On 2015-11-03 09:52:34 +0100, Simon Riggs wrote:
> On 3 November 2015 at 09:35, Andres Freund <and...@anarazel.de> wrote:
> > > With this commit - bde39eed0cafb82bc94c40e95d96b5cf47b6f719, it is not
> > possible
> > > to execute Notify commands inside a parallel worker. Can't we change
> > > it as disable both listen and notify commands inside a background worker?
> >
> > Well, parallel workers are something different from general background
> > workers. I don't see why it'd make sense to allow listen/notify there,
> > given the rest of the restrictions?
> >
> What are the restrictions and/or where are they documented? Thanks

There's a section in README.parallel:
> Instead, we take a more pragmatic approach. First, we try to make as many of
> the operations that are safe outside of parallel mode work correctly in
> parallel mode as well.  Second, we try to prohibit common unsafe operations
> via suitable error checks.  These checks are intended to catch 100% of
> unsafe things that a user might do from the SQL interface, but code written
> in C can do unsafe things that won't trigger these checks.  The error checks
> are engaged via EnterParallelMode(), which should be called before creating
> a parallel context, and disarmed via ExitParallelMode(), which should be
> called after all parallel contexts have been destroyed.  The most
> significant restriction imposed by parallel mode is that all operations must
> be strictly read-only; we allow no writes to the database and no DDL.  We
> might try to relax these restrictions in the future.

Basically the restriction is that, for now, while a parallelized
statement is in progress you can't write data and no DDL is possible.


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