Hi, folks!

I am reading this documentation[1], and it has a sentence that I don’t quite 
understand: "The index columns (key values) can be either simple columns of the 
underlying table or expressions over the table rows.”, I am thinking that for 
the index of expressions, aren’t those expressions over table column? e.g., 
“CREATE INDEX idx_lower_last_name ON users(LOWER(last_name))”, “last_name" is a 
column rather than a row.


[1] https://www.postgresql.org/docs/17/index-api.html#INDEX-API

Regards, Steve.

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