Re: SERIALIZABLE with parallel query

On Tue, Sep 19, 2017 at 11:42 AM, Thomas Munro <
thomas(dot)munro(at)enterprisedb(dot)com> wrote:

> On Fri, Sep 1, 2017 at 5:11 PM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> > On Wed, Jun 28, 2017 at 11:21 AM, Thomas Munro
> > <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> >> [ssi-parallel-v5.patch]
> >
> > Rebased.
>
> Rebased again.
>

During testing of this patch, I found some behavior difference
with the support of parallel query, while experimenting with the provided
test case in the patch.

But I tested the V6 patch, and I don't think that this version contains
any fixes other than rebase.

Test steps:

CREATE TABLE bank_account (id TEXT PRIMARY KEY, balance DECIMAL NOT NULL);
INSERT INTO bank_account (id, balance) VALUES ('X', 0), ('Y', 0);

Session -1:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT balance FROM bank_account WHERE id = 'Y';

Session -2:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET max_parallel_workers_per_gather = 2;
SET force_parallel_mode = on;
set parallel_setup_cost = 0;
set parallel_tuple_cost = 0;
set min_parallel_table_scan_size = 0;
set enable_indexscan = off;
set enable_bitmapscan = off;

SELECT balance FROM bank_account WHERE id = 'X';

Session -1:

update bank_account set balance = 10 where id = 'X';

Session -2:

update bank_account set balance = 10 where id = 'Y';
ERROR: could not serialize access due to read/write dependencies among
transactions
DETAIL: Reason code: Canceled on identification as a pivot, during write.
HINT: The transaction might succeed if retried.

Without the parallel query of select statement in session-2,
the update statement in session-2 is passed.

Regards,
Hari Babu
Fujitsu Australia