Re: Better error reporting from extension scripts (Was: Extend ALTER OPERATOR)

I wrote:
> It looks like if we did want to suppress that, the right fix is to
> make gram.y track statement start locations more honestly, as in
> 0002 attached (0001 is the same as before).

I did a little bit of further work on this:

* I ran some performance checks and convinced myself that indeed
the more complex definition of YYLLOC_DEFAULT has no measurable
cost compared to the overall runtime of raw_parser(), never mind
later processing.  So I see no reason not to go ahead with that
change.  I swapped the two patches to make that 0001, and added
a regression test illustrating its effect on pg_stat_statements.
(Without the gram.y change, the added slash-star comment shows
up in the pg_stat_statements output, which is surely odd.)

* I concluded that the best way to report the individual statement
when we're able to do that is to include it in an errcontext()
message, similar to what spi.c's _SPI_error_callback does.
Otherwise it interacts badly if some more-tightly-nested error
context function has already set up an "internal error query",
as for example SQL function errors will do if you enable
check_function_bodies = on.

So here's v3, now with commit messages and regression tests.
I feel like this is out of the proof-of-concept stage and
might now actually be committable.  There's still a question
of whether reporting the whole script as the query is OK when
we have a syntax error, but I have no good ideas as to how to
make that terser.  And I think you're right that we shouldn't let
perfection stand in the way of making things noticeably better.

			regards, tom lane