Couldn't this kind of thing be done directly with PL/pgSQL?
you can use PL/pgSQL - but there are some limits
* maintenance large plpgsql functions
I agree with large but that would not necessarily mean complex. Also, some functions could be in SQL, and just the logic with PL/pgSQL.
* the plpgsql functions or anonymous functions create a transaction borders - what should not be wanted
Hmmm... If something fails when installing an extension, a transaction border is probably a good thing? Also, the interaction of \if with possible BEGIN/COMMIT can lead to strange states.
* I cannot to use psql variables simply in plpgsql code (plpgsql has not any macro language)
You can call a function with a variable as an argument:
\set foo 1234 SELECT some_function(:foo);
- so some patterns are implemented in plpgsql less readable
Which pattern?
you can use parameters for functions, but you cannot it for DO statement (simply).
for example, if you can create a script that create some plpgsql function for specified PostgreSQL version, then using PLpgSQL for this purpose is suboptimal due less readability and maintainability
I'm not that sure about the qualitative assessment, "DO" looks pretty neat to me.
I can do with DO almost all work, but the result is not readable - I have to play with session variables, I have to play with more levels of custom string separator than is necessary
Moreover this is to be balanced with creating a scope/block/nesting system in psql which is currently alien to it and would bring its own pitfalls: psql is really intrinsically line/statement oriented, that would not be the case with what you're proposing as this logic would be deeply changed, and people would get stuck within a non-closed \if, this would interact with \i in possibly strange ways, and so on (say you include a file with a non closed \if, and then everything you type seems to be ignored, or a BEGIN is done in an if but the COMMIT was in another which was not activated because the conditions where not consistent...).
Basically, it looks like a potential Pandora box which is best left with its lid on.
I don't propose full psql scripting - my proposal is much more enhancing the current psql macro possibilities.
the implementation of \if_version_gt is pretty simple - needs few lines of new code
