>So, what all this means ? Using large index for sorting small resultset is ineffective ?

Something like that. At prepare time, Firebird doesn't know whether your query will return 500 or 100000 records. If it returns 100000 records, then the index is beneficial, if it returns 500, then it is better to not use it. Though I was surprised to see that eliminating the index sped up your query 10-fold.

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