UNION 与 SELECT DISTINCT 和 UNION ALL 性能

Question

这两者在性能方面有什么区别吗？

-- eliminate duplicates using UNION
SELECT col1,col2,col3 FROM Table1 
UNION SELECT col1,col2,col3 FROM Table2 
UNION SELECT col1,col2,col3 FROM Table3 
UNION SELECT col1,col2,col3 FROM Table4 
UNION SELECT col1,col2,col3 FROM Table5       
UNION SELECT col1,col2,col3 FROM Table6       
UNION SELECT col1,col2,col3 FROM Table7       
UNION SELECT col1,col2,col3 FROM Table8       

-- eliminate duplicates using DISTINCT    
SELECT DISTINCT * FROM
(     
    SELECT col1,col2,col3 FROM Table1 
    UNION ALL SELECT col1,col2,col3 FROM Table2 
    UNION ALL SELECT col1,col2,col3 FROM Table3 
    UNION ALL SELECT col1,col2,col3 FROM Table4 
    UNION ALL SELECT col1,col2,col3 FROM Table5       
    UNION ALL SELECT col1,col2,col3 FROM Table6       
    UNION ALL SELECT col1,col2,col3 FROM Table7       
    UNION ALL SELECT col1,col2,col3 FROM Table8       
) x   

Answer 1

Union 和 Union all 之间的区别在于 UNION ALL

 不会消除重复行，而是只是从所有表中提取符合您的查询细节的所有行并将它们合并到一个表中。

UNION 语句有效地对结果集执行 SELECT DISTINCT

。

如果您选择 Distinct from Union All 结果集，则输出将

等于 Union 结果集。

编辑：

CPU 成本性能：

让我用例子来解释：

我有两个疑问。一个是 Union，另一个是 Union All

SET STATISTICS TIME ON
GO
 
select distinct * from (select * from dbo.user_LogTime
union all
select * from dbo.user_LogTime) X 
GO

SET STATISTICS TIME OFF

SET STATISTICS TIME ON
GO
 
select * from dbo.user_LogTime
union
select * from dbo.user_LogTime
GO

SET STATISTICS TIME OFF

我确实在 SMSS 的同一查询窗口中运行了两者。 让我们看看 SMSS 中的

执行计划：

发生的情况是，使用

Union All 和 Distinct 的查询将比使用 Union 的查询花费更多的 CPU 成本。

准时表现：

UNION ALL

：

(1172 row(s) affected)

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 39 ms.

UNION

：

(1172 row(s) affected)

SQL Server Execution Times:
   CPU time = 10 ms,  elapsed time = 25 ms.

所以

Union在性能方面比Union All with Distinct要好得多

Answer 2

另一个说明四种可能情况的例子：

/* with each case we should expect a return set:
(1) DISTINCT UNION {1,2,3,4,5} - is redundant with case (2)
(2) UNION {1,2,3,4,5} - more efficient?
(3) DISTINCT UNION ALL {1,2,2,3,3,4,4,5} 
(4) UNION ALL {1,1,2,2,2,3,3,4,4,5} 
*/

declare @t1 table (c1 varchar(15));
declare @t2 table (c2 varchar(15));

insert into @t1 values ('1'),('1'),('2'),('3'),('4');

insert into @t2 values ('2'),('2'),('3'),('4'),('5');


select DISTINCT * from @t1 --case (1)
UNION
select DISTINCT * from @t2 order by c1

select * from @t1 --case (2)    
UNION
select * from @t2 order by c1

select DISTINCT * from @t1 --case (3)
UNION ALL
select DISTINCT * from @t2 order by c1

select * from @t1 --case (4)   
UNION ALL
select * from @t2 order by c1

Answer 3

UNION DISTINCT和UNION ALL之间的比较

此查询用于为下游系统创建带有附加备用 ID 的扩展员工表。 此示例来自 mySQL 8.0.20 环境。

对于下面显示的数据和查询，测试产生了显着差异：

UNION ALL       8.983 sec  
UNION DISTINCT 15.344 sec

为了显示此示例的规模和复杂性，表大小和查询代码如下所示

hqsource 600K records
accountingemppos 180K 
accountingposld 200K
emp_no_accountingnumeric 20
First UNION block is approx 550K records, second approx 50K

               SELECT a.`emp_no_imported` AS `emp_no`,
              a.`supervisor_emp_no`,
              a.`first name`,
              a.`middle name`,
              a.`last name`,
              a.`jobtitle`,
              a.`status`,
              CASE WHEN rida.`accounting_emp_no` IS NOT NULL THEN
                    rida.`accounting_emp_no`
              ELSE 
                    a.`emp_no_imported`
              END AS `accounting_id`,
              CASE WHEN epfp.`emp_no` IS NOT NULL THEN
                        CASE WHEN `sridf`.`emp_no` IS NOT NULL THEN
                                `sridf`.`accounting_emp_no`
                            ELSE
                                epfp.`emp_no`
                            END
                    ELSE
                        CASE WHEN epp.`emp_no` IS NOT NULL THEN
                            CASE WHEN `srids`.`emp_no` IS NOT NULL THEN
                                `srids`.`accounting_emp_no`
                            ELSE                                        
                                epp.`emp_no`
                            END
                    ELSE
                        CASE WHEN `srida`.`emp_no` IS NOT NULL THEN
                            `srida`.`accounting_emp_no`
                        ELSE
                            a.`supervisor_emp_no`
                        END
                    END
                END AS `accounting_s_emp_no`,
                ep.`emp_no` AS `traas_emp_no`,
                epp.`emp_no` AS `traas_parent_emp_no`
       FROM `hqsource`.hq_people a
       LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `rida` ON `rida`.emp_no = a.`emp_no_imported`
       LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `srida` ON `srida`.emp_no = a.`supervisor_emp_no`

