Re: BUG #17812: LOCK TABLE IN ACCESS EXCLUSIVE MODE with a view returns an empty tuple set

Hi,

On Tue, Feb 28, 2023 at 03:12:38AM +0000, PG Bug reporting form wrote:
> The following bug has been logged on the website:
>
> Bug reference:      17812
> Logged by:          Zhenglong Li
> Email address:      smartkeyerror@gmail.com
> PostgreSQL version: 15.2
> Operating system:   Ubuntu 20.04
> Description:
>
> Step to reproduce the behavior
>
> We need 2 sessions to reproduce this bug.
>
> Firstly, we create a simple view that just fetches all the data from
> table1.
>
> And then we start a transaction and lock table1 with AccessExclusive Mode in
> Read Committed Transaction Isolation Level.
>
> After that, we try to use CTAS to create a temp table table2 using the data
> from table1 in session2, and this will be blocked by AccessExclusive Lock.
>
> Finally, we insert some data into table1 in session1 and commit it, session2
> will continue, but there is no data in table2.
>
> ```sql
> [...]
> TRUNCATE TABLE table1;
> [...]

This is not a bug, this is a documented corner case.  In postgres TRUNCATE is
not fully MVCC, see
https://www.postgresql.org/docs/current/sql-truncate.html and
https://www.postgresql.org/docs/current/mvcc-caveats.html:

"TRUNCATE is not MVCC-safe. After truncation, the table will appear empty to
concurrent transactions, if they are using a snapshot taken before the
truncation occurred".

I guess the difference between referencing the table rather than the view is
that the query get stuck at execution time rather than planning time, meaning
that you do get a snapshot older than the INSERT in the session 1.  Change the
TRUNCATE with a DELETE and you will get the same behavior for both cases.