Query Optimisation (ORDER BY)

I have an SQL query similar to below:

SELECT NAME,
       MY_FUNCTION(NAME) -- carries out some string manipulation
  FROM T开发者_Go百科ITLES
  ORDER BY NAME; -- has an index.

The TITLES table has approximately 12,000 records. At the moment the query takes over 5 minutes to execute but if I remove the ORDER BY clause then it executes within a couple of seconds.

Does anyone have any suggestions on how to be speed up this query.

If MY_FUNCTION is deterministic (i.e. always returns the same result for the same input value) then you could create an index on (NAME, MY_FUNCTION(NAME)) and it may help (or may not!)

In comments under the question, you say that it takes 2 seconds "to return N rows without the ORDER BY". That makes sense: without the ORDER BY you will just get the first N rows encountered, as soon as they are encountered. With the ORDER BY, the first N rows are returned only after the results have been sorted into the correct order.

If the query is being used in a situation where getting the first N rows fast is important (e.g. an online report with pagination) then you could try adding a FIRST_ROWS or FIRST_ROWS_n hint to the query, to try to persuade it to use the index. See Choosing an Optimizer Goal

Use the EXPLAIN statement to see where the issue is

EXPLAIN SELECT NAME, MY_FUNCTION(NAME) FROM TITLES ORDER BY NAME;

Sounds weird. What's name column type?

Have you checked for defective hardware errors? Maybe (just maybe) your query with the order by clause is using your index, and your index is located in a defective disk (it could be in a different disk from the table if they are located in different tablespaces).