Oracle Exadata HCC Removes the 255 Columns Limit

Posted in: Technical Track

It is a little known fact that Oracle table compression doesn’t work with tables that have more than 255. It’s has been reported and compression limitations in general here.

As a follow up from my previous post on Exadata Design, where I question the use of dimensions for certain attributes in data warehouses, I figured I should test whether HCC works with tables that have more than 255 columns.

And it does.
Here’s my test case.

Setup:

create table ctest (
col_1  varchar2(30) default 'JUSTSOMEDATA',
col_2  varchar2(30) default 'JUSTSOMEDATA',
...
col_255  varchar2(30) default 'JUSTSOMEDATA'
);
insert into ctest (col_1) select 'OTHERDATA' from dba_objects;
insert /*+APPEND*/ into ctest select * from ctest;
commit;
insert /*+APPEND*/ into ctest select * from ctest;
commit;

Build various scenarios (NOTE: second create adds the 256th column):

/*REGULAR*/
create table ctest_cmp compress as select * from ctest;
create table ctest_cmp1 compress as select t.*, 'NEW' col_256 from ctest t;

/*HCC QUERY*/
create table ctest_hcc compress for query low as select * from ctest;
create table ctest_hcc1 compress for query low  as select t.*, 'NEW' col_256 from ctest t;

/*HCC ARCHIVE*/
create table ctest_hccA compress for archive low as select * from ctest;
create table ctest_hccA1 compress for archive low  as select t.*, 'NEW' col_256 from ctest t;

And the results:

SQL> select round((bytes)/1024/1024) as mb ,  segment_name from user_segments t where segment_name like 'CTEST%' order by segment_name;

        MB SEGMENT_NAME
---------- ----------------------------
       824 CTEST
         3 CTEST_CMP
       824 CTEST_CMP1
         1 CTEST_HCC
         1 CTEST_HCC1
         1 CTEST_HCCA
         1 CTEST_HCCA1

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About the Author

An Oracle ACE with a deep understanding of databases, application memory, and input/output interactions, Christo is an expert at optimizing the performance of the most complex infrastructures. Methodical and efficiency-oriented, he equates the role of an ATCG Principal Consultant in many ways to that of a data analyst: both require a rigorous sifting-through of information to identify solutions to often large and complex problems. A dynamic speaker, Christo has delivered presentations at the IOUG, the UKOUG, the Rocky Mountain Oracle Users Group, Oracle Open World, and other industry conferences.

1 Comment. Leave new

Exadata « Oracle Scratchpad
November 21, 2010 2:30 pm

[…] Column limit removed […]

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