2016-12-19 18:30 GMT+01:00 Robert Haas <robertmh...@gmail.com>: > On Sat, Dec 17, 2016 at 3:39 PM, Corey Huinker <corey.huin...@gmail.com> > wrote: > >> To implement \while, we'd also need to remember previous commands so > >> that when we reach the end of the loop, we can rewind and put all of > >> those commands back on the stack to be executed again, or perhaps to > >> be skipped if the \while condition turns out now to be false. > > > > This might be what you meant when you said "those commands back on the > > stack", but I think we'd have to remember not a list of commands, but the > > raw string of bytes from the start of the \while to the \endwhile (or > > equivalent), because any psql vars within that block could themselves be > a > > non-parameter part of a command: > > > > -- this is how I fake an 'exit 0' now: > > \set work_needs_to_be_done t > > select > > case > > when :'work_needs_to_be_done'::boolean then '' > > else '\q' > > end as cmd > > \gset > > :cmd > > > > -- ridiculous example to illustrate complications in remembering past > > commands > > select > > case > > when random() < 0.5 then '\ir my_script.sql' > > when random() < 0.7 'select * from a_table; select count(*) from > > another_table;' > > else 'select null as foo;' > > end as cmd > > \gset > > :cmd > > > > And even then, things get complicated, because an \ir include which > makes it > > this iteration might not make it the next, and the \endwhile might have > been > > inside that include, or vice-versa, an included file starts a \while it > > doesn't finish. > > I see your point. Just out of curiosity, why in the world don't you > use something other than psql for scripting? I mean, if you accessed > the data from Perl or Python or > $INSERT_YOUR_FAVORITE_SCRIPTING_LANGUAGE_HERE, you could do all of > this stuff very easily without any contortions. I've always thought > of psql as something that's fine for interactive use and goofy trivial > scripting but not really suitable for serious work. I grant that you > seem to be making it serve the purpose, but, man. >
The integration of any scripting environment with SQL is much more less than in psql - just some easy scenarios are in psql natural. It is similar why some people like me, prefer PLpgSQL against Java, Perl, ... years ago there was a bash integrated with postgres - for me nice idea, but this project is dead. > > > So maybe what we store is a stack of buffers that are currently open > (STDIN > > being captured as a buffer only when a \while starts, everything else > being > > files), and additionally have a stack of positions where a \while started > > (buffer_id, position in buffer). > > Yeah, sounds about right. > > > Additionally, we could assert that all \while-\endwhile pairs must > happen in > > the same MainLoop (aka file), and mismatches are an error. > > Sounds prudent. > > > I'm happy to keep sketching out what control structures might look like > and > > how to implement them. But I'm also happy to leave while/for loops out > for > > now. > > Right, I think that while/for can be left for another time, as long as > the plan doesn't preclude doing it someday. > > -- > Robert Haas > EnterpriseDB: http://www.enterprisedb.com > The Enterprise PostgreSQL Company >
