On 12 April 2016 at 13:51, 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. >
Yuck. You'd ping-pong between two parsers, and have to try to exchange sensible starting states. Point taken. So even that seemingly not-that-bad restricted option turns out to be far from it, which just goes to show what a pit of snakes parser extensibility is... -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
