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