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).