Oracle where子句的性能不佳

时间:2013-04-04 17:56:47

标签: sql database performance oracle oracle11g

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);

此表有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

我在两个字段上构建了组合索引。问题是当my_number很小时,查询的性能很好,但是当my_number增加时,查询时间会不断增加。如果my_number更大,则需要花费相当长的时间才能完成。有人有办法改进这个查询吗?方式可以包括重新设计my_table。谢谢。

5 个答案:

答案 0 :(得分:2)

如果您将架构更改为:

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);

然后,您只能在start列上编制索引。

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。

答案 1 :(得分:0)

你做完了吗?

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

答案 2 :(得分:0)

我认为你应该制作两个索引,一个在开始列,一个在结束列。 然后选择不使用between选项,但选择大于start然后小然后结束。 然后,您将为每个where子句使用索引。

我希望这有助于提升绩效。

答案 3 :(得分:0)

这是一个试图欺骗甲骨文表现得像竞争对手的情况,而且无法访问甲骨文,我只是在猜测。也许自我加入可以做到这一点?每列上的索引

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,但直截了当的解决方案是Joe Frambach。它愚弄Postgres,我确实只做索引搜索。

BTW,Postgres对end作为列名非常不满。我希望你的真实桌子不会在那里使用保留字。

答案 4 :(得分:0)

为每列创建索引并使用此查询:

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
    );

呸。写这个可能是更好的方法。或者您可能希望将其包装在函数中。

问题

测试数据(带有非保留字列名称):

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;
/

低数字几乎是瞬间的 - 0.015秒

select start_num from my_table where 2 between start_num and end_num;

较大的数字较慢 - 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只有一个点。

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。以天真的方式使用索引并没有帮助。事实上,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;

<强>解决方案

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;
/

对于任何数字,结果都是即时的:

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
    );

该计划看起来很棒 - 这可能是您可以获得的最佳表现。

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)