PG Bug reporting form <noreply@postgresql.org> writes: > The array_to_string function should have a volatility of immutable,
Nope. It invokes an arbitrary datatype I/O function, which might only be stable. As an example:
regression=# begin; BEGIN regression=*# select array_to_string(array[now()], ',');
That feels wrong. It's not like we are passing the "now()" function to the function and invoking it later. So far as array_to_string is concerned it is being given a literal value.
By your argument, isfinite(timestamp) should be stable but it is immutable (it's the first example I stumbled across, I suspect there are quite a few more I could contrive).
regression=*# set timezone to UTC; SET regression=*# select array_to_string(array[now()], ','); array_to_string ------------------------------- 2022-01-10 14:47:06.605304+00 (1 row)
[...]
Ideally, perhaps, we'd determine the volatility level of polymorphic functions like array_to_string() based on the actual input data types --- but it's hard to see how to do that.
In short, that doesn't make sense. The volatility level of a function is only determined by the implementation code of said function. The function invoking expression volatility level depends upon the most volatile behavior of all functions used in the expression. That we should be doing if we aren't already.
