表和索引在数据压缩前后空间占用情况的对比

创建压缩表

create table t_compress as select * from dba_objects;

创建非压缩表

create table t_uncompress as select * from dba_objects;

占用空间对比

select segment_name,extent_id,bytes from user_extents where segment_name in ('T_COMPRESS')

select segment_name,extent_id,bytes from user_extents where segment_name in ('T_UNCOMPRESS')

可见未压缩的表extent数目为21 压缩过的表为17 占用空间前者是后者的3倍。

索引的压缩：先建表

创建索引

create table t1 as select * from dba_objects;

create table t2 as select * from dba_objects;

create index ind_t1_compress on t1(object_name) compress;


create index ind_t2_uncompress on t2(object_name);

查询占用空间情况

select segment_name,extent_id,bytes from user_extents where segment_name in ('IND_T1_COMPRESS');
select segment_name,extent_id,bytes from user_extents where segment_name in ('IND_T2_UNCOMPRESS');

可能是测试的数据不够多 这里面发现两个占得空间几乎一样，但是查询叶子节点的数目

发现压缩过后叶子节点减少了五分之一

select index_name,compression,leaf_blocks from user_indexes where index_name in('IND_T1_COMPRESS','IND_T2_UNCOMPRESS')

列重复率高的情况下：先建表

create table t1 as select object_id,object_name from dba_objects;

create table t2 as select 1 object_id,object_name from dba_objects;

create index ind_t1_compress on t1(object_id) compress;


create index ind_t2_compress on t2(object_id) compress;

创建压缩的索引，对比

select segment_name,extent_id,bytes from user_extents where segment_name in ('IND_T1_COMPRESS');


select segment_name,extent_id,bytes from user_extents where segment_name in ('IND_T2_COMPRESS');

可见重复率高的t2表的索引比t1占用的空间少了6个extents