SQLite查询优化
lectA的结果遍历完 EofB: 当selectB的结果遍历完 下面就是四种操作的执行过程: 执行顺序UNION ALLUNIONEXCEPTINTERSECTAltB:outA, nextAoutA, nextAoutA,nextAnextAAeqB:outA, nextAnextAnextAoutA, nextAAgtB:outB, nextBoutB, nextBnextBnextBEofA:outB, nextBoutB, nextBhalthaltEofB:outA, nextAoutA, nextAoutA,nextAhalt 2. 如果可能的话,可以把一个用到GROUP BY