You can also collect statistics on constant expressions that refer to built-in functions.
The Optimizer remembers the materialized values for the CURRENT_TIMESTAMP, CURRENT_DATE and USER functions when statistics are collected and then substitutes the stored materialized values when the statistics are used.
For other built-in functions like SESSION and CURRENT_ROLE, the Optimizer substitutes the values based on the session parameters at the time the statistics are used. Even though the database allows you to specify any built-in functions, you should restrict your collecting of statistics to built-in functions other than CURRENT_TIMESTAMP, CURRENT_DATE and USER in the expressions.
In the following request, the Optimizer substitutes the value for CURRENT_DATE for the CASE expression and then remembers it. When the statistics are used for this expression, the Optimizer substitutes the saved value for CURRENT_DATE.
COLLECT STATISTICS COLUMN CASE WHEN CAST(o_orderdatetime AS DATE AT 0) = CURRENT_DATE THEN 'orders-today' ELSE 'orders-old' END AS stats_orders_today_and_old ON orders;