Re: Slow planing...

[Mladen Marinović <mladen.marinovic@xxxxxxxx>]

After a couple of hours of trying different stuff, set enable_mergejoin  = off made the planning time look better: Planning time: 0.322 ms

Any ideas why this helps?

Regards,

Mladen Marinović

On Fri, Dec 6, 2019 at 11:14 AM Mladen Marinović <mladen.marinovic@xxxxxxxx> wrote:

Hi,

Since this morning our system is running slower than usual. It turns out that some queries take a very long time to plan ( > 1 second). The problem occurs when joining bigger tables. There are no partition for the used tables. The problem has a time correlation with the last autovacuum/autoanalyse this morning, but manual vacuuming and analysing did not fix the problem.

An example explain is:

EXPLAIN ANALYSE
SELECT 1
FROM table_a a
  LEFT JOIN table_b bON b.a_id= a.id
WHERE a.object_id=13
  AND a.timestamp<'2019-12-06'
  AND a.timestamp>'2019-12-03'

Nested Loop Left Join  (cost=1.28..18137.57 rows=6913 width=4) (actual time=0.043..90.016 rows=14850 loops=1)
  ->  Index Scan using uq_object_id_timestamp on table_a a  (cost=0.70..7038.49 rows=6913 width=8) (actual time=0.028..21.832 rows=14850 loops=1)
        Index Cond: ((object_id = 13) AND (timestamp < '2019-12-06'::timestamp with time zone) AND (timestamp > '2019-12-03'::timestamp with time zone))
  ->  Index Only Scan using table_b_a_id on table_b b  (cost=0.57..1.60 rows=1 width=8) (actual time=0.004..0.004 rows=0 loops=14850)
        Index Cond: (a_id = a.id)
        Heap Fetches: 0
Planning time: 1908.550 ms
Execution time: 91.004 ms

The same query on a similar parallel system takes 5ms for planing (PG 9.4.).

Is there a way to detect why the planing is taking this long?

The database is a 9.6.1 with 32GB of shared_buffers, and 1GB of maintanance_work_mem, and machine CPU is below 80% all the time.

Regards,

Mladen Marinović