2 left joins causes seqscan

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От Willy-Bas Loos
Тема 2 left joins causes seqscan
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Msg-id CAHnozTi3jjaYKk5+u6uYm7w_QTbPWhEHdYEU2=iobkTkQM7JyQ@mail.gmail.com
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Ответы Re: 2 left joins causes seqscan  (Kevin Grittner <kgrittn@ymail.com>)
Список pgsql-general
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Hi,

Today i ran into a situation where a second left join on an indexed field would prevent the index from being used, even though the index is clearly more efficient.
Removing either of the 2 joins would cause that the planner will use the index again.
I tested this on postgres 9.1 and 9.3 on my ubuntu (amd64) laptop.

--Here's the test data:

create table a (id serial primary key, field1 text);
create table b (id integer, title text, lang integer);
create index b_title_lowerto on b using btree (lower(title) text_pattern_ops);
vacuum analyze;

with x as (
insert into a
select generate_series(1,40000) as id
returning id
)
insert into b
select id, translate((random()*100*id)::text, '1234567890.', 'abcdefghij'), 1
from x;
update a set field1=translate(id::text, '1234567890.', 'abcdefghij');
insert into b
select b2.id, translate((random()*100*b2.id)::text, '1234567890.', 'abcdefghij'), 2
from b b2;

--Here's the query that doesn't use the index on "b":

select a.field1, b1.title , b2.title
from a
left join b b1 on b1.id = a.id and b1.lang=1
left join b b2 on b2.id = a.id and b2.lang=2
where (lower(b1.title) like'abcd%' or lower(b2.title) like 'abcd%')

--plan:
Hash Right Join  (cost=4298.60..7214.76 rows=8 width=35)
  Hash Cond: (b1.id = a.id)
  Filter: ((lower(b1.title) ~~ 'abcd%'::text) OR (lower(b2.title) ~~ 'abcd%'::text))
  ->  Seq Scan on b b1  (cost=0.00..1510.00 rows=40176 width=19)
        Filter: (lang = 1)
  ->  Hash  (cost=3798.60..3798.60 rows=40000 width=24)
        ->  Hash Right Join  (cost=1293.00..3798.60 rows=40000 width=24)
              Hash Cond: (b2.id = a.id)
              ->  Seq Scan on b b2  (cost=0.00..1510.00 rows=39824 width=19)
                    Filter: (lang = 2)
              ->  Hash  (cost=793.00..793.00 rows=40000 width=9)
                    ->  Seq Scan on a  (cost=0.00..793.00 rows=40000 width=9)



--Here's the query that does use the index on "b":

select a.field1, b1.title
from a
left join b b1 on b1.id = a.id and b1.lang=1
where lower(b1.title) like 'abcd%'
union
select a.field1, b2.title
from a
left join b b2 on b2.id = a.id and b2.lang=2
where lower(b2.title) like 'abcd%'

--plan:
HashAggregate  (cost=98.31..98.39 rows=8 width=20)
  ->  Append  (cost=4.74..98.27 rows=8 width=20)
        ->  Nested Loop  (cost=4.74..49.10 rows=4 width=20)
              ->  Bitmap Heap Scan on b b1  (cost=4.45..15.82 rows=4 width=19)
                    Filter: ((lang = 1) AND (lower(title) ~~ 'abcd%'::text))
                    ->  Bitmap Index Scan on b_title_lowerto  (cost=0.00..4.45 rows=3 width=0)
                          Index Cond: ((lower(title) ~>=~ 'abcd'::text) AND (lower(title) ~<~ 'abce'::text))
              ->  Index Scan using a_pkey on a  (cost=0.29..8.31 rows=1 width=9)
                    Index Cond: (id = b1.id)
        ->  Nested Loop  (cost=4.74..49.10 rows=4 width=20)
              ->  Bitmap Heap Scan on b b2  (cost=4.45..15.82 rows=4 width=19)
                    Filter: ((lang = 2) AND (lower(title) ~~ 'abcd%'::text))
                    ->  Bitmap Index Scan on b_title_lowerto  (cost=0.00..4.45 rows=3 width=0)
                          Index Cond: ((lower(title) ~>=~ 'abcd'::text) AND (lower(title) ~<~ 'abce'::text))
              ->  Index Scan using a_pkey on a a_1  (cost=0.29..8.31 rows=1 width=9)
                    Index Cond: (id = b2.id)


As you can see, the second query is far more efficient, even though it scans both tables twice to combine the results.
Is this some glitch in the query planner?

Cheers,
--
Willy-Bas Loos

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