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Thread: Removing redundant check for transaction in progress in check_safe_enum_use

Removing redundant check for transaction in progress in check_safe_enum_use

From
Zhihong Yu
Date:
Hi,
I was looking at :
Relax transactional restrictions on ALTER TYPE ... ADD VALUE (redux).

In check_safe_enum_use():

+   if (!TransactionIdIsInProgress(xmin) &&
+       TransactionIdDidCommit(xmin))
+       return;

Since the condition would be true only when TransactionIdDidCommit() returns true, I think the call to TransactionIdIsInProgress is not needed.
If transaction for xmin is committed, the transaction cannot be in progress at the same time.

Please see the simple patch for removing the redundant check.

Thanks

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Re: Removing redundant check for transaction in progress in check_safe_enum_use

From
Matthias van de Meent
Date:
On Sun, 4 Jul 2021, 03:40 Zhihong Yu, <zyu@yugabyte.com> wrote:
>
> Hi,
> I was looking at :
> Relax transactional restrictions on ALTER TYPE ... ADD VALUE (redux).
>
> In check_safe_enum_use():
>
> +   if (!TransactionIdIsInProgress(xmin) &&
> +       TransactionIdDidCommit(xmin))
> +       return;
>
> Since the condition would be true only when TransactionIdDidCommit() returns true, I think the call to
TransactionIdIsInProgressis not needed.
 
> If transaction for xmin is committed, the transaction cannot be in progress at the same time.

I'm not sure that removing the !TransactionIdIsInProgress-check is
correct. The comment in heapam_visibility.c:13 explains that we need
to check TransactionIdIsInProgress before TransactionIdDidCommit in
non-MVCC snapshots, and I'm fairly certain that check_safe_enum_use()
is not guaranteed to run only in MVCC snapshots (at least its
documentation does not warn against non-MVCC snapshots).

Kind regards,

Matthias van de Meent