This is the only provided interface which allows you to query the
database. If you wish to use the simpler "query" function, you need to
use the Sql.Sql generic SQL-object.
It returns a postgres_result object (which conforms to the
Sql.sql_result standard interface for accessing data). I
recommend using query() for simpler queries (because it is
easier to handle, but stores all the result in memory), and
big_query() for queries you expect to return huge amounts of
data (it's harder to handle, but fectches results on demand).