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What's new in 12.2: CELLMEMORY Part 3

The Cellmemory Stats in RDBMS  The RDBMS stats for Cellmemory are designed to closely follow the pattern used by the Inmemory stats  Query Stats   Each column in each one MB of disk blocks will be rewritten into one IMC format Column CU in flash and a set of Column CUs comprise an overall Compression Unit so these stats reflect the number of 1 MB rewrites that were processed (not the number of column CUs). "cellmemory IM scan CUs processed for query"- #1 MB chuncks scanned in...

Friday, April 14, 2017 | Read More

What's new in 12.2: CELLMEMORY Part 2

Question: do I need to know anything in this blog post? Answer: No, it is a true cache and CELLMEMORY works automatically Question: so why should I read this blog post? Answer:  because you like to keep a toolkit of useful ways to control the system when needed The DDL The Exadata engineering team has done a lot of work to make the flash cache automatically handle a variety of very different workloads happening simultaneously which is why we now typically discourage users from...

Monday, March 27, 2017 | Read More

What's new in 12.2: CELLMEMORY Part 1

Many people know that in we introduced a ground-breaking columnar cache that rewrote 1 MB chunks of HCC format blocks in the flash cache into pure columnar form in a way that allowed us to only do the I/O for the columns needed but also to recreate the original block when that was required. This showed up in stats as "cell physical IO bytes saved by columnar cache". But in we had also introduced Database In-Memory (or DBIM) that rewrote heap blocks into pure...

Friday, March 24, 2017 | Read More

More tricks with OPT_PARAM

Did you know you can set most parameters for the execution of a single statement without using an Alter Session by using an OPT_PARAM hint? For example, regular parameters (here forcing the storage clause in the query plan): SQL> select /*+ OPT_PARAM('cell_offload_plan_display' 'always') */ col1 From table1 and underscore parameters: SQL> select /*+ OPT_PARAM('_arch_comp_dbg_scan',2048) */ n_name from nation However if you try conflicting settings that set a different value...

Tuesday, March 21, 2017 | Read More

Improvements to HCC with wide tables in 12.2

HCC Compression Unit Sizing  Since the beginning Oracle has provided four compression levels to offer a trade-off between the compression ratio and various other factors including table scans and the performance of single-row retrieval. I can not emphasize enough that the various trade offs mean that YMMV with the different levels and you should always test what works best with your application and hardware when choosing the level. Historically people have rarely used Query...

Friday, March 17, 2017 | Read More

Why are HCC stats being bumped when Smart Scanning row major data in 12.2?

In 12.2, there is a stat "cell blocks pivoted" that points to a new optimization. When Smart Scan processes data, it has to create a new synthetic block that only contains the columns that are needed and the rows that pass the offloaded predicates. If Smart Scan has to create a new block, why would you create a new row-major block when we can just as easily create uncompressed HCC block? It is roughly the same amount of work but once the synthetic block is returned to the...

Friday, February 3, 2017 | Read More

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