Re: Parser extensions (maybe for 10?)

On Tue, Apr 19, 2016 at 1:49 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> On 12 April 2016 at 06:51, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> Craig Ringer <craig(at)2ndquadrant(dot)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
> error.
>
>
that's interesting. I'd add parse_error_handler, which actually processes
syntax error.

SET client_language_path = 'foo, postgresql, parse_error_handler'

> 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/
> <http://www.2ndquadrant.com/>
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