Exadata混合列压缩如何处理INSERT和UPDATE

Hybrid Columnar Compression混合列压缩是Exadata数据库一体机的核心功能之一，与普通的高级压缩特性(advanced compression)不同，Hybrid columnar compression (HCC) 仅仅在Exadata平台上可用。使用HCC的情况下数据压缩存放在CU(compression unit压缩单位中)，一个CU单位包括多个数据库块，这是出于单数据块不利于以列值压缩算法的考量所决定的，当一个CU包含多个block时可以实现较优的列值压缩算法。

同时对于普通的INSERT/UPDATE操作，需要造成对行级数据的压缩降级，即在经历UPDATE/INSERT后原本HCC压缩的行可能变成普通高级压缩的水平。

hybrid columnar compression与数据仓库行为的批量初始化导入(bulk initial load)配合，直接路径导入(direct load)例如ALTER TABLE MOVE, IMPDP或直接路径插入(append INSERT)，使用HCC的前提是这些数据将不会被频繁修改或从不被修改。

当你更新混合列压缩启动的表中的数据行时，相关整个的压缩单位CU中的数据将被锁住。被更新的这些数据将不得不从原HCC压缩级别降级到例如无压缩或for OLTP压缩的水准。

我们来看以下例子：

SQL*Plus: Release 11.2.0.2.0 Production on Wed Sep 12 06:14:53 2012

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
With the Partitioning, Automatic Storage Management, OLAP, Data Mining
and Real Application Testing options

SQL> grant dba to scott;

Grant succeeded.

SQL> conn scott/oracle
Connected.
SQL> 
SQL> create table hcc_maclean tablespace users compress for query high as select * from dba_objects;

Table created.

  1* select rowid,owner,object_name,dbms_rowid.rowid_block_number(rowid) from hcc_maclean where owner='MACLEAN'
SQL> /

ROWID                          OWNER                          OBJECT_NAME          DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID)
------------------------------ ------------------------------ -------------------- ------------------------------------
AAAThuAAEAAAHTJAOI             MACLEAN                        SALES                                               29897
AAAThuAAEAAAHTJAOJ             MACLEAN                        MYCUSTOMERS                                         29897
AAAThuAAEAAAHTJAOK             MACLEAN                        MYCUST_ARCHIVE                                      29897
AAAThuAAEAAAHTJAOL             MACLEAN                        MYCUST_QUERY                                        29897
AAAThuAAEAAAHTJAOh             MACLEAN                        COMPRESS_QUERY                                      29897
AAAThuAAEAAAHTJAOi             MACLEAN                        UNCOMPRESS                                          29897
AAAThuAAEAAAHTJAOj             MACLEAN                        CHAINED_ROWS                                        29897
AAAThuAAEAAAHTJAOk             MACLEAN                        COMPRESS_QUERY1                                     29897

8 rows selected.

select dbms_rowid.rowid_block_number(rowid),dbms_rowid.rowid_relative_fno(rowid) from hcc_maclean where owner='MACLEAN';

session A:

update hcc_maclean set OBJECT_NAME=OBJECT_NAME||'DBM' where rowid='AAAThuAAEAAAHTJAOI';

session B:

update hcc_maclean set OBJECT_NAME=OBJECT_NAME||'DBM' where rowid='AAAThuAAEAAAHTJAOJ';

SQL> select sid,wait_event_text,BLOCKER_SID from v$wait_chains;

       SID WAIT_EVENT_TEXT                                                  BLOCKER_SID
---------- ---------------------------------------------------------------- -----------
        13 enq: TX - row lock contention                                            136
       136 SQL*Net message from client

可以看到session A block B，这验证了HCC压缩后update row所在CU会造成整个CU被锁住的说法

SQL> alter system checkpoint;

System altered.

SQL> /     

System altered.

