重写SQL查询以优化性能

Question

我有一个查询，大约需要2秒钟才能加载：

SELECT OUTPUT_VAL.NEXTVAL VAR1_R_ID,A.R_ID,A.VAR1,A.SEQU,A.OUTPUT,B.VAR1 DATATYPE_VAR1 
FROM
( 
    SELECT A.R_ID,A.VAR1,A.SEQU,A.OUTPUT,B.D_TYPE
    FROM
    (
        select A.R_ID, 2484 VAR1,1 SEQU, A.USER OUTPUT 
        from R_TB_1 A 
        WHERE A.R_ID BETWEEN 2457854437 AND 2458854437
        union all
        select A.R_ID, A.MEM_VAR1 VAR1,1 SEQU, MEM_OUTPUT OUTPUT 
        from R_TB_1 A 
        WHERE A.R_ID BETWEEN 2457854437 AND 2458854437
    ) A 
    LEFT JOIN VAR1_TABLE B 
        ON A.VAR1=B.VAR1
) A 
LEFT JOIN VAR1_TABLE B 
    ON A.D_TYPE=B.VAR1_NAME;

如何重写它以提高性能？

Answer 1

所以，你有一个表VAR1_TABLE至少有三列， VAR1 ， D_TYPE和VAR1_NAME ...你选择有行VAR1从另一个表列，那么你加入VAR1_TABLE上MEM_VAR1 = VAR1 ，然后你再加入D_TYPE = VAR1_NAME ？ 您能解释一下这部分吗，因为这对我来说毫无意义...为什么将D_TYPE与VAR1_NAME进行比较？ 仅仅因为您可以做到并且查询运行没有错误并不意味着就可以了。

假设表R_TB_1有很多行（您似乎选择了100,000行，这是整个表的一小部分），那么UNION ALL表示该表被扫描了两次。 如果您的版本至少是Oracle 11.1，则最好只在CTE中选择一次，然后再基于CTE进行合并...。 （顺便说一句，每当您提出问题时，请声明您的Oracle版本！）如果您使用的是Oracle 10或更低版本，则将需要像现在一样的子查询。

像这样：

with 
     Z ( R_ID, MEM_VAR1, USER, MEM_OUTPUT ) as (
       select R_ID, MEM_VAR1, USER, MEM_OUTPUT
       from   R_TB_1
       where  R_ID between 2457854437 and 2458854437
     ),
     A ( R_ID, VAR1, SEQU, OUTPUT ) as (
       select R_ID, 2484    , 1, USER
         from Z
       union all
       select R_ID, MEM_VAR1, 1, MEM_OUTPUT
         from Z 
     )
select  -- your joins from A to the other table here; A is defined in the WITH clause

Answer 2

WITH temp AS (
SELECT OUTPUT_VAL.NEXTVAL VAR1_R_ID,A.R_ID,A.VAR1,A.SEQU,A.OUTPUT,B.VAR1 DATATYPE_VAR1 FROM

( 
SELECT A.R_ID,A.VAR1,A.SEQU,A.OUTPUT,B.D_TYPE FROM
(
SELECT A.R_ID, 2484 VAR1,1 SEQU, A.USER OUTPUT from R_TB_1 A WHERE A.R_ID BETWEEN 2457854437 AND 2458854437

union ALL

select A.R_ID, A.MEM_VAR1 VAR1,1 SEQU, MEM_OUTPUT OUTPUT from R_TB_1 A WHERE A.R_ID BETWEEN 2457854437 AND 2458854437
)
A LEFT JOIN VAR1_TABLE B ON A.VAR1=B.VAR1 ) 
A LEFT JOIN VAR1_TABLE B ON A.D_TYPE=B.VAR1_NAME
)
SELECT * FROM temp