Where clause causing a query to slow down in a cursor

Question

I have a table "the_table" with about 10,000 rows and four columns (id, description, inventory, and category).
This query returns all rows immediately:
    select id
         , description
         , inventory
    from the_table
    where category =  nvl(null, category);
However, when it is put into a cursor, like this:
(p_user_category is a user-defined variable which can be null if the user wants all of the rows)
    c_results  sys_refcursor;
    open c_results for
        select id
             , description
             , inventory
        from the_table
        where category =  nvl(p_user_category, category);
    fetch c_results into v_id, v_description, v_inventory;
    close c_results;
then it takes five minutes to return even just one row when p_user_category is null.
However, if I change the where clause to:
    where (p_user_category is null or category = p_user_category)
then it returns all rows immediately.
It started being a problem right around the time of the most recent update - I am running 11.2.0.2.0 (64-bit production).

Answer 1

The optimizer is smart enough to recognise that null is null, so "nvl(null,category)" collapses to "category", and your predicate "category = nvl(null, category)" is transformed to "category is not null" (if it's allowed to be null) or simply disappears - so the SQL test is not the same as the PL/SQL run.
In PL/SQL your manual rewrite is not logicall the same as the original unless you have declared category to be non-null because your first disjunct will allow rows with a null category to be reported, while the original query would lose them.
Check the execution plans for the SQL and the PL/SQL versions.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
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