how to extract the page "address" from the ctid

Hello,

while waiting for the new BRIN and improved GIN Indexes :-)

I'd like to get a rough feeling, how some column's values are distributed on the internal pages.

e.g.:

SELECT c, count(*) FROM
(
SELECT substring(ctid::text, '^.([^,]+)'),
count(distinct myColumn) as c
FROM myTable
GROUP BY substring(ctid::text, '^.([^,]+)')
)foo
GROUP BY c order by 1 desc;

Is there a quicker way to extract the page reference from the ctid?

regards,

Marc Mamin