abs() is simply called once per output value of the generate_series SRF. That seems fairly equivalent to me to what's going on in your example case.
Fair point. Both of these are premised on two related facts:
One, the select generate_series(1,3) function call causes multiple rows to be generated where there would usually be only one. In short, SRF function calls and non-SRF function calls exhibit different behaviors on the output.
Two, composition results in an inside-to-outside execution order: the SRF is evaluated first, the additional rows added, then the outer function (abs or the subscript function respectively in these examples) is evaluated for whatever rows are now present in the result.
Is the above something one can learn from our documentation?
Is this syntax we are discouraging users from using and thus intentionally not documenting it? I do get this impression but, frankly, given the utility of " [ generate_series(...) ] " I find it hard to recommend something different to get the same result yet don't have a place to point and say "here is how and why it works".