12C新特性：IN-MEMORY实战（2）执行计划的不同点与v$im_segments

查看inmemory的表，执行计划中会有相应的标识例如全表扫描为TABLE ACCESS INMEMORY FULL，如下：

SQL> create table test1 tablespace users as select * from dba_objects;

Table created.

SQL> alter table test1 inmemory;
SQL> set autotrace on
SQL>  set autotrace traceonly
SQL> select count(*) from test1;

Execution Plan
----------------------------------------------------------
Plan hash value: 3896847026

-----------------------------------------------------------------------------
| Id  | Operation                   | Name  | Rows  | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |       |     1 |    15   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE             |       |     1 |            |          |
|   2 |   TABLE ACCESS INMEMORY FULL| TEST1 | 72573 |    15   (0)| 00:00:01 |
-----------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        552  bytes sent via SQL*Net to client
        388  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

在gv$im_segments中可以看到inmemory使用情况

SQL> col SEGMENT_NAME for a5
SQL> col POPULATE_STATUS for a10
SQL> col INMEMORY_DISTRIBUTE for a10
SQL> col INMEMORY_DUPLICATE for a20
SQL> set linesize 200
SQL> select INST_ID,SEGMENT_NAME,INMEMORY_SIZE/1024/1024,BYTES/1024/1024,BYTES_NOT_POPULATED/1024/1024,INMEMORY_DISTRIBUTE,INMEMORY_DUPLICATE,POPULATE_STATUS from gv$im_segments;

   INST_ID SEGME INMEMORY_SIZE/1024/1024 BYTES/1024/1024 BYTES_NOT_POPULATED/1024/1024 INMEMORY_D INMEMORY_DUPLICATE   POPULATE_S
---------- ----- ----------------------- --------------- ----------------------------- ---------- -------------------- ----------
         1 TEST1                  4.4375       11.015625                             0 AUTO       NO DUPLICATE         COMPLETED

INMEMORY_SIZE：segment在in-memory中占用的内存数量，单位bytes
BYTES：在内存中展示的segment，对应的磁盘上数据量（不包括元数据块），单位bytes
BYTES_NOT_POPULATED：磁盘的segment中未填充到内存中的部分的大小，单位bytes。
结合test1表的情况就是：在硬盘上为11MB的segment，带入in-memory压缩后占用内存的大小为4.4MB，该test1表的数据还有0MB没带入到内存中。