Re: LIMIT OFFSET with DB view vs plain SQL

[Raj Gandhi <raj01gandhi@xxxxxxxxx>]

Any other idea how to resolve the performance issue with the database view?

On Fri, Mar 29, 2019 at 7:38 PM Raj Gandhi <raj01gandhi@xxxxxxxxx> wrote:

Merlin,  I tried the hack you suggested but that didn't work. Planner used the same path.

The same query works much faster when using the raw SQL instead of  DB view:

Here is the definition of DB View ‘job’

 SELECT w.id,

    w.parent_id,

    w.status AS state,

    w.percent_complete AS progress_percentage,

    w.start_time,

    w.end_time,

    w.est_completion_time AS estimated_completion_time,

    w.root_id,

    w.internal AS is_internal,

    w.order_id AS step_order,

    c.resource_type,

    c.resource_id,

    c.id AS command_id,

    c.client_cookie,

    c.user_name AS "user",

    c.metadata,

    c.client_address,

    response_body(r.*, w.*) AS response_body

   FROM work_unit w

     LEFT JOIN command c ON c.work_unit_id = w.id

     LEFT JOIN command_response r ON r.command_id::text = c.id::text;

 

 

Query that uses the DB view:

SELECT id, start_time

FROM job

order by id  LIMIT 101 OFFSET 0;

 

Explain plan: https://explain.depesz.com/s/gzjQ

 Query using the raw SQL

<SQL from Job DB View definition>

ORDER BY id LIMIT 101 OFFSET 0;

 

Explain plan:https://explain.depesz.com/s/KgwO

 

On Fri, Mar 29, 2019 at 11:26 AM Merlin Moncure <mmoncure@xxxxxxxxx> wrote:

On Thu, Mar 28, 2019 at 5:44 PM Raj Gandhi <raj01gandhi@xxxxxxxxx> wrote:
>
> + pgsql-performance
>
> On Thu, Mar 28, 2019 at 6:41 PM Raj Gandhi <raj01gandhi@xxxxxxxxx> wrote:
>>
>> Hi everyone,
>>
>>
>>
>> I’m using LIMIT offset with DB view. Looks like query planner is applying the LIMIT for DB view at the end after processing all rows.
>>
>> When running same SQL that was used to create the DB view, LIMIT is applied earlier so the query is much faster.
>>
>>
>>
>> Explain plan using DB view
>>
>> https://explain.depesz.com/s/gzjQ
>>
>>
>>
>> Explain plan using raw SQL
>>
>> https://explain.depesz.com/s/KgwO
>>
>>
>>
>> In both tests LIMIT was 100 with offset  = 0.
>>
>> Is there any way to force DB view to apply limit earlier?

huh. OFFSET does indeed force a materialize plan.   This is a widely
used tactic to hack the planner ('OFFSET 0').

Maybe try converting your query from something like:

SELECT * FROM foo LIMIT m OFFSET N;
to
WITH data AS
(
  SELECT * FROM foo LIMIT m + n
)
SELECT * FROM foo OFFSET n;

I didn't try this, and it may not help, but it's worth a shot.

merlin