Thinking on this for a moment, perhaps better than pg_get_indexdef is pulling column ordinals out of the indexprs column noted in the first answer, and specifically the varattno field. regexp_matches for that (and only that :)) I threw together the following, going back to the definition in the columns table of information_schema:
SELECT (SELECT JSON_AGG(columns.column_name)
FROM information_schema.columns
WHERE table_schema = 'public'
AND table_name = i.indrelid::regclass::text
AND ordinal_position IN (SELECT matches[1]::INTEGER
FROM regexp_matches(i.indexprs::TEXT,
'varattno (\d)',
'g') as matches)) AS argument_columns
, pg_get_indexdef(att.attrelid, att.attnum, true)
, i.indrelid::regclass::text AS table
, c.relname AS index_nameq
, i.indisunique AS is_unique
, att.attname as column_names
FROM pg_catalog.pg_namespace n
JOIN pg_catalog.pg_class c ON c.relnamespace = n.oid
JOIN pg_catalog.pg_attribute att ON att.attrelid = c.oid
JOIN pg_catalog.pg_index i ON i.indexrelid = c.oid
WHERE n.nspname !~ '^pg_'
AND c.relkind IN ('r', 't', 'i')
and that produces:
| argument_columns | pg_get_indexdef | table | index_nameq | is_unique | column_names |
|---|---|---|---|---|---|
| ["content"] | "substring"(content, 1, 5) | file_lookup_4k | date5_index | 0 | substring |
| ["file_id", "sequence_no"] | abs(file_id - sequence_no) | file_lookup_4k | date6_index | 0 | abs |
| ["file_id"] | abs(file_id) | file_lookup_4k | date2_index | 0 | abs |
| ["sequence_no"] | round(sequence_no::double precision) | file_lookup_4k | date3_index | 0 | round |
| ["sequence_no"] | round(sequence_no::double precision) | file_lookup_4k | date4_index | 0 | round |
I feel that this is far more in line with what we would both want.
Note that I joined only to the attname table, because that is really all I need