Re: Array type confusion

Peter Eisentraut <peter_e@gmx.net> writes:
> The question is how to determine when a type is an array type (and not
> using the leading-underscore convention). A comment in pg_type.h says:
>  * typelem is 0 if this is not an array type.  If this is an array
>  * type, typelem is the OID of the type of the elements of the array
>  * (it identifies another row in Table pg_type).
> The reverse seems to be false. If typelem is not 0, then the type is not
> necessarily an array type. For example, the typelem entries of text,
> bpchar, and name point to char (the single-byte variant), while box and
> lseg claim to be arrays of "point".

I don't think that the typelem values presently given in pg_type for
these datatypes are necessarily sacrosanct.  In fact, some of these
demonstrably don't work.

AFAICT, the array-subscripting code supports two cases: genuine arrays
(variable-size, variable number of dimensions, array header data) and
fixed-length pseudo-array types like oidvector.  So, for example,
the fact that oidvector is declared with typelem = oid makes it possible
to write things like "select proargtypes[1] from pg_proc", even though
oidvector is a basic type and not a genuine array.

The way array_ref tells the difference is that typlen = -1 means a
real array, typlen > 0 means one of the pseudo-array types.  It does
not work to subscript a varlena type that's not really an array.
For example, you get bogus results if you try to subscript a text value.

I believe we need to remove the typelem specifications from these
varlena datatypes:
  17 | bytea  25 | text 602 | path 705 | unknown1042 | bpchar1043 | varchar

since subscripting them doesn't work and can't work without additional
information provided to array_ref.

If we do that then your type formatter can distinguish "real" array
types as being those with typelem != 0 and typlen < 0.  If typlen > 0
then treat it as a basic type regardless of typelem.
        regards, tom lane