_id = 1;

 

                                                         QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
 Nested Loop Left Join  (cost=0.15..13.77 rows=4 width=76) (actual time=0.031..0.045 rows=3 loops=1)
   ->  Seq Scan on document doc  (cost=0.00..1.05 rows=4 width=44) (actual time=0.012..0.018 rows=3 loops=1)
         Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
         Rows Removed by Filter: 1
   ->  Index Scan using folder_pkey on folder f  (cost=0.15..3.17 rows=1 width=36) (actual time=0.005..0.006 rows=1 loops=3)
         Index Cond: (doc.folder_id = id)
 Planning time: 0.267 ms
 Execution time: 0.094 ms
(8 rows)

 

 

Is the a way to write a query which uses an index efficiently for such a schema?

 

I'd like to eliminate the Filter: ((folder_id IS NOT NULL) OR (owner_id = 1)) and rather have "index cond" insted, is that possible?

 

your example is partially broken - ANALYZE and hashjoin and seqscan penalization are missing - index scan is not used due too small table sizes
 

I tested 9.5, probably same as 9.4 and there indexes are used

postgres=# set enable_hashjoin to off;
SET
Time: 0.473 ms
postgres=# set enable_seqscan to off;
SET
Time: 0.904 ms
postgres=# explain select f.id, f.name, doc.id, doc.owner_id, doc.name
FROM document doc left outer join folder f ON doc.folder_id = f.id
WHERE doc.folder_id is not null OR doc.owner_id = 1;
                                           QUERY PLAN                                          
────────────────────────────────────────────────────────────────────────────────────────────────
 Merge Left Join  (cost=0.26..24.38 rows=3 width=32)
   Merge Cond: (doc.folder_id = f.id)
   ->  Index Scan using document_folder_idx on document doc  (cost=0.13..12.20 rows=3 width=23)
         Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
   ->  Index Scan using folder_pkey on folder f  (cost=0.13..12.16 rows=2 width=13)
 Planning time: 0.663 ms
(6 rows)
 

default 9.2, 9.3, ...

postgres=# explain select f.id, f.name, doc.id, doc.owner_id, doc.name
FROM document doc left outer join folder f ON doc.folder_id = f.id
WHERE doc.folder_id is not null OR doc.owner_id = 1;
                             QUERY PLAN                             
─────────────────────────────────────────────────────────────────────
 Hash Left Join  (cost=1.04..2.12 rows=3 width=32)
   Hash Cond: (doc.folder_id = f.id)
   ->  Seq Scan on document doc  (cost=0.00..1.05 rows=3 width=23)
         Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
   ->  Hash  (cost=1.02..1.02 rows=2 width=13)
         ->  Seq Scan on folder f  (cost=0.00..1.02 rows=2 width=13)
(6 rows)
 

and 9.2 after hashjoin and indexscan penalization

postgres=# explain select f.id, f.name, doc.id, doc.owner_id, doc.name
FROM document doc left outer join folder f ON doc.folder_id = f.id
WHERE doc.folder_id is not null OR doc.owner_id = 1;
                                           QUERY PLAN                                          
────────────────────────────────────────────────────────────────────────────────────────────────
 Merge Left Join  (cost=0.00..24.62 rows=3 width=32)
   Merge Cond: (doc.folder_id = f.id)
   ->  Index Scan using document_folder_idx on document doc  (cost=0.00..12.32 rows=3 width=23)
         Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
   ->  Index Scan using folder_pkey on folder f  (cost=0.00..12.28 rows=2 width=13)
(5 rows)

Time: 2.258 ms
 

What is your PostgreSQL?
 

Regards
 

Pavel
 

P.S. ten years ago I had a similar issue - "OR" predikates can be replaced by UNION
 

you can try:
 

SELECT * FROM

(SELECT * FROM doc
  WHERE folder_id IS NOT NULL

UNION

  SELECT * FROM doc

  WHERE owner_id = 1) s

  LEFT JOIN folder ON s.folder_id = folder.id
 

or some similar magic

 

select f.id, f.name, doc.id, doc.owner_id, doc.name

FROM document doc left outer join folder f ON doc.folder_id = f.id

WHERE doc.folder_id is not null OR doc.owner_id = 1;

--

Andreas Joseph Krogh

CTO / Partner - Visena AS

Mobile: +47 909 56 963

andreas@visena.com

www.visena.com