You can collect statistics on deterministic UDFs.
For example, suppose you create the following UDF.
CREATE FUNCTION months_between(Date1 TIMESTAMP, Date2 TIMESTAMP) RETURNS FLOAT LANGUAGE C NO SQL DETERMINISTIC SPECIFIC months_between_tt EXTERNAL NAME 'CS!months_between_tt!$PGMPATH$/months_between_tt.c' PARAMETER STYLE SQL;
The following request collects statistics on the UDF months_between().
COLLECT STATISTICS COLUMN months_between(BEGIN(policy_duration), END(policy_duration)) AS Stats_MthsBetweenBegAndEnd ON Policy_Types;