no rows selected
execution plan
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0 select statement optimizer=choose
1 0 table access (by index rowid) of 'emp'
2 1 index (range scan) of 'emptype' (non-unique)
虽然empno是唯一性索引,但是由于它所做的是范围比较, 等级要比非唯一性索引的等式比较低!
