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Database Storage Optimization best practices, tips and tricks and guidance from Database Compression Product Management

Critical Statistics for Hybrid Columnar Compression

Guest Author

There are various Hybrid Columnar Compression (HCC) session level statistics available to help identify space savings, Exadata Offload benefits and other query benefits with this feature. Since the statistic descriptions didn’t make it into the Oracle Reference manual and have been confusing for many since they were introduced in 11.2, I thought it was a good idea to list few of the critical ones that can help you get an idea of benefits of HCC.

Statistic Name

Description

EHCC Analyzer Calls

Number of times HCC Analyzer was called to pick the optimal compression algorithms

EHCC Archive High CUs compressed

Number of HCC CUs compressed at Archive High level

EHCC Archive High CUs decompressed

Number of HCC CUs decompressed at Archive High level. Every decompression is counted

EHCC Archive Low CUs compressed

Number of HCC CUs compressed at Archive Low level

EHCC Archive Low CUs decompressed

Number of HCC CUs decompressed at Archive Low level. Every decompression is counted

EHCC Query High CUs compressed

Number of HCC CUs compressed at Query High level

EHCC Query High CUs decompressed

Number of HCC CUs decompressed at Query High level. Every decompression is counted

EHCC Query Low CUs compressed

Number of HCC CUs compressed at Query Low level

EHCC Query Low CUs decompressed

Number of HCC CUs decompressed at Query Low level. Every decompression is counted

EHCC CUs compressed

Total number of HCC CUs compressed

EHCC CUs decompressed

Total number of HCC CUs decompressed. Every decompression is counted

EHCC Rows Compressed

Total number of rows compressed using HCC compression algorithms

EHCC Rows Not Compressed

Total number of rows not compressed with HCC

EHCC Total Rows Decompressed

Total number of rows decompressed counting every decompression

EHCC Compressed Length

Total number of bytes compressed using HCC compression

EHCC Decompressed Length

Total number of bytes decompressed. Every decompression is counted

EHCC Check CUs Decompression

Number of times CUs decompressed by block checking

EHCC Dump CUs Decompressed

Number of times CUs decompressed for block dumps

EHCC ROWID CUs Decompressed

Number of times ROWID CUs were decompressed

Cell CUS sent compressed

Number of CUs sent compressed from Exadata cells to server to be processed

Cell CUs sent head uncompressed

Number of CUs sent uncompressed from Exadata cells to server to be processed

Cell CUs sent head piece

Number of CUs sent head piece from Exadata cells to server to be processed

EHCC CUs no rows pass minmax

Number of CUs for which can be pruned since no row passes min-max values

EHCC Columns Decompressed

Number of column CUs decompressed due to selective decompression

EHCC Pieces Buffered for Decompression

Number of CUs buffered for decompression

EHCC Used on ZFS Tablespace

Tracks HCC usage on ZFSSA storage

EHCC Used on Pillar Tablespace

Tracks HCC usage on Pillar storage

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