Re: POC: Lock updated tuples in tuple_update() and tuple_delete()

On Fri, 1 Jul 2022 at 16:49, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>
> Hackers,
>
> When working in the read committed transaction isolation mode
> (default), we have the following sequence of actions when
> tuple_update() or tuple_delete() find concurrently updated tuple.
>
> 1. tuple_update()/tuple_delete() returns TM_Updated
> 2. tuple_lock()
> 3. Re-evaluate plan qual (recheck if we still need to update/delete
> and calculate the new tuple for update)
> 4. tuple_update()/tuple_delete() (this time should be successful,
> since we've previously locked the tuple).
>
> I wonder if we should merge steps 1 and 2. We could save some efforts
> already done during tuple_update()/tuple_delete() for locking the
> tuple. In heap table access method, we've to start tuple_lock() with
> the first tuple in the chain, but tuple_update()/tuple_delete()
> already visited it. For undo-based table access methods,
> tuple_update()/tuple_delete() should start from the last version, why
> don't place the tuple lock immediately once a concurrent update is
> detected. I think this patch should have some performance benefits on
> high concurrency.
>
> Also, the patch simplifies code in nodeModifyTable.c getting rid of
> the nested case. I also get rid of extra
> table_tuple_fetch_row_version() in ExecUpdate. Why re-fetch the old
> tuple, when it should be exactly the same tuple we've just locked.
>
> I'm going to check the performance impact. Thoughts and feedback are welcome.

The patch does not apply on top of HEAD as in [1], please post a rebased patch:
=== Applying patches on top of PostgreSQL commit ID
eb5ad4ff05fd382ac98cab60b82f7fd6ce4cfeb8 ===
=== applying patch
./0001-Lock-updated-tuples-in-tuple_update-and-tuple_del-v1.patch
patching file src/backend/executor/nodeModifyTable.c
...
Hunk #3 FAILED at 1376.
...
1 out of 15 hunks FAILED -- saving rejects to file
src/backend/executor/nodeModifyTable.c.rej

[1] - http://cfbot.cputube.org/patch_41_4099.log

Regards,
Vignesh