On Tue, Apr 19, 2016 at 1:49 PM, Simon Riggs <si...@2ndquadrant.com> wrote:
> 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. > > 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/> > PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services >
