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  • index unique scan

    2014-01-26 20:29:00
    INDEX UNIQUE SCAN 索引唯一扫描。单块读 只可能发生在unique index/primary key 等值查找 等待事件:db file sequential read 但是你几乎看不到,因为只读一条数据 HINT 无需指定,有索引会自动走INDEX UNIQUE ...

       INDEX UNIQUE SCAN 索引唯一扫描。单块读 只可能发生在unique index/primary key 等值查找
                         等待事件:db file sequential read 但是你几乎看不到,因为只读一条数据
                         HINT 无需指定,有索引会自动走INDEX UNIQUE SCAN


    唯一索引等值扫描,需要几个I/O?

    需要索引高度个IO

    转载于:https://www.cnblogs.com/zhaoyangjian724/p/3797938.html

    展开全文
  • Add Index unique

    2021-01-06 14:21:45
    <div><h1>880 <pre><code>python ks.DataFrame({'...[1, 1, 3]).index.unique() Int64Index([1, 3], dtype='int64') </code></pre>该提问来源于开源项目:databricks/koalas</p></div>
  • Index Unique Scan唯一索引(B-tree) Index unique scan is one of the most efficient ways of accessing data. This access method is used for returning the data from B-tree indexes. The optimizer c




    Index Unique Scan唯一索引(B-tree)







    Index unique scan is one of the most efficient ways of accessing data. This access method is used for returning the data from B-tree indexes. The optimizer chooses a unique scan when all columns of a unique (B-tree) index are specified with equality conditions.








    Index Range Scan 范围(升序 相同值根据ROWID升)







    Index range scan is a common operation for accessing selective data. It can be bounded (bounded on both sides) or unbounded (on one or both sides). Data is returned in the ascending order of index columns. Multiple rows with identical values are sorted (in ascending order) by the ROWIDs.








    Bitmap Indexes(节省空间;不适合修改多的事务操作;不适合范围条件的查询,适合相等条件的查询;适用类型手机号;可以有NULL值跟其他索引不一样)







    The purpose of an index is to provide pointers to the rows in a table that contain a given key value. In a regular index, this is achieved by storing a list of rowids for each key corresponding to the rows with that key value. Oracle stores each key value repeatedly with each stored rowid. In a bitmap index, a bitmap for each key value is used instead of a list of rowids.




     




    http://docs.oracle.com/cd/B19306_01/server.102/b14220/schema.htm#sthref984




    两篇还不错的文章:
    http://blog.csdn.net/robinson1988/article/details/4980611


    http://www.cnblogs.com/tracy/archive/2011/09/02/2163462.html

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  • 索引唯一性扫描(INDEX UNIQUE SCAN)是针对唯一性索引(UNIQUE INDEX)的扫描,它仅仅适用于where条件里是等值查询的目标SQL。因为扫描的对象是唯一性索引,所以索引唯一性扫描的结果至多只会返回一条记录。 Emp表...
     

         索引唯一性扫描(INDEX UNIQUE SCAN)是针对唯一性索引(UNIQUE INDEX)的扫描,它仅仅适用于where条件里是等值查询的目标SQL。因为扫描的对象是唯一性索引,所以索引唯一性扫描的结果至多只会返回一条记录。

    Emp表中empno为primary key,对应一个unique index


    SCOTT@PDBORCL> select * from emp where empno=7369;


    执行计划
    ----------------------------------------------------------
    Plan hash value: 2949544139

    --------------------------------------------------------------------------------------
    | Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
    --------------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT            |        |     1 |    38 |     1   (0)| 00:00:01 |
    |   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     1 |    38 |     1   (0)| 00:00:01 |
    |*  2 |   INDEX UNIQUE SCAN         | PK_EMP |     1 |       |     0   (0)| 00:00:01 |
    --------------------------------------------------------------------------------------

    Predicate Information (identified by operation id):
    ---------------------------------------------------

       2 - access("EMPNO"=7369)


    统计信息
    ----------------------------------------------------------
             69  recursive calls
              0  db block gets
             95  consistent gets
             21  physical reads
              0  redo size
            898  bytes sent via SQL*Net to client
            533  bytes received via SQL*Net from client
              1  SQL*Net roundtrips to/from client
              8  sorts (memory)
              0  sorts (disk)
              1  rows processed

