Donald Burleson Click here for more Oracle News Headlines

Built-in functions and the CBO

Oracle created function-based indexes (FBI’s) to ensure that indexes are used when you must include a built-in function on an indexed column in your SQL.  For example, the following query would not be able to use an index on the mydate column:

    . . . where to_char(mydate) = ’10-1-07’

If it’s necessary to change an indexed column with a function, you can create a function-based index on the column:

   create index
    mydate_to_char
    on
       mytab (to_char(mydate));

It’s always a good idea to re-work your SQL to remove built-in function from any where clause predicates:

Bad:

   where to_num(char_column) = 123

   where to_char(date_column) = ’10-1-07’

Good:

   where char_column = ‘123’

   where date_column = to_date(’10-1-07’);

The “index invalidation also extends to partitioned table where you use a built-in function on a partitioned table key.

Partition pruning and predicate functions

Partitioning pruning refers to the ability of the cost-based optimizer to exclude irrelevant table partitions when servicing an SQL query.  This blogger has published some excellent advice on the issue of using built-in functions on partition key columns. Essentially, the use of a “predicate” built-in function (e.g. trunc, substr, to_char, &c), will disable partition pruning unless a check constraint is placed on the column:

“When you place a predicate on a function of a partition key column then the optimizer is prevented from pruning unless there is a check constraint to indicate logical equivalence between the column and the function of the column.”

“The partitioning scheme acts effectively as a series of check constraints on the individual partitions that allow table access to be avoided when it can be deduced that a predicate does not resolve to any partitions at all.”

This comment by Pete S. confirms that built-in functions can be problematic when used on table partition keys:

“Elsewhere, this week he has made an interesting observation on the use of functions on partition keys in queries . . . TRUNC prevents the use of partition elimination and therefore negates a major performance enhancing feature in an Oracle DW.”

Also, see these insightful notes on predicate pushing.

The moral of this story is that you should avoid the use of built-in functions on partitioned table keys whenever possible, but if you must use a function predicate on a partition key, make sure to create a corresponding check constraint.

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