Explain the DETERMINISTIC clause on a PL/SQL function

Marking a function DETERMINISTIC tells the optimiser it may reuse a prior result for the same arguments within a single SQL statement, and it is required for a function used in a function-based index or a materialised view. The promise must actually hold — no dependence on table data that can change, session settings, SYSDATE, sequences or package state — or you get wrong results and stale indexes. Pure computational transforms (formatting, hashing, math) are the right candidates.

CREATE OR REPLACE FUNCTION full_name(p_first VARCHAR2, p_last VARCHAR2)
  RETURN VARCHAR2
  DETERMINISTIC
IS
BEGIN
  RETURN TRIM(p_last || ', ' || p_first);
END;
/
-- enables a function-based index:
CREATE INDEX emp_full_name_ix ON employees (full_name(first_name, last_name));