default permissible datatype conversion matrix

You can get a list using the psql client with

\dCS

This will show all casts defined between system data types.

In addition, all types can be cast to and from text using the type input and output functions.

As a supplement to Laurenz' post...

From the docs:

\dC[+] [ pattern ]

Lists type casts. If pattern is specified, only casts whose source or target types match the pattern are listed. If + is appended to the command name, each object is listed with its associated description.

\set ECHO_HIDDEN on reveals \dCS+ to be executing...

SELECT pg_catalog.format_type(castsource, NULL) AS "Source type",
       pg_catalog.format_type(casttarget, NULL) AS "Target type",
       CASE WHEN c.castmethod = 'b' THEN '(binary coercible)'
            WHEN c.castmethod = 'i' THEN '(with inout)'
            ELSE p.proname
       END AS "Function",
       CASE WHEN c.castcontext = 'e' THEN 'no'
            WHEN c.castcontext = 'a' THEN 'in assignment'
            ELSE 'yes'
       END AS "Implicit?",
       d.description AS "Description"
FROM pg_catalog.pg_cast c LEFT JOIN pg_catalog.pg_proc p
     ON c.castfunc = p.oid
     LEFT JOIN pg_catalog.pg_type ts
     ON c.castsource = ts.oid
     LEFT JOIN pg_catalog.pg_namespace ns
     ON ns.oid = ts.typnamespace
     LEFT JOIN pg_catalog.pg_type tt
     ON c.casttarget = tt.oid
     LEFT JOIN pg_catalog.pg_namespace nt
     ON nt.oid = tt.typnamespace
     LEFT JOIN pg_catalog.pg_description d
     ON d.classoid = c.tableoid AND d.objoid = c.oid AND d.objsubid = 0
WHERE ( (true  AND pg_catalog.pg_type_is_visible(ts.oid)
) OR (true  AND pg_catalog.pg_type_is_visible(tt.oid)
) )
ORDER BY 1, 2;

...which points us to a few more interesting docs pages.

• pg_cast - the base system table listing permissible casts
• pg_type - the base system table reference for datatypes
• format_type() - a prettifier function for pg_type. Helpful in mapping proper names to aliases as seen on the datatypes reference table