Oracle where子句性能不佳

Question

我有以下Oracle表:

create table my_table(
   start int,
   end int
);
insert into my_table values(1, 3);
insert into my_table values(5, 7);
insert into my_table values(11, 200);
insert into my_table values(311, 5000);
insert into my_table values(60004, 60024);
insert into my_table values(123213, 12312312);

这个表有1M行并存储数字范围('start'， 'end')，所有数字都是唯一的，它没有重复的范围，任何数字只能在这个表的一个范围内，我有以下查询，它传递一个变量my_number来标识范围的'开始'。

 execute immediate 
    'select start from my_table where :1 between start and end' using my_number

我已经建立了两个字段的组合索引。问题是，当my_number较小时，查询性能良好，但当my_number增加时，查询时间不断增加。如果my_number大得多，则需要花费相当长的时间来完成。有人有办法改进这个问题吗?方法包括重新设计my_table。谢谢。

Answer 1

如果您将模式更改为:

create table my_table(
   start int,
   range_size int
);
insert into my_table values(1, 2);
insert into my_table values(5, 2);
insert into my_table values(11, 189);
insert into my_table values(311, 4689);
insert into my_table values(60004, 20);
insert into my_table values(123213, 12300001);

则只能在start列上建立索引。

execute immediate 
    'select start from (select start, range_size from my_table where start < :1 order by start asc limit 1) tmp where :1 < start+range_size' using my_number

这可能会有一些性能提升

Answer 2

你做了吗?

create table my_table(
start int,
end int
constraint PK_comp primary key (start, end)
) ;

Answer 3

我认为你应该做两个索引，一个在开始列，一个在结束列。然后不要选择between选项，而是选择大于开始，小于结束的位置。然后，您将为每个where子句使用索引。

Answer 4

这是一个试图愚弄Oracle的案例，使其表现得像其竞争对手一样，而没有访问Oracle的权限，我只是猜测。也许自连接可以做到这一点?在每个列上分别设置索引，

SELECT t1.start
FROM my_table t1 JOIN my_table t2
ON t1.start=t2.start AND t2."end"=t1."end"
AND t1.start <= :1
AND t2.end >= :1

这看起来很傻，但Joe Frambach的解决方案很简单。它欺骗Postgres，我有，只做索引搜索。

顺便说一句，Postgres对end作为列名很不满意。

Answer 5

为每一列创建一个索引，并使用如下查询:

select start_num
from my_table
where
    start_num =
    (
        --Last start <= number
        select start_num
        from
        (
            select start_num
            from my_table
            where :1 >= start_num
            order by start_num desc
        )
        where rownum = 1
    ) and
    end_num =
    (
        --First end >= number
        select end_num
        from
        (
            select end_num
            from my_table
            where :1 <= end_num
            order by end_num
        )
        where rownum = 1
    );

恶心。可能有更好的写法。或者你可以把它封装在一个函数中。

测试数据(非保留字列名):

drop table my_table;
create table my_table(
   start_num int,
   end_num int
);
insert into my_table select level*2,level*2+1 from dual connect by level <= 1000000;
commit;
create index my_table_index on my_table(start_num, end_num);
begin
    dbms_stats.gather_table_stats(user, 'MY_TABLE', no_invalidate => false);
end;
/

低数字几乎是瞬时的- 0.015秒

select start_num from my_table where 2 between start_num and end_num;

数字越大越慢- 0.125秒

select start_num from my_table where 1000000 between start_num and end_num;

范围扫描和全表扫描之间有一个点

explain plan for select start_num from my_table where 402741 between start_num and end_num;
select * from table(dbms_xplan.display);
Plan hash value: 3804444429
------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |   160K|  1570K|   622   (2)| 00:00:08 |
|*  1 |  TABLE ACCESS FULL| MY_TABLE |   160K|  1570K|   622   (2)| 00:00:08 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("START_NUM"<=402742 AND "END_NUM">=402742)

explain plan for select start_num from my_table where 402742 between start_num and end_num;
select * from table(dbms_xplan.display);

Plan hash value: 3804444429
------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |   160K|  1570K|   622   (2)| 00:00:08 |
|*  1 |  TABLE ACCESS FULL| MY_TABLE |   160K|  1570K|   622   (2)| 00:00:08 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("START_NUM"<=402742 AND "END_NUM">=402742)

但问题不在于Oracle不使用索引。以一种幼稚的方式使用索引并没有帮助。实际上，这甚至更慢，为0.172秒:

select /*+ index(my_table my_table_index) */ start_num
from my_table
where 1000000 between start_num and end_num;

<<p> 解决方案/strong>

创建新索引:

drop index my_table_index;
create index my_table_index1 on my_table(start_num);
create index my_table_index2 on my_table(end_num);
begin
    dbms_stats.gather_table_stats(user, 'MY_TABLE', no_invalidate => false);
end;
/

结果仍然是即时的，对于任何数字:

select start_num
from my_table
where
    start_num =
    (
        --Last start <= number
        select start_num
        from
        (
            select start_num
            from my_table
            where 1000000 >= start_num
            order by start_num desc
        )
        where rownum = 1
    ) and
    end_num =
    (
        --First end >= number
        select end_num
        from
        (
            select end_num
            from my_table
            where 1000000 <= end_num
            order by end_num
        )
        where rownum = 1
    );

这个计划看起来很棒——这可能是你能得到的最好的性能。

Plan hash value: 522166032
--------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |                 |     1 |    10 |    10   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS BY INDEX ROWID   | MY_TABLE        |     1 |    10 |     4   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN             | MY_TABLE_INDEX2 |     1 |       |     3   (0)| 00:00:01 |
|*  3 |    COUNT STOPKEY               |                 |       |       |            |          |
|   4 |     VIEW                       |                 |     3 |    39 |     3   (0)| 00:00:01 |
|*  5 |      INDEX RANGE SCAN          | MY_TABLE_INDEX2 |     3 |    18 |     3   (0)| 00:00:01 |
|*  6 |   COUNT STOPKEY                |                 |       |       |            |          |
|   7 |    VIEW                        |                 |     2 |    26 |     3   (0)| 00:00:01 |
|*  8 |     INDEX RANGE SCAN DESCENDING| MY_TABLE_INDEX1 |   500K|  2929K|     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("START_NUM"= (SELECT "START_NUM" FROM  (SELECT "START_NUM" "START_NUM" FROM 
              "MY_TABLE" "MY_TABLE" WHERE "START_NUM"<=1000000 ORDER BY "START_NUM" DESC) 
              "from$_subquery$_002" WHERE ROWNUM=1))
   2 - access("END_NUM"= (SELECT "END_NUM" FROM  (SELECT "END_NUM" "END_NUM" FROM 
              "MY_TABLE" "MY_TABLE" WHERE "END_NUM">=1000000 ORDER BY "END_NUM") "from$_subquery$_004" 
              WHERE ROWNUM=1))
   3 - filter(ROWNUM=1)
   5 - access("END_NUM">=1000000)
   6 - filter(ROWNUM=1)
   8 - access("START_NUM"<=1000000)