    SCOTT@PDBORCL>

    View Code

    image

    从上述显示内容可以看出,"select * from emp where empno=7369"的执行计划走的是索引唯一性扫描

    转载于:https://my.oschina.net/wangshengzhuang/blog/785041

    展开全文
  • index unique scan ,是因为建了唯一索引, unique如 :create unique index AA_INDEX on AA_TEST (SID, ITEMVALUE)在执行语句中 where子句必须包括 唯一索引中的所有字段,而不用管select是否是索引中的字段.analyze ...

    index unique scan ,是因为建了唯一索引, unique

    如 :create unique index AA_INDEX on AA_TEST (SID, ITEMVALUE)

    在执行语句中 where子句必须包括 唯一索引中的所有字段,而不用管select是否是索引中的字段.

    analyze table aa_test compute statistics;

    select sid,itemvalue from aa_test t where sid=1;

    这个用 index range scan

    select sid,itemvalue from aa_test t where itemvalue ='100';

    这个用index full scan

    也就是说如果在where子句中没有选择 唯一索引的前置索引字段,而是唯一索引的其他的字段,就会变成

    index full scan

    而对其他索引(不是唯一索引)中,只用索引中的某个字段(不是前置),只是 index range scan,不是index full scan,

    从某些文档中知道 index full scan是index range scan的一个极限,当从index的叶子从左找到右时,就是index full scan.

    下面是强制用 跳跃索引

    select /*+ index_ss(aa_t inde_aa_t) */ aa_t.object_type
    from aa_t where object_id =2

     

     

    from :http://space.itpub.net/12712263/viewspace-606960

    展开全文
  • INDEX UNIQUE SCAN 如果表上有唯一索引, 搜索索引列时会用上INDEX UNIQUE SCAN 原来Index Unique Scan和I...
  • index unique scan 与index range scan等的区别  存取Oracle当中扫描数据的方法(一) Oracle 是一个面向Internet计算环境的数据库。它是在数据库领域一直处于领先地位的甲骨文公司的产品。可以说Oracle关系...
  • 关于oracle index unique scan/index range scan和mysql range/const/ref/eq_ref type的区别 关于ORACLE index unique scan和index range scan区别在于是否索引是唯一的,如果=操作谓词有唯一索引则使用unique s...
  • 一开始给一个列设置了unique,然后将该列重命名,之后想要移除该列unique的时候发生error:check that column/key exists 原因:虽然,名字和非空性已经改完成,但是这个unique属性的所有者还是old_column不是new_...
  • Index Unique Scan (213)

    2007-11-17 15:24:55
    Index unique scan is one of the most efficient ways of accessing data. This accessmethod is used for returning the...
  • Index Unique Scan 索引唯一扫描   (page 78)  An index unique scan is chosen when a predicate contains a condition using a column defined with a UNIQUE or PRIMARY KEY index. These types of ...
  • 高效的SQL(Index unique scan最优化)   1、 实验表 test 唯一约束ID,test1 主键 ID 。(唯一约束与主键会自动创建索引)。另外建立一张没有约束的表test2与test1数据相同,但是没有主键约束。 Test表 ...
  • (1) 索引唯一扫描(index unique scan) 通过唯一索引查找一个数值经常返回单个ROWID。如果存在UNIQUE 或PRIMARY KEY 约束(它保证了语句只存取单行)的话,Oracle经常实现唯一性扫描。 使用唯一性约束的例子:      ...
  • 12:29:15 SQL> create table t1(a int); 表已创建。 已用时间:00: 00: 00.31 12:29:21 SQL> create unique index te...
  • Question:What is between between "unique index vs non-unique index". Which one is faster. The customer using non-unique and sql is getting delay If we change tp non-unique. Is i...
  • unique index

    2006-08-18 08:50:00
    unique index 是指字段中的值除了为NULL外, 不能重复. 而非unique index则无此限制. 其实unique index在性能上也比较好,找到第一个符合条件的终止索引扫描......
  • Unique Index Name

    2020-11-30 14:59:24
    <div><p>Is a unique index name required for a shared server? <p>If I create an index on an application and use the same on another will they clash?</p><p>该提问来源于开源项目:FriendsOfSymfony/...
  • Unique Index Selection

    2020-12-08 23:02:49
    <div><p>I want to be able to select unique index number, and popup all the information related to row. </p><p>该提问来源于开源项目:hkalbertl/jquery.appendGrid</p></div>
  • Unique index migration

    2021-01-06 23:41:55
    CREATE UNIQUE INDEX "index_clients_on_group_id_and_deleted_at" ON "clients" ("group_id", "COALESCE(deleted_at, false)") </code></pre> <p>The <code>COALESCE</code> ...

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