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51.63. pg_user_mapping
  The catalog pg_user_mapping stores the mappings from local user to remote. Access to this catalog is restricted from normal users, use the view pg_user_mappings instead.
Table 51.64. pg_user_mapping Columns
| Name | Type | References | Description | 
|---|---|---|---|
oid | 
        oid | 
        Row identifier (hidden attribute; must be explicitly selected) | |
umuser | 
        oid | 
        pg_authid.oid | 
        OID of the local role being mapped, 0 if the user mapping is public | 
umserver | 
        oid | 
        pg_foreign_server.oid | 
        The OID of the foreign server that contains this mapping | 
umoptions | 
        text[] | 
        User mapping specific options, as “keyword=value” strings | 
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 https://www.postgresql.org/docs/10/catalog-pg-user-mapping.html