Query to show all tables used (and how many times) by your materialized views in a schema

SELECT ad.detailobj_owner
, ad.detailobj_name
, count(1) as occurrence
FROM all_mview_detail_relations ad
WHERE ad.owner = 'YOURSCHEMA'
GROUP BY ad.detailobj_owner, ad.detailobj_name
ORDER BY ad.detailobj_owner, ad.detailobj_name