Odd unfamiliar Postgres SQL syntax

Hey Guys

Can someone help me understand the following SQL? What does the notation
(i.keys).n and (i.keys).x mean... as indicated by the <<<=== in the query.
Any help would be appreciated.

SELECT
NULL AS TABLE_CAT
, n.nspname AS TABLE_SCHEM
, ct.relname AS TABLE_NAME
, a.attname AS COLUMN_NAME
, (i.keys).n AS KEY_SEQ <<<==============
, ci.relname AS PK_NAME
FROM
pg_catalog.pg_class ct
JOIN pg_catalog.pg_attribute a
ON (ct.oid = a.attrelid)
JOIN pg_catalog.pg_namespace n
ON (ct.relnamespace = n.oid)
JOIN (
SELECT
i.indexrelid
, i.indrelid
, i.indisprimary
, information_schema._pg_expandarray(i.indkey) AS keys
FROM pg_catalog.pg_index i
) i
ON (a.attnum = (i.keys).x AND a.attrelid = i.indrelid) <<<===========
JOIN pg_catalog.pg_class ci
ON (ci.oid = i.indexrelid)
WHERE
n.nspname = 'edw'
AND ct.relname = 'campaign_dim'
AND i.indisprimary
ORDER BY
table_name
, pk_name
, key_seq;

--
Sonny.
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