From: Tom Lane <tgl@sss.pgh.pa.us>
To: Craig Ringer <craig@2ndquadrant.com>
cc: Arcadiy Ivanov <arcadiy@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Parser extensions (maybe for 10?)
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References: <570C7B5A.9070309@gmail.com> <5688.1460437831@sss.pgh.pa.us>
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Comments: In-reply-to Craig Ringer <craig@2ndquadrant.com>
	message dated "Tue, 12 Apr 2016 13:22:11 +0800"
Date: Tue, 12 Apr 2016 01:51:59 -0400
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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.

			regards, tom lane


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