[HACKERS] array casts that worked in 8.3 no longer work

[J. Greg Davidson]

Dear PostgreSQL Hackers,

I have a couple of user defined types, which have reference semantics.
One is more specialized, so by Liskov I can upcast both single values
and arrays of values.

-- no problem:
CREATE CAST (derived_refs AS base_refs) WITHOUT FUNCTION AS IMPLICIT;

-- ok with pg-8.3, error with pg-8.4-b2:
CREATE CAST (_derived_refs AS _base_refs) WITHOUT FUNCTION AS IMPLICIT;
-- ERROR:  42P17: array data types are not binary-compatible
-- LOCATION:  CreateCast, functioncmds.c:1648

Reading the comment in functioncmds:
  /*
   * We know that composite, enum and array types are never binary-
   * compatible with each other.  They all have OIDs embedded in them.
   */
I'm guessing that what I was doing was not safe although I have been
getting away with it quite nicely for several years.

Unfortunately I do this all over the place (I have a master ref type
and lots of specializations of it).  Is there an efficient (and maybe
even easy) way to legally convert such arrays?

Thanks for any suggestions you may have,

_Greg

J. Greg Davidson

P.S. If you want a more complete example to refer to, here it is:


CREATE TYPE base_refs;

CREATE OR REPLACE
FUNCTION base_ref_in(cstring) RETURNS base_refs
AS 'pg-array-problem.so' LANGUAGE 'c' STRICT;

CREATE OR REPLACE
FUNCTION base_ref_out(base_refs) RETURNS cstring
AS 'pg-array-problem.so' LANGUAGE 'c' STRICT;

CREATE TYPE base_refs (
  INTERNALLENGTH = 8,
  ALIGNMENT = double,
  input = base_ref_in,
  output = base_ref_out,
  PASSEDBYVALUE
);

CREATE TYPE derived_refs;

CREATE OR REPLACE
FUNCTION derived_ref_in(cstring) RETURNS derived_refs
AS 'pg-array-problem.so' LANGUAGE 'c' STRICT;

CREATE OR REPLACE
FUNCTION derived_ref_out(derived_refs) RETURNS cstring
AS 'pg-array-problem.so' LANGUAGE 'c' STRICT;

CREATE TYPE derived_refs (
  INTERNALLENGTH = 8,
  ALIGNMENT = double,
  input = derived_ref_in,
  output = derived_ref_out,
  PASSEDBYVALUE
);

-- ** Safe Upcasts

-- no problem:
CREATE CAST (derived_refs AS base_refs)
WITHOUT FUNCTION AS IMPLICIT;

-- error with pg-8.4-b2:
CREATE CAST (_derived_refs AS _base_refs)
WITHOUT FUNCTION AS IMPLICIT;



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