The optimizer may choose not to merge a view for many reasons, including cost or validity restrictions. Otherwise, the optimizer uses cost to make the determination. For certain simple views in which merging always leads to a better plan, the optimizer automatically merges the view without considering cost. View merging can improve plans by enabling the optimizer to consider additional join orders, access methods, and other transformations.įor example, after a view has been merged and several tables reside in one query block, a table inside a view may permit the optimizer to use join elimination to remove a table outside the view. In view merging, the optimizer merges the query block representing a view into the query block that contains it.
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