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
