Re: Postgresql 10.3 , query never completes if LIMIT clause isspecified and paralle processing is on - Mailing list pgsql-general

On 03/21/2018 05:09 PM, Alessandro Aste wrote:
> Hi there, we are using postgresql 10.3 and we're facing an issue with a
> query. The query (full query below)  completes only  when: 
> 
> 1 - LIMIT 10 is removed
> or
> 2 -  show max_parallel_workers_per_gather  is set to 0, so parallel
> processing is disabled.
> 
> With  max_parallel_workers_per_gather   set to the default value (8) I'm
> not even able to get the query plan.
> 
> Notes: 
> 
>   * We're experiencing the issue in any server of ours but I've
>     reproduced the issue in a fresh restored database with full
>     vacuum/reindex of the tables.
>   * We didn't touch any parameter concering the parallel processing,
>     we're running the defaults: 
> 
> 
> cmdstaging=# show max_parallel_workers_per_gather ;
>  max_parallel_workers_per_gather
> ---------------------------------
>  8
> (1 row)
> 
> cmdstaging=# show max_worker_processes ;
>  max_worker_processes
> ----------------------
>  8
> (1 row)
> 
> cmdstaging=# show max_parallel_workers;
>  max_parallel_workers
> ----------------------
>  8
> (1 row)
> 
> 
> 
> 
> The query completes only omitting the LIMIT clause or when I disable
> parallel processing: 
> 
>  id   | vendor_id | gaa |   pop_name    | pop_status | pop_location |
> pop_provider_id | pop_provider |     pop_street      | pop_city | pop
> _postal_code | pop_state | pop_country | pop_country_id
>
--------+-----------+-----+---------------+------------+--------------+-----------------+--------------+---------------------+----------+----
> -------------+-----------+-------------+----------------
>  684807 |     12346 |     | GTT/POP/LON1T | Active     | LON1T        | 
>          12288 | Telehouse UK | 14 Coriander Avenue | London   | E14
>  2AA         |           | GB          |            219
> (1 row)
> 
> Time: 4374.759 ms (00:04.375)
> cmdstaging=# show max_parallel_workers_per_gather ;
>  max_parallel_workers_per_gather
> ---------------------------------
>  0
> (1 row)
> 
> Time: 0.097 ms
> 
> 
> Otherwise it just keep running for forever. 
> 

When you say "running forever" is it actually using CPU, or does it get
stuck on something?

> 
> This is the full query: 
> 
> 
> SELECT * FROM (
> SELECT
> seg.circuit_id AS id,
> vendor_gtt_pop.vendor_id,
> CASE WHEN vendor_gtt_pop.push_to_gaa = 1 THEN 'Y' END as gaa,
> pop.gii_circuitid AS pop_name,
> cst.label AS pop_status,
> seg.a_company_name AS pop_location,
> seg.vendor_id AS pop_provider_id,
> seg.vendor_name AS pop_provider,
> cs.address1 AS pop_street,
> cs.city AS pop_city,
> cs.postal_code AS pop_postal_code,
> cs.state AS pop_state,
> csc.code AS pop_country,
> cs.country_id AS pop_country_id
> FROM (
> SELECT c.gii_circuitid, max(so.id <http://so.id>) AS service_order_id
> FROM service_order so
> join circuit c on c.product_id=so.product_id
> join master_service_order mso on mso.id <http://mso.id>=so.master_service_order_id
> WHERE NOT (so.ordertype_id = 2 AND so.status <> 999) AND
> NOT (so.ordertype_id = 3 AND so.status <> 999) AND
> c.status >= 20 AND
> c.status not in (160,999) AND
> mso.client_id=11615 AND
> c.service_description=28 AND
> c.status!=160
> GROUP BY c.gii_circuitid
> ) pop
> JOIN service_order so ON so.id <http://so.id> = pop.service_order_id
> left JOIN client_site cs on cs.id <http://cs.id>=so.a_site_id
> left JOIN country csc on csc.id <http://csc.id>=cs.country_id
> JOIN circuit c ON so.product_id=c.product_id
> JOIN circuit_status cst ON cst.id <http://cst.id>=c.status
> JOIN (
> SELECT c.id <http://c.id> AS circuit_id, sg.id <http://sg.id> AS segment_id, c.pop_support_vendor_id AS vendor_id,
> v.name <http://v.name> AS vendor_name, sg.a_company_name
> FROM segment sg
> JOIN circuit_layout cl ON cl.segment_id = sg.id <http://sg.id> AND cl.ordinal = 1
> JOIN circuit c ON c.id <http://c.id> = cl.circuit_id
> JOIN vendor v ON v.id <http://v.id> = c.pop_support_vendor_id
> ) seg ON seg.circuit_id = c.id <http://c.id>
> JOIN vendor_gtt_pop on vendor_gtt_pop.gtt_pop_id = seg.circuit_id
> ) foo where vendor_id = 12346 AND (1=1) ORDER BY pop_name ASC,id LIMIT 10
> 
> 
> 
> Execution plan with  max_parallel_workers_per_gather =0 , 
> max_parallel_workers_per_gather =8 and no LIMIT clause : 
> 
> 
We really need to see the execution plan that causes issues, i.e.
max_parallel_workers_per_gather=8 with LIMIT clause. Plain explain
(without analyze), at least.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services