The oldest I can find is from 1998 (PostgreSQL 6.3), but it was probably in use even before.
> Postgres offers substantial additional power by incorporating the following four additional basic concepts in such a way that users can easily extend the system:
classes
inheritance
types
functions
Other features provide additional power and flexibility:
constraints
triggers
rules
transaction integrity
These features put Postgres into the category of databases referred to as object-relational
The oldest I can find is from 1998 (PostgreSQL 6.3), but it was probably in use even before.
> Postgres offers substantial additional power by incorporating the following four additional basic concepts in such a way that users can easily extend the system:
classes inheritance types functions
Other features provide additional power and flexibility:
constraints triggers rules transaction integrity
These features put Postgres into the category of databases referred to as object-relational
https://www.postgresql.org/docs/6.3/c0101.htm