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For the first two queries all it has to do is scan in the clustered index to the first entry for that value of IDUkazatel - because of the order of the index that row will be the lowest value for cas for that value of IDUkazatel. In the second query this optimisation is not value and it is probably seeking to the first row for IDUkazatel=24 then scanning ...


Most likely, either: The state of the database (e.g. statistics) changed since the cached plan was first compiled, causing a recompilation; or Your SSMS connection uses different settings (e.g. ANSI_NULLS) from those used by the connection that cached the plan (so it could not be reused); or It was an ad-hoc batch and the text of your query did not exactly ...

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