Yes, the order of the fields in the "WHERE" statement is important for the right index to be chosen.
The database system sometimes does not use a suitable index for a selection, even if there is one. The index used depends on the optimizer used for the database system. You should therefore check if the index you created is also used for the selection (see How to Check if an Index is Used).).
Creating an additional index could also have side effects on the performance. This is because an index that was used successfully for selection might not be used any longer by the optimizer if the optimizer estimates (sometimes incorrectly) that the newly created index is more selective.
The indexes on a table should therefore be as disjunct as possible, that is they should contain as few fields in common as possible. If two indexes on a table have a large number of common fields, this could make it more difficult for the optimizer to choose the most selective index.
How to Check if an Index is Used
Procedure
Open a second session and choose System ® Utilities ® Performance trace.
The Trace Requests screen appears.
Select Trace on.
The SQL trace is activated for your user, that is all the database operations under your user are recorded.
In the first window, perform the action in which the index should be used.
If your database system uses a cost-based optimizer, you should perform this action with as representative data as possible. A cost-based optimizer tries to determine the best index based on the statistics.
In the second session, choose Trace off and then Trace list.
Result
The format of the generated output depends on the database system used. You can determine the index that the database used for your action with the EXPLAIN function for the critical statements (PREPARE, OPEN, REPOPEN).
Best Regards,
Vibha
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Bluesky
