假设一个表，10个列，字段列长度分布均匀。
一个表10G，那么一个列的索引+rowid,肯定是在1G+
例如有个场景：一个客户数据库，一个业务表，记录STATUS状态列，欠费A,正常B。
如果没到月底，只统计欠费的少量数据，如何创建相匹配的索引。
如果一个表有1个亿，欠费数量只有500时！ 如果常规创建索引SQL>create index cc_status on A(status); 索引将非常大！
此事，可以使用如下测试方式，进行索引瘦身：
1.优点，索引变小；
2.缺点：或者说使用索引的SQL需要改写或者与该函数索引相匹配。
SQL> conn tt/tt Connected. SQL> create table b as select * from dba_objects; SQL> select count(*) from b where status=‘INACTIVE‘; COUNT(*) ---------- 0 SQL> update b set status=‘INACTIVE‘ where rownum=1; update b set status=‘INACTIVE‘ where rownum=1 * ERROR at line 1: ORA-12899: value too large for column "TT"."B"."STATUS" (actual: 8, maximum: 7) SQL> alter table b modify STATUS varchar2(10); Table altered. SQL> update b set status=‘INACTIVE‘ where rownum=1; 1 row updated. SQL> commit; Commit complete. SQL> create index cc_ind_status on b(case when STATUS=‘INACTIVE‘ then 1 else null end); Index created. SQL> select count(*) from b where STATUS=‘INACTIVE‘; COUNT(*) ---------- 1 Execution Plan ---------------------------------------------------------- Plan hash value: 749587668 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 7 | 345 (1)| 00:00:05 | | 1 | SORT AGGREGATE | | 1 | 7 | | | |* 2 | TABLE ACCESS FULL| B | 14 | 98 | 345 (1)| 00:00:05 | --------------------------------------------------------------------------- select count(*) from b where (case when STATUS=‘INACTIVE‘ then 1 else null end)=1; SQL> select count(*) from b where (case when STATUS=‘INACTIVE‘ then 1 else null end)=1 COUNT(*) ---------- 1 Execution Plan ---------------------------------------------------------- Plan hash value: 1978997881 ----------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 3 | | | |* 2 | INDEX RANGE SCAN| CC_IND_STATUS | 1 | 3 | 1 (0)| 00:00:01 | ----------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access(CASE "STATUS" WHEN ‘INACTIVE‘ THEN 1 ELSE NULL END =1) Note ----- - dynamic sampling used for this statement (level=2)
二、对比大小
SQL> create index cc_ind_status_2 on b(status);
SQL>select SEGMENT_NAME,BYTES/1024/1024 from user_segments where SEGMENT_NAME like ‘CC_IND_STA%‘;
SEGMENT_NAME BYTES/1024/1024
-------------------- ---------------
CC_IND_STATUS .0625
CC_IND_STATUS_2 2
SQL> analyze table b compute statistics;
SQL> select INDEX_NAME,NUM_ROWS from user_indexes where INDEX_NAME like ‘CC_IND_STA%‘;
INDEX_NAME NUM_ROWS
------------------------------------------------------------ ----------
CC_IND_STATUS 1
CC_IND_STATUS_2 86341
原文：https://www.cnblogs.com/lvcha001/p/13357038.html