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 error. 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/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
