Mittwoch, 5. Oktober 2011

non-Exadata HCC

Oracles Hybrid Columnar Compression was one of the big new features of Oracle 11.2. Unfortunately someone decided this feature should only be available in Exadata. Even Oracle tried to explain it with technical limitations, it was more or less obvious that's just wrong. There are some reasons for this:

  • The Database is doing the HCC compression all the time
  • The Database must be able doing the HCC decompression in case the storage cell can not or want not.
  • Beta-testers where very sure, HCC worked there without any special hardware.
  • Jonathan Lewis shows there are situations, where also an ordinary database creates tables with HCC
But fact is: Oracle decided to disable HCC for general usage. As there is no different database software in Exadata database servers, the decision whether process the statement or throw a ORA-64307: hybrid columnar compression is only supported in tablespaces residing on Exadata storage must be done by any switch within the software.

Here my collection of informations I have about these switch:

  • Kerry Osborne described in the book Expert Oracle Exadata in pages 46 to 48 how the ASM DiskGroup attribute cell.smart_scan_capable=TRUE is only possible on Exadata, and necessary for any kind of smart scan - so also for HCC.
  • Cern has published a paper about Compression in Oracle - in the Appendix (pages 42 to 44) they show how to change this attribute. Not so easy and it corrupts the ASM DG. 
  • Jonathan Lewis mentioned there might be a switch in DBMS_COMPRESSION.GET_COMPRESSION_RATIO which disables the switch for the purpose of the temporary compressed tables. He did not go into details, but I decided to investigate into that direction.
The package DBMS_COMPRESSION uses prvt_compression, and there in GET_COMPRESSION_RATIO it calls PRVT_COMPRESSION.CHECK_HLI(1); to disable this switch and PRVT_COMPRESSION.CHECK_HLI(0); to enables it at the end again. CHECK_HLI just calls the kernel function KDZCHECKHI with it's parameter, nothing more. Unfortunately it can not be called from outside of PRVT_COMPRESSION. That's the way I started to investigate: I removed the line   PROCEDURE CHECK_HLI (HLID    IN NUMBER); from the package body and inserted it into the package header. (by doing so, I left the path of supported system - don't do this if you care your system!). Now I can call CHECK_HLI:
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production 
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,Data Mining and Real Application Testing options

SQL> exec sys.prvt_compression.CHECK_HLI(1);

PL/SQL procedure successfully completed.

SQL> create table bx_dba_objects Compress For Archive Low as select * from dba_objects;

Table created.

prvt_compression.CHECK_HLI works per session; so a logon-trigger comes to my mind.
To enable the check again, the parameter is 0 instead of 1.

Update:
parallel processes does not inherit this feature:
(a slightly different testcase, but same setup in general)
SQL> select /*+ PARALLEL 8 */ count(*) from test_user.DBMS_TABCOMP_TEMP_CMP; 
select /*+ PARALLEL 8 */ count(*) from test_user.DBMS_TABCOMP_TEMP_CMP
*
ERROR at line 1:
ORA-12801: error signaled in parallel query server P008, instance av2l904t:VAX1
(1)
ORA-64307: hybrid columnar compression is only supported in tablespaces
residing on Exadata storage

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