Обсуждение: Re: valid casts to anyarray

Philip Carlsen <plcplc@gmail.com> writes:
> 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()').

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.

If you follow that code down you eventually find that it expects
get_element_type() to succeed, and that says

 * NB: this only succeeds for "true" arrays having array_subscript_handler
 * as typsubscript.  For other types, InvalidOid is returned independently
 * of whether they have typelem or typsubscript set.

which is mechanized as an IsTrueArrayType() check.

> 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.

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.

            regards, tom lane

> 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