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  • December 16, 2015

Double counting of HCC stats prior to next major release

Roger Macnicol
Software Architect

HCC stats have been confusing for many since their introduction in 11.2 both because of the lack of a common naming convention and because of double counting.

When we did DBIM we spent a lot of time getting the stats right both in terms of naming convention that lent itself to straightforward wildcard searches, and in contents. For the next major release, the introduction of the IMC format Columnar Cache meant that the time had come to revamp the HCC stats more in line with the DBIM stats.

The double counting came from the fact that the decompression library (kdzd) had a stats container that was updated for each operation and the final tally was copied into the stat in a one-to-one correspondence. Each CU and each column that was processed on the cell was matched by the set of calls that looked at the CUs in the synthetic blocks returned by Smart Scan to see if they were secondarily compressed or not (which sometime happens to squeeze the results into the output buffer) consequently each CU and each column incremented the same stat twice. 

In the next major release these will be all been split into pairs of stats 'HCC scan [ cell | rdbms ] %'.

This table provides a handy mapping for anyone looking to find where their favourite stats have gone:

 Previous Stat  New RDBMS Stat  New Cell Stat
 EHCC Analyze CUs Decompressed  HCC analyze table CUs
 EHCC Analyzer Calls  HCC analyzer calls
 EHCC Archive High CUs Compressed Conventional  HCC load conventional CUs archive high
 EHCC Archive High CUs Compressed Direct  HCC load direct CUs archive high
 EHCC Archive Low CUs Compressed Conventional  HCC load conventional CUs archive high
 EHCC Archive Low CUs Compressed Direct  HCC load direct CUs archive low
 EHCC Query High CUs Compressed Conventional  HCC load conventional CUs query high
 EHCC Query High CUs Compressed Direct  HCC load direct CUs query high
 EHCC Query Low CUs Compressed Conventional  HCC load conventional CUs query low
 EHCC Query Low CUs Compressed Direct  HCC load direct CUs query low
 EHCC CU Row Pieces Compressed Conventional  HCC load conventional CUs row pieces
 EHCC CU Row Pieces Compressed Direct  HCC load direct CUs row pieces
 EHCC CUs Compressed Conventional  HCC load conventional CUs 
 EHCC CUs Compressed Direct  HCC load direct CUs 
 EHCC Rows Compressed Conventional  HCC load conventional rows
 EHCC Rows Compressed Direct  HCC load direct rows
 EHCC Rows Not Compressed Conventional  HCC load conventional rows not compressed
 EHCC Rows Not Compressed Direct  HCC load direct rows not compressed
 EHCC Compressed Length Compressed Conventional  HCC load conventional bytes compressed
 EHCC Compressed Length Compressed Direct  HCC load direct bytes compressed
 EHCC Decompressed Length Compressed Conventional  HCC load conventional bytes uncompressed
 EHCC Decompressed Length Compressed Direct  HCC load direct bytes uncompressed
 EHCC Compressed Length Compressed Conventional  HCC load conventional bytes compressed
 EHCC Compressed Length Compressed Direct  HCC load direct bytes compressed
 EHCC Attempted Block Compressions  HCC block compressions attempted
 EHCC Block Compressions  HCC block compressions completed



Table 2. Query Stats

Previous Stat  New RDBMS Stat  New Cell Stat
 EHCC Archive High CUs Decompressed  HCC scan rdbms CUs archive high  HCC scan cell CUs archive high
 EHCC Archive LowCUs Decompressed  HCC scan rdbms CUs archive low  HCC scan cell CUs archive low
 EHCC Query High CUs Decompressed  HCC scan rdbms CUs query high  HCC scan cell CUs query high
 EHCC Query Low CUs Decompressed  HCC scan rdbms CUs query low  HCC scan cell CUs query loq
 EHCC CUs Decompressed  HCC scan rdbms CUs decompressed  HCC scan cell CUs decompressed
 EHCC Check CUs Decompressed  HCC block check CUs
 EHCC Dump CUs Decompressed  HCC block dump CUs
 EHCC Rowid CUs Decompressed  HCC fetch by rowid CUs
 EHCC Normal Scan CUs Decompressed  HCC scan rdbms CUs normal
 EHCC Turbo Scan CUs Decompressed  HCC scan rdbms CUs turbo
 cell CUs processed for compressed  HCC scan cell CUs processed for compressed
 cell CUs processed for uncompressed  HCC scan cell CUs processed for uncompressed
 cell CUs sent compressed  HCC scan cell CUs sent compressed
 cell CUs sent head piece  HCC scan cell CUs sent head piece
 cell CUs sent uncompressed  HCC scan cell CUs sent uncompressed
 EHCC DML CUs Decompressed  HCC DML CUs
 EHCC Conventional DMLs  HCC DML conventional
 EHCC CUs all rows pass minmax  HCC scan rdbms CUs optimized read  HCC scan cell CUs optimized read
 EHCC CUs no rows pass minmax  HCC scan rdbms CUs pruned  HCC scan cell CUs pruned
 EHCC CUs some rows pass minmax <removed> <removed>
 EHCC Columns Decompressed  HCC scan rdbms CUs columns accessed  HCC scan cell CUs columns accessed
 EHCC Compressed Length Decompressed  HCC scan rdbms bytes compresse  HCC scan cell bytes compresse
 EHCC Decompressed Length Decompressed  HCC scan rdbms bytes decompressed  HCC scan cell bytes decompressed
 EHCC Pieces Buffered for Decompression  HCC scan rdbms CUs row pieces accessed  HCC scan cell CUs row pieces accessed
 EHCC Preds all rows pass minmax  HCC scan rdbms CUs predicates received  HCC scan cell CUs predicates received
 EHCC Preds no rows pass minmax  HCC scan rdbms CUs predicates applied  HCC scan cell CUs predicates applied
 EHCC Preds some rows pass minmax  HCC scan rdbms CUs predicates optimized  HCC scan cell CUs predicates optimized
 EHCC Total Columns for Decompression  HCC scan rdbms columns theoretical max  HCC scan cell columns theoretical max
 EHCC Total Pieces for Decompression  HCC scan rdbms CUs row pieces total  HCC scan cell CUs row pieces total
 EHCC Total Rows for Decompression  HCC scan rdbms rows  HCC scan cell rows
 EHCC Used on ZFS Tablespace  HCC usage ZFS
 EHCC Used on Pillar Tablespace  HCC usage pillar



In follow up entries I'll be looking at how the new in 12.2 Columnar Cache works, what the new Columnar Cache stats mean, and how Set membership in Storage Index works, 

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