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MDEV-9712 Performance degradation of nested NULLIF
10.1 introduced a problem: Execution time for various recursive stages (walk, update_used_table, and propagate_equal_fields) in NULLIF is O(recursion_level^2), because complexity is doubled on every recursion level when we copy args[0] to args[2]. This change fixes to avoid unnecessary recursion in: - Item_func_nullif::walk - Item_func_nullif::update_used_tables - Item_func_nullif::propagate_equal_fields when possible.
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Alexander Barkov
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May 5, 2016
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