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PostgreSQL stores relation information in a separate file by using OID. You can find a list of those files in the PostgreSQL data directory.
It creates a file for relation like table, index, sequence and toast table. As it stores with OID (Object ID), then it’s hard to locate a particular relation file.
Once you get the OID and location of your table, you can quickly estimate other items like a size of a table, FSM file, VM file, toast information.
Use pg_relation_filepath() to get a location of your table:
SELECT *FROM pg_relation_filepath('Table_Name')