Re: How to optimize WHERE column_a IS NOT NULL OR column_b = 'value'

På mandag 11. august 2014 kl. 21:34:57, skrev Pavel Stehule <pavel.stehule@gmail.com>:

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;

 

I see turning enable_seqscan to off results in BitmapOr in 9.3:

 

loff=# select version();
                                                   version                                                   
-------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.3.2 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.8.1-10ubuntu9) 4.8.1, 64-bit
(1 row)

loff=# show enable_seqscan ;
 enable_seqscan
----------------
 off
(1 row)

loff=# explain analyze  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=103.08..141.89 rows=1095 width=76) (actual time=0.052..0.057 rows=3 loops=1)

   Hash Cond: (doc.folder_id = f.id)

   ->  Bitmap Heap Scan on document doc  (cost=21.10..44.85 rows=1095 width=44) (actual time=0.027..0.028 rows=3 loops=1)
         Recheck Cond: ((folder_id IS NOT NULL) OR (owner_id = 1))
         ->  BitmapOr  (cost=21.10..21.10 rows=1100 width=0) (actual time=0.018..0.018 rows=0 loops=1)
               ->  Bitmap Index Scan on document_folder_idx  (cost=0.00..16.36 rows=1094 width=0) (actual time=0.012..0.012 rows=2 loops=1)
                     Index Cond: (folder_id IS NOT NULL)
               ->  Bitmap Index Scan on document_owner_idx  (cost=0.00..4.20 rows=6 width=0) (actual time=0.005..0.005 rows=2 loops=1)
                     Index Cond: (owner_id = 1)
   ->  Hash  (cost=66.60..66.60 rows=1230 width=36) (actual time=0.011..0.011 rows=2 loops=1)
         Buckets: 1024  Batches: 1  Memory Usage: 1kB
         ->  Index Scan using folder_pkey on folder f  (cost=0.15..66.60 rows=1230 width=36) (actual time=0.005..0.006 rows=2 loops=1)
 Total runtime: 0.125 ms
(13 rows)

 

 

 

In 9.4-beta2 it results in an index-scan with a filter:

 

andreak=# select version();
                                                 version
---------------------------------------------------------------------------------------------------------
 PostgreSQL 9.4beta2 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2, 64-bit
(1 row)

andreak=# show enable_seqscan ;
 enable_seqscan
----------------
 off
(1 row)

andreak=# explain analyze 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=0.28..18.92 rows=4 width=76) (actual time=0.032..0.044 rows=3 loops=1)
   ->  Index Scan using document_folder_idx on document doc  (cost=0.13..6.20 rows=4 width=44) (actual time=0.018..0.024 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=0.003..0.004 rows=1 loops=3)

         Index Cond: (doc.folder_id = id)

 Planning time: 0.260 ms

 Execution time: 0.094 ms
(8 rows)

 

 

I have quite large dataset (and some additional joins) in my prod-data and hoped that I could solve this with one index-scan on an index on document-table to avoid 2 index-scans and OR-ing the results.

 

Thanks for help!

 

--

Andreas Joseph Krogh

CTO / Partner - Visena AS

Mobile: +47 909 56 963

andreas@visena.com

www.visena.com