SQL> alter system dump datafile 4 block 29897
  2  ;

  Block header dump:  0x010074c9
 Object id on Block? Y
 seg/obj: 0x1386e  csc: 0x00.1cad7e  itc: 3  flg: E  typ: 1 - DATA
     brn: 0  bdba: 0x10074c8 ver: 0x01 opc: 0
     inc: 0  exflg: 0

 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0xffff.000.00000000  0x00000000.0000.00  C---    0  scn 0x0000.001cabfa
0x02   0x000a.00a.00000430  0x00c051a7.0169.17  ----    1  fsc 0x0000.00000000
0x03   0x0000.000.00000000  0x00000000.0000.00  ----    0  fsc 0x0000.00000000

avsp=0x14
tosp=0x14
        r0_9ir2=0x0
        mec_kdbh9ir2=0x0
                      76543210
        shcf_kdbh9ir2=----------
                  76543210
        flag_9ir2=--R-----      Archive compression: Y
                fcls_9ir2[0]={ }
0x16:pti[0]     nrow=1  offs=0
0x1a:pri[0]     offs=0x30
block_row_dump:
tab 0, row 0, @0x30
tl: 8016 fb: --H-F--N lb: 0x2  cc: 1          ==>整个CU指向ITL 0x02
nrid:  0x010074ca.0
col  0: [8004]
Compression level: 02 (Query High)
 Length of CU row: 8004
kdzhrh: ------PC CBLK: 1 Start Slot: 00
 NUMP: 01
 PNUM: 00 POFF: 7984 PRID: 0x010074ca.0
CU header:
CU version: 0   CU magic number: 0x4b445a30
CU checksum: 0xf8faf86e
CU total length: 8694
CU flags: NC-U-CRD-OP
ncols: 15
nrows: 995
algo: 0
CU decomp length: 8487   len/value length: 100111
row pieces per row: 1
num deleted rows: 1
deleted rows: 904,
START_CU:

我们可以使用如下方式衡量row的压缩情况：

SQL> select DBMS_COMPRESSION.GET_COMPRESSION_TYPE('SCOTT','HCC_MACLEAN','AAAThuAAEAAAHTJAOk') from dual;

DBMS_COMPRESSION.GET_COMPRESSION_TYPE('SCOTT','HCC_MACLEAN','AAATHUAAEAAAHTJAOK'
--------------------------------------------------------------------------------
                                                                               4

COMP_NOCOMPRESS CONSTANT NUMBER := 1;
COMP_FOR_OLTP CONSTANT NUMBER := 2;
COMP_FOR_QUERY_HIGH CONSTANT NUMBER := 4;
COMP_FOR_QUERY_LOW CONSTANT NUMBER := 8;
COMP_FOR_ARCHIVE_HIGH CONSTANT NUMBER := 16;
COMP_FOR_ARCHIVE_LOW CONSTANT NUMBER := 32;

COMP_RATIO_MINROWS CONSTANT NUMBER := 1000000;
COMP_RATIO_ALLROWS CONSTANT NUMBER := -1;

上表列出了压缩类型的常数值，例如COMP_FOR_QUERY_HIGH是4，COMP_FOR_QUERY_LOW 是8

这里我们从上述查询GET_COMPRESSION_TYPE指定rowid的情况下得到的是4说明该列以COMP_FOR_QUERY_HIGH形式压缩：

SQL>  update hcc_maclean set OBJECT_NAME=OBJECT_NAME||'DBM' where owner='MACLEAN';

8 rows updated.

SQL> commit;

Commit complete.




SQL>  select DBMS_COMPRESSION.GET_COMPRESSION_TYPE('SCOTT','HCC_MACLEAN',rowid) from HCC_MACLEAN where owner='MACLEAN';

DBMS_COMPRESSION.GET_COMPRESSION_TYPE('SCOTT','HCC_MACLEAN',ROWID)
------------------------------------------------------------------
                                                                 1
                                                                 1
                                                                 1
                                                                 1
                                                                 1
                                                                 1
                                                                 1
                                                                 1

8 rows selected.

以上更新一定量数据后可以看到COMPRESSION_TYPE由COMP_FOR_QUERY_HIGH降级为COMP_NOCOMPRESS，这说明这张表虽然compress for query high但部分数据在更新后实际不再被压缩。

在11g中这些非压缩态复萌的数据行不会自动升级成HCC状态。必要的时候手动作 ALTER TABLE MOVE或在线重定义以便将非压缩态的数据转换回HCC状态。

SQL>  ALTER TABLE hcc_MACLEAN move COMPRESS FOR ARCHIVE HIGH;

Table altered.

SQL> select DBMS_COMPRESSION.GET_COMPRESSION_TYPE('SCOTT','HCC_MACLEAN',rowid) from HCC_MACLEAN where owner='MACLEAN';

DBMS_COMPRESSION.GET_COMPRESSION_TYPE('SCOTT','HCC_MACLEAN',ROWID)
------------------------------------------------------------------
                                                                16
                                                                16
                                                                16
                                                                16
                                                                16
                                                                16
                                                                16
                                                                16

8 rows selected.