I don't see indication that the capability described below exists in Postgres (or any RDBMS), so this is likely a feature request --
For column types that are frequently defined in tables, and which are typically indexed, it would be helpful to be able to specify in the type definition of the column that its addition/creation into a table should be automatically accompanied by the creation of an associated index.
Auto-index generation for a specific column type would be overridable for specific tables, as you wouldn't always want to auto-create an index at the time of column addition to a table. In the case of CREATE TABLE, this would be less of a concern, and more of a concern for ALTER TABLE ADD COLUMN.
Such a configuration would also specify the type of index.
What do you think ?
It is relatively dangerous, I think. What is typical type that can be indexed?