The primary key, is the key (column) that uniquely identifies a row. It is often put behind a unique constraint and indexed for fast access. Now, because it is unique, the primary index is often…

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Hussein Nasser

A primary key uniquely identifies a row and is typically indexed for fast access. Primary indexes are often clustered, meaning they store full rows. Secondary indexes only store row identifiers, requiring an additional lookup of the primary key to retrieve full rows. Postgres is an exception as it does not use a real primary clustered index; all indexes, including primary, point to the row identifier or tuple id. The tuple id in Postgres is unique and allows direct row access with minimal I/O.

The Internals of a Primary Index