mysql union，不同的查询优化

Question

所以我有一个简单的查询如下:

Select DISTINCT A.col1 as col1 
    FROM Table A join Table B on(A.col2 = B.col2)
    JOIN table C on(C.col1 = A.col3)
UNION
SELECT DISTINCT A.col1 as col1 
    FROM Table A 
    JOIN Table B on (A.col2 = B. col2) where A.col4 != 'somevalue';

因为它是可见的，表 A 和表 B 每次都在相同的列上执行连接。我无法找到一种方法来利用这些详细信息并加快查询速度

Answer 1

尝试像这样将连接转换为 EXISTS 和子查询逻辑:

SELECT DISTINCT A.col1 AS col1
FROM TABLE A
WHERE EXISTS
  (SELECT 1 FROM TABLE B WHERE A.col2 = B.col2
  )
AND EXISTS
  (SELECT 1 FROM TABLE C WHERE A.col1 = C.col3
  )
UNION
SELECT DISTINCT A.col1 AS col1
FROM TABLE A
WHERE EXISTS
  (SELECT 1 FROM TABLE B WHERE A.col2 = B.col2
  )
AND A.col4 != 'somevalue');