valid casts to anyarray

[Philip Carlsen]

Hi list

I've been down a rabbit hole today trying to understand what exactly makes
a type a valid candidate for an ANYARRAY function argument (e.g., something
you can 'unnest()').

My reading has led me across such functions as 'get_promoted_array_type',
'IsTrueArrayType', 'can_coerce_type', and 'check_generic_type_consistency',
and this has led me to believe that any type which has a valid (i.e.,
non-zero?) pg_type.typelem defined should be applicable.

However, I cannot seem to be able to call 'unnest' on a 'point':

postgres=# select unnest(point(1,2));
ERROR:  function unnest(point) does not exist

... even though according to 'pg_catalog.pg_type' the type 'point' does
indeed look very array-like (it should be equivalent to an float8 array).
The only difference I can spot is that it has
'typsubscript=raw_array_subscript_handler', as opposed to
typsubscript=array_subscript_handler' which is what 'IsTrueArrayType'
checks for.

Can anyone here perhaps enlighten me as to how I can tell if a type is a
valid ANYARRAY (and bonus points to point out the check I must have missed
in the (parser?) source code)?

-Philip

Re: valid casts to anyarray

[Philip Carlsen]

> Per the comments for check_generic_type_consistency:
>
>  * 2) All arguments declared ANYARRAY must have the same datatype,
>  *which must be a varlena array type.
>

This must be exactly the bit that I missed during my reading - thanks!

> It has to not only have an element type, but have a standard array
> header, else we don't know how to do a lot of operations on it.
>
> Type "point" and related animals are sort of a poor man's array,
> which is supported for basic subscripting operations, but it's not
> generic enough to be reasonable to consider as an ANYARRAY.

This is what I had started to suspect.

I also found the source commentary in c.h (via
https://doxygen.postgresql.org/c_8h_source.html, though apparently the
comments do not make it to the generated doxygen docs) was helpful in
understanding just what exactly the term 'varlena' refers to.
The docs also refer to this term at times, but it never makes concrete
what it actually means (IIRC).

This was a very enlightening answer - Thanks!

-Philip