Oracle: 索引与 … where a is null

摘自《Troubleshooting Oracle Performance》

假设 a 列上建了一条索引

1. 对于B-树索引

   … where a is null 会导致全表扫描,因为B-树索引不存储NULL值


       … where a is null and b = xx 仍会使用索引


      … where a is null 仍然会使用索引,因为位图索引会存储NULL值

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