Re: [PERFORM] Query with no result set, really really slow addingORBDER BY / LIMIT clause - Mailing list pgsql-performance

2017-04-20 9:19 GMT+02:00 Marco Renzi <renzi.mrc@gmail.com>:

Hi!, i've currently a big problem using  ORBDER BY / LIMIT in a query with no result set.
If i add the order by/limit clause it runs really really slow.

QUERY 1 FAST:
--------------------------------

SELECT  fase.id
FROM            tipofase
JOIN       fase
ON         (fase.tipofase = tipofase.id)
WHERE   tipofase.agendafrontoffice = true

EXPLAIN ANALYZE:

Nested Loop  (cost=0.43..790.19 rows=14462 width=4) (actual time=0.079..0.079 rows=0 loops=1)

  ->  Seq Scan on tipofase  (cost=0.00..3.02 rows=1 width=4) (actual time=0.077..0.077 rows=0 loops=1)
        Filter: agendafrontoffice
        Rows Removed by Filter: 102
  ->  Index Only Scan using fase_test_prova_4 on fase  (cost=0.43..595.59 rows=19158 width=8) (never executed)
        Index Cond: (tipofase = tipofase.id)
        Heap Fetches: 0
Planning time: 0.669 ms
Execution time: 0.141 ms

---

It's perfect because it starts from tipofase, where there are no agendafrontoffice = true

fase_test_prova_4 is a btree index ON (fase.tipofase, fase.id)
fase.id is PRIMARY key on fase, 
tipofase.id is PRIMARY key on tipofase, 
fase.tipofase is FK on tipofase.id
and tipofase.agendafrontoffice is a boolean. 

I've also created a btree index on tipofase.agendafrontoffice.

**fase** is a large table with 1.475.146 records. There are no rows in the table matching tipofase.agendafrontoffice = true, so the result set is empty(QUERY 1) 

QUERY 2 SLOW(WITH limit and order by):
--------------------------------


SELECT  fase.id
FROM            tipofase
JOIN       fase
ON         (fase.tipofase = tipofase.id)
WHERE   tipofase.agendafrontoffice = true
ORDER BY        fase.id DESC limit 10 offset 0

Limit  (cost=0.43..149.66 rows=10 width=4) (actual time=173853.131..173853.131 rows=0 loops=1)
  ->  Nested Loop  (cost=0.43..215814.25 rows=14462 width=4) (actual time=173853.130..173853.130 rows=0 loops=1)
        Join Filter: (fase.tipofase = tipofase.id)
        ->  Index Scan Backward using test_prova_2 on fase  (cost=0.43..193684.04 rows=1475146 width=8) (actual time=1.336..173128.418 rows=1475146 loops=1)
        ->  Materialize  (cost=0.00..3.02 rows=1 width=4) (actual time=0.000..0.000 rows=0 loops=1475146)
              ->  Seq Scan on tipofase  (cost=0.00..3.02 rows=1 width=4) (actual time=0.000..0.000 rows=0 loops=1)
                    Filter: agendafrontoffice
                    Rows Removed by Filter: 102
Planning time: 0.685 ms
Execution time: 173853.221 ms

I am afraid so is not possible to solve this issue by one query. In this case the planner expects early stop due finding few values. But because there are not any value, the LIMIT clause has not any benefit in executor time, but the planner is messed. Maybe try to increase LIMIT to some higher value .. 1000, 10000 so planner don't fall to this trap. PostgreSQL statistics are about most common values, but the values without any occurrence are not well registered by statistics.

Regards

Pavel

 

Really really slow..... looks like the planner is not doing a good job.
PostgreSQL 9.4.1, compiled by Visual C++ build 1800, 64-bit


I also run VACUUM  AND VACUUM ANALYZE on both table
I tried to play with the 
"alter table tipofase alter column agendafrontoffice set statistics 2"
but nothing.

Thanks in advance Marco

--

-------------------------------------------------------------------------------------------------------------------------------------------
Ing. Marco Renzi
OCA - Oracle Certified Associate Java SE7 Programmer
OCP - Oracle Certified Mysql 5 Developer

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