Oracle-DB: Apparent cardinality problem with expressions indexed by a function based index
Recently a problem occured using function based indexes: The optimizer switched to full table scan on a table with 3.2 billion rows although the used function expression is covered by a function based index. The expression often evaluates to NULL ond so this index has only 4.9 million rows ( < 1% of the table rows ) and a low selectivity of approx. 4 rows per key. Let's consider the simple case. The table INVOICE has two interesting columns: - CUSTOMER_ID 38 mio distinct values, not null, avg. 84 rows/key - OPEN_AMOUNT not null, 4,8 mio. records > 0 A function based index was created to quickly identify the small portion of rows of a certain customer where the OPEN_AMOUNT is > 0 CREATE INDEX IX_OPENAMOUNT ON INVOICE(CASE WHEN Open_Amount > 0 THEN Customer_ID END); This SQL is used to select the rows of a certain customer where the OPEN_AMOUNT is > 0 SELECT ID FROM Invoice WHERE CASE WHEN Open_Amount > 0 THEN Customer_ID END = :B1 ; The executi