On 12 April 2016 at 06:51, Tom Lane <t...@sss.pgh.pa.us> wrote:

> Craig Ringer <cr...@2ndquadrant.com> writes:
> > The other area where there's room for extension without throwing out the
> > whole thing and rebuilding is handling of new top-level statements. We
> can
> > probably dispatch the statement text to a sub-parser provided by an
> > extension that registers interest in that statement name when we attempt
> to
> > parse it and fail. Even then I'm pretty sure it won't be possible to do
> so
> > while still allowing multi-statements. I wish we didn't support
> > multi-statements, but we're fairly stuck with them.
> Well, as I said, I've been there and done that.  Things get sticky
> when you notice that those "new top-level statements" would like to
> contain sub-clauses (e.g. arithmetic expressions) that should be defined
> by the core grammar.  And maybe the extension would also like to
> define additions to the expression grammar, requiring a recursive
> callback into the extension.  It gets very messy very fast.

As Tom says, we can't easily break it down into multiple co-operating
pieces, so lets forget that as unworkable.

What is possible is a whole new grammar... for example if we imagine

 SET client_language_path = 'foo, postgresql'

Works similar to search_path, but not userset. We try to parse incoming
statements against the foo parser first, if that fails we try postgresql.
The default setting would be simply 'postgresql', so no match -> syntax

We could make that easier by making the postgresql parser a plugin itself.
So to produce a new one you just copy the files, modify them as needed then
insert a new record into pg_language as an extension.

Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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