在使用压缩之前,我们可以估算一下使用压缩能够拥有多大的效果。 11gr2以前可以使用dbms_comp_advisor,具体代码已经在附件中给出。只需要执行两个文件dbmscomp.sql和prvtcomp.plb,然后使用DBMS_COMP_ADVISOR.getratio存储过程即可。不再详细描述。 SQL set
在使用压缩之前,我们可以估算一下使用压缩能够拥有多大的效果。
11gr2以前可以使用dbms_comp_advisor,具体代码已经在附件中给出。只需要执行两个文件dbmscomp.sql和prvtcomp.plb,然后使用DBMS_COMP_ADVISOR.getratio存储过程即可。不再详细描述。
SQL> set serveroutput on SQL> execdbms_comp_advisor.getratio('SH','SALES',10) Sampling table: SH.SALES Sampling percentage: 10% Estimated compression ratio for the advancedcompression option is : 2.96
11gr2以后系统会自带一个dbms_compression的包,用来代替dbms_comp_advisor提供服务。
_sys@FAKE> desc dbms_compression PROCEDURE GET_COMPRESSION_RATIO ArgumentName Type In/Out Default? ----------------------------------------------------- ------ -------- SCRATCHTBSNAME VARCHAR2 IN OWNNAME VARCHAR2 IN TABNAME VARCHAR2 IN PARTNAME VARCHAR2 IN COMPTYPE NUMBER IN BLKCNT_CMP BINARY_INTEGER OUT BLKCNT_UNCMP BINARY_INTEGER OUT ROW_CMP BINARY_INTEGER OUT ROW_UNCMP BINARY_INTEGER OUT CMP_RATIO NUMBER OUT COMPTYPE_STR VARCHAR2 OUT SUBSET_NUMROWS NUMBER IN DEFAULT FUNCTION GET_COMPRESSION_TYPE RETURNS NUMBER ArgumentName Type In/Out Default? ----------------------------------------------------- ------ -------- OWNNAME VARCHAR2 IN TABNAME VARCHAR2 IN ROW_ID ROWID IN PROCEDURE INCREMENTAL_COMPRESS ArgumentName Type In/Out Default? ----------------------------------------------------- ------ -------- OWNNAME VARCHAR2(30) IN TABNAME VARCHAR2(128) IN PARTNAME VARCHAR2(30) IN COLNAME VARCHAR2 IN DUMP_ON NUMBER IN DEFAULT AUTOCOMPRESS_ON NUMBER IN DEFAULT WHERE_CLAUSE VARCHAR2 IN DEFAULT
重点看GET_COMPRESSION_RATIO这个存储过程,它可以预估表的压缩比例。
可以使用以下的匿名块执行。
DECLARE blkcnt_comp PLS_INTEGER; blkcnt_uncm PLS_INTEGER; row_comp PLS_INTEGER; row_uncm PLS_INTEGER; comp_ratio number; comp_type VARCHAR2(30); username varchar2(30) := '&USER'; tablename varchar2(30) := '&TB' ; BEGIN dbms_compression.get_compression_ratio('&Usedtbs', username, tablename, NULL, dbms_compression.COMP_FOR_OLTP, blkcnt_comp, blkcnt_uncm, row_comp, row_uncm, comp_ratio, comp_type); dbms_output.put_line('Sampling table: '||username||'.'||tablename); dbms_output.put_line('Estimated compression ratio: ' ||TO_CHAR(comp_ratio)); dbms_output.put_line('Compression Type: ' || comp_type); END; /
执行效果:
/ Enter value for user: DEXTER old 8: username varchar2(30) :='&USER'; new 8: username varchar2(30) :='DEXTER'; Enter value for tb: ACCOUNT old 9: tablename varchar2(30) :='&TB' ; new 9: tablename varchar2(30) :='ACCOUNT' ; Enter value for usedtbs: USERS old 11: dbms_compression.get_compression_ratio('&Usedtbs', new 11: dbms_compression.get_compression_ratio('USERS', Sampling table: DEXTER.ACCOUNT Estimated compression ratio: 1 Compression Type: "Compress For OLTP" PL/SQL procedure successfully completed.
因为表中的重复值非常少,上文中Estimated compression ratio: 1,表示没有任何压缩效果。
高级压缩,基于块内的压缩。所以就算有重复值,但是没有在一个块中,那么高级压缩还是无法起作用。
这里重点介绍一个参数 COMPTYPE,它一共有6个选项,分别是
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;
Query high 以下都是HCC(HybridColumnar Compression)的内容,因为与Exadata的存储节点相关,所以在非Exadata一体机环境无法使用。不过有意思的是,你可以在普通环境下使用get_compression_ratio来预估压缩的比例。
11gr2以前compression-advisor存储过程下载地址:
http://download.csdn.net/detail/renfengjun/7514723

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