Generally, the data of a normal table in a database is stored in the storage space of the database, whereas the data of an external table is stored in an external storage service. When you create an external table, you must specify the path to and format of the relevant data files. After you create an external table, you can read data from files in the external storage service by using the external table.
An external table can be linked, aggregated, and sorted with another table like a normal table. External tables differ from normal tables in the following aspects:
The data of external tables is stored in external files, whereas that of normal tables is stored in databases.
External tables are read-only and can be used in query statements. However, you cannot perform Data Manipulation Language (DML) operations on external tables.
You cannot add constraints or create indexes on external tables.
Generally, the access speed of an external table is lower than that of a normal table.