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  • [ORALCE]SQL 优化案例之 索引的聚合因子clustering factor

    索引查询要尽可能的避免回表,如不可避免,要关注聚合因子是否过大,聚合因子过大,回表代价高,产生的bufer 多

    第一步,构造有序列x在表"colocated ",无序列x在表"disorganized

    --构造有序列x在表"colocated "
    drop table colocated  purge;
    create table colocated (x int,y varchar2(80));
    begin
        for i in 1 .. 100000
        loop
            insert into colocated(x,y)
            values (i,rpad(dbms_random.random,75,'*'));
        end loop;
    end;
    /
    
    alter table colocated add constraint colocated_pk primary key (x);
    begin
        dbms_stats.gather_table_stats( 'SYS', 'colocated', cascade => true);
    end;
    /
    --disorganized 列x完全无序
    drop table disorganized purge;
    create table disorganized as select x,y from colocated order by y;
    alter table disorganized add constraint disorganized_pk primary key(x);
    begin
        dbms_stats.gather_table_stats('SYS','disorganized',cascade=>true);
    end;
    /

    第二步 比较

    第三步  查看clustering_factor

    col index_name for A15
    select
        a.table_name,
        a.index_name,
        a.blevel,
        a.leaf_blocks,
        b.num_rows,
        b.blocks,
        a.clustering_factor,
        trunc(a.clustering_factor / b.num_rows,2) cluster_rate
    from dba_indexes a, dba_tables b
    where index_name in('COLOCATED_PK','DISORGANIZED_PK') and a.table_name=b.table_name;
    
    TABLE_NAME          INDEX_NAME      BLEVEL LEAF_BLOCKS   NUM_ROWS  BLOCKS   CLUSTERING_FACTOR    CLUSTER_RATE
    ----------------- --------------- ------ -----------   --------- -------    ------------------   ------------
    COLOCATED          COLOCATED_PK        1     208           100000     1191           1190                 .01
    DISORGANIZED       DISORGANIZED_PK     1     208           100000     1191          99913                 .99




     

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  • 原文地址:https://www.cnblogs.com/tingxin/p/12852630.html
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