脚本之家 服务器常用软件
快捷导航
您的位置：首页 → 数据库 → Mysql → union all语句优化

SQL中Union All语句优化改写经验分享

 更新时间：2026年10月31日 10:05:08   作者：zcn126  
改写UNION ALL的核心思路是：默认用它替代 UNION,把过滤条件下推到每个分支,并让每个分支独立走索引,本文介绍SQL中Union All语句优化改写经验分享,感兴趣的朋友跟随小编一起看看吧

1、问题

前面几个案例是因为提前生成HEAP TABLE，条件没有提前过滤导致性能问题，今天想跟大家分享一个案例是适合做HEAP TABLE优化的。原语句如下：

SELECT * 
  FROM(SELECT INNER_TABLE.*, 
               ROWNUM OUTER_TABLE_ROWNUM 
          FROM (SELECT T1.CODE,
              case when T1.CODE='-9999' then '合计'
              else t2.cname end cname,
              nvl(firstnum,0)firstnum,
              nvl(secondnum,0)secondnum,
              nvl(thirdnum,0)thirdnum,
              nvl(fourthnum,0)fourthnum,
              nvl(fifthnum,0)fifthnum
              from ( select '-9999' code,
                 sum(case when status=1 then 1 else 0 end) firstnum,
                 sum(case when status=2 then 1 else 0 end) secondnum,
                 sum(case when status=3 then 1 else 0 end) thirdnum,
                 sum(case when status=4 then 1 else 0 end) fourthnum,
                 sum(case when status=5 then 1 else 0 end) fifthnum
                 from t1
                 where c1 like 'A1%'
                 union all
           select code,
                 sum(case when status=1 then 1 else 0 end) firstnum,
                 sum(case when status=2 then 1 else 0 end) secondnum,
                 sum(case when status=3 then 1 else 0 end) thirdnum,
                 sum(case when status=4 then 1 else 0 end) fourthnum,
                 sum(case when status=5 then 1 else 0 end) fifthnum
                 from t1
                 where c1 like 'A1%'
                 group by code) t1 left join t2 on t1.code=t2.code
               ORDER BY T1.CODE)INNER_TABLE ) OUTER_TABLE 
 WHERE OUTER_TABLE_ROWNUM<=200AND OUTER_TABLE_ROWNUM >0

计划：

从计划上对t1表过滤重复两次，从语句中可以看到

select '-9999' code,
                 sum(case when status=1 then 1 else 0 end) firstnum,
                 sum(case when status=2 then 1 else 0 end) secondnum,
                 sum(case when status=3 then 1 else 0 end) thirdnum,
                 sum(case when status=4 then 1 else 0 end) fourthnum,
                 sum(case when status=5 then 1 else 0 end) fifthnum
                 from t1
                 where c1 like 'A1%'
                 union all
           select code,
                 sum(case when status=1 then 1 else 0 end) firstnum,
                 sum(case when status=2 then 1 else 0 end) secondnum,
                 sum(case when status=3 then 1 else 0 end) thirdnum,
                 sum(case when status=4 then 1 else 0 end) fourthnum,
                 sum(case when status=5 then 1 else 0 end) fifthnum
                 from t1
                 where c1 like 'A1%'
                 group by code

是union all两部分，一部分是合计，另一部分是分组统计，此时我想到可以先分组统计，再把分组统计结果进行合计，分组统计的结果可以放到临时表中即heap table,从数学角度上讲就是提取公因子。

2、改写

SELECT * 
  FROM(SELECT INNER_TABLE.*, 
               ROWNUM OUTER_TABLE_ROWNUM 
          FROM (SELECT T1.CODE,
              case when T1.CODE='-9999' then '合计'
              else t2.cname end cname,
              nvl(firstnum,0)firstnum,
              nvl(secondnum,0)secondnum,
              nvl(thirdnum,0)thirdnum,
              nvl(fourthnum,0)fourthnum,
              nvl(fifthnum,0)fifthnum
              from (with temp as ( 
           select code,
                 sum(case when status=1 then 1 else 0 end) firstnum,
                 sum(case when status=2 then 1 else 0 end) secondnum,
                 sum(case when status=3 then 1 else 0 end) thirdnum,
                 sum(case when status=4 then 1 else 0 end) fourthnum,
                 sum(case when status=5 then 1 else 0 end) fifthnum
                 from t1
                 where c1 like 'A1%'
                 group by code)
                 select '-9999' code, sum(firstnum) firstnum,
                 sum(secondnum) secondnum, sum(thirdnum)thirdnum,
                 sum(fourthnum) fourthnum,sum(fifthnum) fifthnum
                 from temp t1
                 union all
                 select * from temp
                 ) t1 
                  left join t2 on t1.code=t2.code
               ORDER BY T1.CODE)INNER_TABLE ) OUTER_TABLE 
 WHERE OUTER_TABLE_ROWNUM<=200AND OUTER_TABLE_ROWNUM >0

计划：

结果符合预期，性能提升一倍以上。

3、小结

使用union all和group by rollup（前面案例中有提过-单表查询-group by rollup优化）进行数据统计的sql优化思路就是提取公因子，利用临时表（HEAP TABLE）减少扫描次数，从而提升性能。
前面讲过优化不是单向，前面是讲HEAP TABLE如何拆解优化，而这次是如何用HEAP TABLE减少扫描次数优化，优化做的事不过是对症下药。

4、附加测试数据

create table t1(id int primary key,c1 varchar2(20),status int,code varchar2(20));
insert into t1 select level,'A'||to_char(round(dbms_random.value(1,10000),0)),round(dbms_random.value(1,5),0),
'0'||to_char(round(dbms_random.value(300,322),0)) from dual connect by level<=300000;
commit;
dbms_stats.gather_table_stats(USER,'T1',null,100);
create table t2(id int primary key,cname varchar2(20),code varchar2(20));
insert into t2 select level,'A'||to_char(round(dbms_random.value(300,322),0)),
'0'||to_char(round(dbms_random.value(300,322),0)) from dual connect by level<=10000;
commit;
dbms_stats.gather_table_stats(USER,'T2',null,100);
create index IDX_DM_T1_C1 on T1(c1);
create index IDX_DM_t2_CODE on T2(CODE);

到此这篇关于SQL中Union All语句优化改写经验分享的文章就介绍到这了,更多相关union all语句优化内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！

您可能感兴趣的文章:

相关文章

最新评论