       LEFT OUTER JOIN `traas`.`accountingemppos_data_extract` ep  ON ep.`emp_no` = a.`emp_no_imported` AND ep.`End` = '2899-12-31' AND ep.`Primary` = 'Y'
       LEFT OUTER JOIN `epe`.`accountingposld_data_extract` p ON p.`RangeGID` = ep.`GID`
       LEFT OUTER JOIN `traas`.`accountingemppos_data_extract` epp  ON epp.`GID` = p.`ParentGID` AND epp.`End` = '2899-12-31' AND epp.`Primary` = 'Y'
       LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `rids` ON `rids`.emp_no = ep.`emp_no`
       LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `srids` ON `srids`.emp_no = epp.`emp_no` AND epp.`End` = '2899-12-31' AND epp.`Primary` = 'Y'

       LEFT OUTER JOIN `epe`.`accountingemppos_data_extract_filtered` epf  ON epf.`emp_no` = a.`emp_no_imported` AND epf.`End` = '2899-12-31' AND epf.`Primary` = 'Y'
       LEFT OUTER JOIN `epe`.`accountingposld_data_extract` pf ON pf.`RangeGID` = epf.`GID` 
       LEFT OUTER JOIN `epe`.`accountingemppos_data_extract_filtered` epfp  ON epfp.`GID` = pf.`ParentGID` AND epfp.`End` = '2899-12-31' AND epfp.`Primary` = 'Y'
       LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `ridf` ON `ridf`.emp_no = epf.`emp_no` AND epf.`End` = '2899-12-31' AND epf.`Primary` = 'Y'
       LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `sridf` ON `sridf`.emp_no = epfp.`emp_no` AND epfp.`End` = '2899-12-31' AND epfp.`Primary` = 'Y'
       WHERE a.`emp_no_imported` REGEXP ('^[a-z]{2}\\d{5}.$') 

UNION ALL
-- UNION DISTINCT

    SELECT a.`emp_no_imported` AS `emp_no`, a.`supervisor_emp_no` AS `s_emp_no`, u.`First_Name`, 'ƒ' AS `MI`, u.`Last_Name`, u.`Job_Title`, NULL AS `status`, 
          CASE WHEN rid.`accounting_emp_no` IS NULL THEN
            ep.`emp_no`
          ELSE 
            rid.`accounting_emp_no`
          END AS `accounting_emp_no`,
          CASE WHEN `srid`.`accounting_emp_no` IS NULL THEN
            epp.`emp_no`
          ELSE
            `srid`.`accounting_emp_no`
          END AS `accounting_s_emp_no`,
          ep.`emp_no` AS `traas_emp_no`,
          epp.`emp_no` AS `traas_parent_emp_no`
    FROM `epe`.`accountingemppos_data_extract_filtered`  ep

    LEFT OUTER JOIN  `hqsource`.`hq_people` a ON a.`emp_no_imported` = ep.`emp_no`
    LEFT OUTER JOIN `epe`.`accountingposld_data_extract` p ON p.`RangeGID` =  ep.`GID`
    LEFT OUTER JOIN `epe`.`accountingemppos_data_extract_filtered` epp ON epp.`GID` = p.`ParentGID`
    LEFT OUTER JOIN `siebel`.`users_all_output` u ON u.`LOGIN` = ep.`emp_no`
    LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `rid` ON `rid`.emp_no = ep.`emp_no`
    LEFT OUTER JOIN `hqsource`.`emp_no_accountingnumeric` `srid` ON `srid`.emp_no = epp.`emp_no`
    WHERE 
        ep.`End` = '2899-12-31' AND
        epp.`End` = '2899-12-31' AND
        p.`End` = '2899-12-31' AND
        ep.emp_no REGEXP ('^F\\d{8}$|^V[0-3]\\d{5}$')
        
ORDER BY LENGTH(accounting_emp_no) ASC, accounting_emp_no ASC

;

两个 UNION 块中每个的 WHERE 子句保证结果是唯一的。 （这个查询已有多年历史并且每天运行。我希望我早点尝试过这个）。

字段名称已被混淆

Answer 4

所以今天我正在编写一个查询，“合并两个具有相同结构的查询，2 个 int cols 和一个 varchar 列 - 总宽度 40 字节..

4,403,063 行到 8,743,056 行中的另一行 生成的不同结果集为 8,141,350 行。

使用 UNION 在 30 到 40 秒之间连接两个查询 对临时表使用 UNION ALL，然后在临时表上使用 SELECT DISTINCT 10秒。

不要假设您会在数百万行中看到与数百或数千行相同的结果