Re: BUG #17949: Adding an index introduces serialisation anomalies.

Hi Thomas,

thank you for the confirmation and analysis. Did you have a chance to
take a more detailed look at the problem?

Best regards,

Artem.

On 30/05/2023 04:29, Thomas Munro wrote:
> Hi,
>
> Reproduced here. Thanks for the reproducer. I agree that something
> is wrong here, but I haven't had time to figure out what, yet, but let
> me share what I noticed so far... I modified your test to add a pid
> column to the locks table and to insert insert pg_backend_pid() into
> it, and got:
>
> postgres=# select xmin, * from locks;
>
> ┌───────┬──────┬───────┐
> │ xmin │ path │ pid │
> ├───────┼──────┼───────┤
> │ 17634 │ xyz │ 32932 │
> │ 17639 │ xyz │ 32957 │
> └───────┴──────┴───────┘
>
> Then I filtered the logs (having turned the logging up to capture all
> queries) so I could see just those PIDs and saw this sequence:
>
> 2023-05-29 00:15:43.933 EDT [32932] LOG: duration: 0.182 ms
> statement: BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE
> 2023-05-29 00:15:43.934 EDT [32957] LOG: duration: 0.276 ms
> statement: BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE
> 2023-05-29 00:15:43.935 EDT [32932] LOG: duration: 1.563 ms
> statement: SELECT * FROM locks WHERE path = 'xyz'
> 2023-05-29 00:15:43.936 EDT [32932] LOG: duration: 0.126 ms
> statement: INSERT INTO locks(path, pid) VALUES('xyz',
> pg_backend_pid())
> 2023-05-29 00:15:43.937 EDT [32957] LOG: duration: 2.191 ms
> statement: SELECT * FROM locks WHERE path = 'xyz'
> 2023-05-29 00:15:43.937 EDT [32957] LOG: duration: 0.261 ms
> statement: INSERT INTO locks(path, pid) VALUES('xyz',
> pg_backend_pid())
> 2023-05-29 00:15:43.937 EDT [32932] LOG: duration: 0.222 ms statement: COMMIT
> 2023-05-29 00:15:43.939 EDT [32957] LOG: duration: 1.775 ms statement: COMMIT
>
> That sequence if run (without overlap) in the logged order is normally
> rejected. The query plan being used (at least when I run the query
> myself) looks like this:
>
> Query Text: SELECT * FROM locks WHERE path = 'xyz'
> Bitmap Heap Scan on locks (cost=4.20..13.67 rows=6 width=36)
> Recheck Cond: (path = 'xyz'::text)
> -> Bitmap Index Scan on locks_path_idx (cost=0.00..4.20 rows=6 width=0)
> Index Cond: (path = 'xyz'::text)