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A query that ran fast on V9.5 was found to run dramatically slower in V10. Comparing the V9.5 execution plan with the V10 execution plan, it is clear that V10 tends to build a Dynamic Index (DIDX) instead of using an existing index to execute a join.
In V10 logic has been introduced to better optimize joins when the restriction imposed by join predicate can be executed using only a few columns of a multi-column index. In some cases it may be more convenient to create a DIDX and use it instead of doing partial key searches on the existing index. However, the logic was using a DIDX in cases where it was not preferred.
The logic has been refined to perform better checks before deciding to use a DIDX.
