2016-12-17 16:26 GMT+01:00 Fabien COELHO <coe...@cri.ensmp.fr>: > > Hello Tom, > > So I think it would be reasonable for somebody to implement \if, >>> \elseif, \endif first, with the argument having to be, precisely, a >>> single variable and nothing else (not even a negator). [...] >>> >> > This seems like a reasonable implementation plan to me, not least because >> it tackles the hard part first. There's no doubt that we can build an >> expression evaluator, but I'm not entirely sure how we're going to wedge >> conditional eval or loops into psql's command reader. >> >> (Or in other words, let's see \while ... \endloop in the minimal proposal >> as well, or at least a sketch of how to get there.) >> > > My 0.02 €: > > I have not seen any use case for a loop... Does someone have something > convincing? I could think of some use in benchmarking (aka in pgbench), but > not psql... But I may lack imagination. > > If one realistic case is found, then from a syntactic point of view > "\while expr ... \endwhile/loop/whatever" looks straightforward enough. >
maybe iteration over cursor can be interesting - but now with with \gexec it is not important. > > However, the implementation issues are pretty different from "if" which > can be managed pretty simply on the fly with a stack and a little > automaton. A loop needs to store its body and evaluate it over and over, > which means having processed the input up to the end of the loop before > proceeding, including nesting and so... it is a much less interactive > friendly construct. > > Note that although "cpp" has an if, but it does not have any loop. > > In my opinion psql should stay at that same simple level: ISTM that the > typical psql-script requirement is to be able to test some things, eg for > installing or upgrading the schema of an application, and for that > variables, expressions server side and maybe client side, and conditions > are mostly enough. A lot of "IF EXISTS" added to many commands recently are > motivated to handle this kind of use-case at the command per command level, > which is not necessarily the right place. > > A while loops turns a simple thing into a potential Turing-complete beast, > without a strong incentive I think that it should be avoided. > +1 Pavel > > -- > Fabien.
