Re: [HACKERS] SERIALIZABLE with parallel query - Mailing list pgsql-hackers

On Tue, Sep 19, 2017 at 1:47 PM, Haribabu Kommi
<kommi.haribabu@gmail.com> wrote:
> 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.

Hi Haribabu,

Thanks for looking at this!

Yeah.  The difference seems to be that session 2 chooses a Parallel
Seq Scan instead of an Index Scan when you flip all those GUCs into
parallelism-is-free mode.  Seq Scan takes a table-level predicate lock
(see heap_beginscan_internal()).  But if you continue your example in
non-parallel mode (patched or unpatched), you'll find that only one of
those transactions can commit successfully.

Using the fancy notation in the papers about this stuff where w1[x=42]
means "write by transaction 1 on object x with value 42", let's see if
there is an apparent sequential order of these transactions that makes
sense:

Actual order: r1[Y=0] r2[X=0] w1[X=10] w2[Y=10] ... some commit order ...
Apparent order A: r2[X=0] w2[Y=10] c2 r1[Y=0*] w1[X=10] c1 (*nonsense)
Apparent order B: r1[Y=0] w1[X=10] c1 r2[X=0*] w2[Y=10] c2 (*nonsense)

Both potential commit orders are nonsensical.  I think what happened
in your example was that a Seq Scan allowed the SSI algorithm to
reject a transaction sooner.  Instead of r2[X=0], the executor sees
r2[X=0,Y=0] (we scanned the whole table, as if we read all objects, in
this case X and Y, even though we only asked to read X).  Then the SSI
algorithm is able to detect a "dangerous structure" at w2[Y=10],
instead of later at commit time.

So I don't think this indicates a problem with the patch.

-- 
Thomas Munro
http://www.enterprisedb.com


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