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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:
INDEX_COMP_PAGESAVE – “percentage Reduction in Index Leaf Page Space”
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!
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