Re: SPI and transactions

On Wed, Nov 18, 2015 at 5:18 AM, Konstantin Knizhnik
<k(dot)knizhnik(at)postgrespro(dot)ru> wrote:
> Hello,
>
> SPI was originally developed for execution SQL statements from C user
> defined functions in context of existed transaction.
> This is why it is not possible to execute any transaction manipulation
> statement (BEGIN, COMMIT, PREPARE,...) using
> SPI_execute:SPI_ERROR_TRANSACTION is returned.
>
> But now SPI is used not only inside UDFs. It is also used in background
> workers. For example in receiver_raw, written by Michael Paquier (I lot of
> thanks Michael, understand logical replication without them will be much
> more difficult).
> Right now transactions have to be started by background worker using
> StartTransactionCommand().
> So receiver_raw is not able to preserve master's transaction semantic
> (certainly it can be implemented).
>
> I wonder whether SPI can be extended now to support transaction manipulation
> functions when been called outside transaction context? Or there are some
> principle problem with it?

I think SPI pretty fundamentally assumes we're inside a transaction,
and that we'll still be at the same transaction nesting depth when we
get done with SPI. For example, SPI_connect() allocates memory in
TopTransactionContext. So I doubt that it will work out well to try
to solve the problem you're aiming to fix in this particular way.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company