Another way is
SELECT DISTINCT field INTO temp_table FROM mytable;SELECT count(*) FROM temp_table;DROP TABLE temp_table;
which is arguably more efficient than the previous solution
for large tables --- it should involve O(n log n) work rather
than O(n^2). For a small table, the overhead of creating and
dropping a table might overshadow the actual work, though.
In 6.5 you can use "INTO TEMP temp_table" and avoid worrying
about having to invent distinct temp table names for concurrent
users of the database.
The SQL-standard "SELECT count(DISTINCT field)" would be even nicer,
of course, but I dunno when someone will get around to it...
regards, tom lane