A
function
is an identifier that instructs PostgreSQL to perform a programmatic operation within
a SQL statement. A function returns a single value from its operation, and that value is then used in the SQL statement where
the function was invoked. This process is similar to the way operators return their results in the location from which they
were called in the query. (In fact, operators are technically pointers to built-in system functions, and are sometimes
called "syntactic sugar" for functions, as they are a syntactically convenient way to call underlying functions.)
