Table Engines
=============

See: [ClickHouse Documentation](https://clickhouse.yandex/reference_en.html#Table+engines)

Each model must have an engine instance, used when creating the table in ClickHouse.

The following engines are supported by the ORM:

- TinyLog
- Log
- Memory
- MergeTree / ReplicatedMergeTree
- CollapsingMergeTree / ReplicatedCollapsingMergeTree
- SummingMergeTree / ReplicatedSummingMergeTree
- ReplacingMergeTree / ReplicatedReplacingMergeTree
- Buffer
- Merge


Simple Engines
--------------

`TinyLog`, `Log` and `Memory` engines do not require any parameters:

    engine = engines.TinyLog()

    engine = engines.Log()
    
    engine = engines.Memory()


Engines in the MergeTree Family
-------------------------------

To define a `MergeTree` engine, supply the date column name and the names (or expressions) for the key columns:

    engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'))

You may also provide a sampling expression:

    engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'), sampling_expr='intHash32(UserID)')

A `CollapsingMergeTree` engine is defined in a similar manner, but requires also a sign column:

    engine = engines.CollapsingMergeTree('EventDate', ('CounterID', 'EventDate'), 'Sign')

For a `SummingMergeTree` you can optionally specify the summing columns:

    engine = engines.SummingMergeTree('EventDate', ('OrderID', 'EventDate', 'BannerID'),
                                      summing_cols=('Shows', 'Clicks', 'Cost'))

For a `ReplacingMergeTree` you can optionally specify the version column:

    engine = engines.ReplacingMergeTree('EventDate', ('OrderID', 'EventDate', 'BannerID'), ver_col='Version')

### Data Replication

Any of the above engines can be converted to a replicated engine (e.g. `ReplicatedMergeTree`) by adding two parameters, `replica_table_path` and `replica_name`:

    engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'),
                               replica_table_path='/clickhouse/tables/{layer}-{shard}/hits',
                               replica_name='{replica}')


Buffer Engine
-------------

A `Buffer` engine is only used in conjunction with a `BufferModel`.
The model should be a subclass of both `models.BufferModel` and the main model. 
The main model is also passed to the engine:

    class PersonBuffer(models.BufferModel, Person):

        engine = engines.Buffer(Person)

Additional buffer parameters can optionally be specified:

        engine = engines.Buffer(Person, num_layers=16, min_time=10, 
                                max_time=100, min_rows=10000, max_rows=1000000, 
                                min_bytes=10000000, max_bytes=100000000)

Then you can insert objects into Buffer model and they will be handled by ClickHouse properly:

    db.create_table(PersonBuffer)
    suzy = PersonBuffer(first_name='Suzy', last_name='Jones')
    dan = PersonBuffer(first_name='Dan', last_name='Schwartz')
    db.insert([dan, suzy])
    
    
Merge Engine
-------------

[ClickHouse docs](https://clickhouse.yandex/docs/en/single/index.html#merge)  
A `Merge` engine is only used in conjunction with a `MergeModel`.   
This table does not store data itself, but allows reading from any number of other tables simultaneously. So you can't insert in it.
Engine parameter specifies re2 (similar to PCRE) regular expression, from which data is selected.

    class MergeTable(models.MergeModel):
        engine = engines.Merge('^table_prefix')


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