Hola Alvaro,
In reply to https://postgr.es/m/alpine.DEB.2.21.1901102211350.27692@lancre
wherein Fabien wrote:
I'm not very happy with the resulting syntax, but IMO the feature is useful.
My initial design was to copy PL/pgSQL "into" with some "\into" orthogonal
to \; and ;, but the implementation was not especially nice and I was told
to use psql's \gset approach, which I did.
If we do not provide \cset, then combined queries and getting results are
not orthogonal, although from a performance testing point of view an
application could do both, and the point is to allow pgbench to test the
performance impact of doing that.
We very briefly discussed this topic at FOSDEM pgday. My feeling on the
general opinion is that there's appreciation for \gset in general, but
that people feel that \cset is too much cruft to take for not enough
additional added value (compared to great value delivered by \gset).
What I'm going to do now is to write a patch to remove the \cset part of
the commit and post it, intending to push at some point next week.
If somebody has grown really fond of \cset, they can work on a patch to
implement it properly, which it isn't now.
My usless 0.02€:
I'm willing to implement it properly. Do you have any advice?
For me the issue comes from the fact that postgres silently ignores empty
queries, i.e. on:
SELECT 1 \; \; SELECT 2;
Three queries are sent, the middle one empty, but two results returned
(PGRES_TUPLES_OK, PGRES_TUPLES_OK) instead of (PGRES_TUPLES_OK,
PGRES_EMPTY_QUERY, PGRES_TUPLES_OK). However, on:
;
You do have an PGRES_EMPTY_QUERY result. How to deal cleanly and simply
with that?
Removing the "skip empty query" optimizations would remove the "it does
not work with empty queries" documentation warning that I understood Tom
complains about. It may have a little impact on "psql" implementation to
keep the "show the last non-empty result" behavior on combined queries.
Providing non-orthogonal features (eg combined queries cannot use some
options, such as -M in pgbench) is as much substandard as awkward
optimizations like the above.
An alternative is to detect whether a query is empty, but that complicates
the lexing phase to detect "\; <spaces and comments only> \;" and adjust
the query count to match variable setting to their queries: one version of
the patch kept the position of embedded semi-colons so as to be able to
check that only spaces appear (comments are removed by the lexer). Because
this was seen as awkward code working around awkward protocol behavior for
an unlikely corner case, it was removed in the reviewing process and the
limitation was documented instead.
--
Fabien.
