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Docs for DoubleDelta and Gorilla column codecs.

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Vasily Nemkov 2019-06-13 12:40:44 +03:00
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docs/en/query_language/create.md
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@ -101,6 +101,8 @@ Possible `level` range: \[3, 12\]. Default value: 9. Greater values stands for b
Greater values stands for better compression and higher CPU usage.
- `Delta(delta_bytes)` - compression approach when raw values are replace with difference of two neighbour values. Up to `delta_bytes` are used for storing delta value.
Possible `delta_bytes` values: 1, 2, 4, 8. Default value for delta bytes is `sizeof(type)`, if it is equals to 1, 2, 4, 8 and equals to 1 otherwise.
- `DoubleDelta` - stores delta of deltas in compact binary form, compressing values down to 1 bit (in the best case). Best compression rates are achieved on monotonic sequences with constant stride, e.g. time samples. Can be used against any fixed-width type. Implementation is based on [Gorilla paper](http://www.vldb.org/pvldb/vol8/p1816-teller.pdf), and extended to support 64bit types. The drawback is 1 extra bit for 32-byte wide deltas: 5-bit prefix instead of 4-bit prefix.
- `Gorilla` - stores (parts of) xored values in compact binary form, compressing values down to 1 bit (in the best case). Best compression rate is achieved when neighbouring values are binary equal. Basic use case - floating point data that do not change rapidly. Implementation is based on [Gorilla paper](http://www.vldb.org/pvldb/vol8/p1816-teller.pdf), and extended to support 64bit types.
Syntax example:
```