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Status Future consideration
Workspace Db2 for z/OS
Created by Guest
Created on Nov 22, 2019

DB2 Catalog and INDEX COMPRESSION statistics (PAGESAVE and BP_UNUSED)

If an index is compressed, it is documented in DB2 catalog table SYSIBM.SYSINDEXES column called ‘COMPRESS’

 

Tablespace compression is documented in SYSTABLESPACE and SYSTABLEPART.  In both tables with column called ‘COMPRESS’

SYSTABLEPART also contains the informative column PAGESAVE  which tells the percentage of pages saved.  Very useful!

 

 

I would like similar information about INDEX COMPRESSION. 

 

Basically, add new columns to table SYSINDEXES:

  1. INDEX_COMP_PAGESAVE – “percentage Reduction in Index Leaf Page Space”
  2. INDEX_COMP_BP_UNUSED – “percentage of Bufferpool Space that is unused to ensure keys fit into compressed buffers”

 

(obviously, you can pick your own DB2 catalog names)

 

 

Why is this additional index compression information useful? 

It can help the DBA review which indexes are compressed and the effectiveness of the compress.  And it can help us confirm that we are using the appropriate bufferpool size for the index (remember, when deciding to compress an index, we must pick 8K, 16K or 32K)

If an index was compressed in the past (by some other DBA) then with this information then I could confirm the index compress actually saves significant space and I could confirm the best bufferpool size was picked!

 

It seem like a logical extension for IBM to include this information about COMPRESSED INDEXES in the Catalog! 

 

DB2 has the equivalent information for tablespaces!  Do the same for indexes!

  • Admin
    Janet Figone
    Reply
    |
    Nov 7, 2023

    Thank you for the additional information, Brian. I notified our SME of your update.

  • Guest
    Reply
    |
    Oct 13, 2023

    To be clear. The two obvious missing statistics are :

    1. INDEX_COMP_PAGESAVE – “percentage Reduction in Index Leaf Page Space”

    2. INDEX_COMP_BP_UNUSED – “percentage of Bufferpool Space that is unused to ensure keys fit into compressed buffers”

    These new column values will tell us the effectiveness for the compressed index for the current bufferpool.

    As a bonus. You should consider adding SOME new cols that essentially give us the same ESTIMATED benefit value that we get from DSN1COMP. This would save us from having to run DSN1COMP. And I believe it is “easy” for you to do this calculation while doing REORG INDEX or REBUILD INDEX or LOAD REPLACE … etc.

    I think this is low cost for you and it would make it obvious when we should compress some indexes!

    Remember, some people in the world are still measured by how much space they allocate. I know index compression is all about space allocation reduction. Index compression is not really for performance… but still reducing space is a measure and I work toward measures

    INDEX_COMP_EST_PAGESAV_BP4K

    INDEX_COMP_EST_BP_UNUSED_BP4K

    INDEX_COMP_EST_PAGESAV_BP8K

    INDEX_COMP_EST_BP_UNUSED_BP8K

    INDEX_COMP_EST_PAGESAV_BP32K

    INDEX_COMP_EST_BP_UNUSED_BP32K

  • Admin
    Janet Figone
    Reply
    |
    May 9, 2023

    Thank you for your comment. We're checking with the development team and will provide an update here when we have more information.

    Sincerely,

    Db2 for z/OS Team

  • Guest
    Reply
    |
    May 9, 2023

    Any update or progress on this request?


    I ran into situation today where I'm trying to implement index compression and have little insight into how it is performing. Only thing I can think to do is check the number of tracks consumed before and after. The tables I am working with eventually in PROD are 15 and 11.6 billion rows. Each table has 2 indexes and I estimate approximately 5,359 gigs. I'd really like to know the results before and after to ensure compression is worthwhile from a storage savings perspective.

  • Admin
    Janet Figone
    Reply
    |
    Dec 14, 2022

    Brian, Thank you for your inquiry. I've asked the SME for an update as to the plans for this idea, and will post the update here as soon as it is received.

    Sincerely,

    The Db2 for z/OS Team

  • Guest
    Reply
    |
    Dec 8, 2022

    The following has been under consideration for 18 months . The fact that it is STILL under consideration provides hope that it might be done. I cannot imagine the effort for IBM is huge. I think the benefit is something. One could argue that it is not worth the effort…. But I would like to see it done. It is the right thing to do.


  • Admin
    Janet Figone
    Reply
    |
    Aug 6, 2021

    Hello Brian, We are reconsidering this Aha idea and therefore placing it in Future Consideration state.

  • Guest
    Reply
    |
    Dec 8, 2020

    does IBM have any feedback on the status of this idea? I feel it is important and I would really like to see it implemented? It is a logical and obvious extension to index compression... why is it not done already?

    What do you think?