The (trivial) big picture is to allow client-side expressions in psql (which as a \if:-) by reusing pgbench expression engine, so that one could write things like
\let i :j + 12 * :k
or
\if :VERSION_NUM < 140000
I still haven't really grasped why this isn't done by embedding a client-side scripting language interpreter, giving us vastly greater capabilities with only the maintenance of the glue instead of writing our own ad-hoc scripting tool. Something confine-able like JavaScript or Lua.
I am primary a psql user, so I'll talk about psql. I don't need there more functionality, than has C macros. Not more. So \if :VERSION_NUM < x is good enough. && || operators are nice to have
For this I don't to join any VM. It is overkill. What scripting functionality we can do in psql now, and probably in long future
a) setting prompt
b) some deployment - version checks
c) implementation of some simple regress scenarios
Can be different if we start new rich TUI client based on ncurses, new features - but it is different story.
More - implementation of simple expression evaluation in psql doesn't break later integration of some VM. Maybe it prepare a way for it.
I can imagine the psql command \lua anylua expression, with possibility to call any lua function with possibility to iterate over SQL result, show any result, and read, write from psql variables - prefer Lua against JavaScript, but same can be done there too.
But this not against to this patch. This patch is not too big, too complex, too hard to maintain - and some simple today issues can be done simple
Fine by me so long as it remains a manageable scope, rather than incrementally turning into some horror scripting language.