Trying to the behavior of a parallel query with with a change in the transaction isolation mode

[Luis Carril]

Hi all,



 I am trying to verify if during a transaction a parallel plan ca be 
generated but later only a sequential one is executed.

 The documentation says the following (in 
https://www.postgresql.org/docs/9.6/static/when-can-parallel-query-be-used.html):

---
Even when parallel query plan is generated for a particular query, there are 
several circumstances under which it will be impossible to execute that plan in 
parallel at execution time. If this occurs, the leader will execute the portion 
of the plan below the Gather node entirely by itself, almost as if the Gather 
node were not present. This will happen if any of the following conditions are 
met:
 [...]

The transaction isolation level is serializable. This situation does not 
normally arise, because parallel query plans are not generated when the 
transaction isolation level is serializable. However, it can happen if the 
transaction isolation level is changed to serializable after the plan is 
generated and before it is executed.

---


 I have tried the following:

BEGIN;
SET TRANSACTION ISOLATION LEVEL READ COMMITTED, READ ONLY;
EXPLAIN (COSTS OFF) SELECT avg(a) FROM parallel_big;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE, READ ONLY;
EXPLAIN ANALYZE SELECT avg(a) FROM parallel_big;
COMMIT;


 But complains that after the first SELECT (even if it is in an EXPLAIN) 
the isolation level cannot be changed, so the transaction is aborted  and the 
SELECT is never executed (even sequentially).


 Is there any way to test the possible behavior described in the 
documentation?


Thanks you very much

Luis M Carril

Re: Trying to the behavior of a parallel query with with a change in the transaction isolation mode

[Luis Carril]

Thanks for the answer it worked, the third analyze in the sequence below show 
multiple workers planned and none launched.


PREPARE st AS SELECT avg(a) FROM parallel_big;
EXPLAIN ANALYZE EXECUTE st;

BEGIN;
SET TRANSACTION ISOLATION LEVEL READ COMMITTED, READ ONLY;
EXPLAIN ANALYZE EXECUTE st;
COMMIT;

BEGIN;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE, READ ONLY;
EXPLAIN ANALYZE EXECUTE st;
COMMIT;
DEALLOCATE st;

Cheers,
Luis M




From: Alvaro Herrera 
Sent: Monday, February 12, 2018 4:19:52 PM
To: Luis Carril
Cc: pgsql-gene...@postgresql.org
Subject: Re: Trying to the behavior of a parallel query with with a change in 
the transaction isolation mode

Luis Carril wrote:

> The transaction isolation level is serializable. This situation does not 
> normally arise, because parallel query plans are not generated when the 
> transaction isolation level is serializable. However, it can happen if the 
> transaction isolation level is changed to serializable after the plan is 
> generated and before it is executed.

> BEGIN;
> SET TRANSACTION ISOLATION LEVEL READ COMMITTED, READ ONLY;
> EXPLAIN (COSTS OFF) SELECT avg(a) FROM parallel_big;
> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE, READ ONLY;
> EXPLAIN ANALYZE SELECT avg(a) FROM parallel_big;
> COMMIT;
>
>
>  But complains that after the first SELECT (even if it is in an EXPLAIN) 
> the isolation level cannot be changed, so the transaction is aborted  and the 
> SELECT is never executed (even sequentially).
>
>
>  Is there any way to test the possible behavior described in the 
> documentation?

I think you would do a PREPARE in a regular transaction, then open a
transaction changing the isolation level to serializable and try the
EXPLAIN EXECUTE there.

--
Álvaro Herrerahttps://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services