再对比一下压缩表和非压缩表在读方面的性能：

SQL> alter session set sql_trace=true;
 
Session altered.
 
SQL> select count(*) from test_nocompress;
 
  COUNT(*)
----------
     64000
 
SQL> select count(*) from test_compress;
 
  COUNT(*)
----------
     64000

查看生成的trace文件
select count(*)
from
 test_nocompress
 
 
call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.01       0.00          0          0          0           0
Fetch        2      0.24       0.23        604        608          0           1
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        4      0.25       0.24        604        608          0           1
 
Misses in library cache during parse: 1
Optimizer goal: CHOOSE
Parsing user id: SYS
 
Rows     Row Source Operation
-------  ---------------------------------------------------
      1  SORT AGGREGATE (cr=608 r=604 w=0 time=238185 us)
  64000   TABLE ACCESS FULL TEST_NOCOMPRESS (cr=608 r=604 w=0 time=157204 us)
 
********************************************************************************
 
select count(*)
from
 test_compress
 
 
call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        2      0.18       0.17         89         94          0           1
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        4      0.18       0.17         89         94          0           1
 

可以看出，压缩表在读性能上还是有明显优势