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125 lines
4.3 KiB
Markdown
125 lines
4.3 KiB
Markdown
Table Engines
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=============
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See: [ClickHouse Documentation](https://clickhouse.yandex/docs/en/table_engines/)
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Each model must have an engine instance, used when creating the table in ClickHouse.
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The following engines are supported by the ORM:
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- TinyLog
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- Log
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- Memory
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- MergeTree / ReplicatedMergeTree
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- CollapsingMergeTree / ReplicatedCollapsingMergeTree
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- SummingMergeTree / ReplicatedSummingMergeTree
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- ReplacingMergeTree / ReplicatedReplacingMergeTree
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- Buffer
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- Merge
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- Distributed
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Simple Engines
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--------------
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`TinyLog`, `Log` and `Memory` engines do not require any parameters:
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engine = engines.TinyLog()
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engine = engines.Log()
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engine = engines.Memory()
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Engines in the MergeTree Family
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-------------------------------
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To define a `MergeTree` engine, supply the date column name and the names (or expressions) for the key columns:
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engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'))
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You may also provide a sampling expression:
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engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'), sampling_expr='intHash32(UserID)')
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A `CollapsingMergeTree` engine is defined in a similar manner, but requires also a sign column:
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engine = engines.CollapsingMergeTree('EventDate', ('CounterID', 'EventDate'), 'Sign')
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For a `SummingMergeTree` you can optionally specify the summing columns:
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engine = engines.SummingMergeTree('EventDate', ('OrderID', 'EventDate', 'BannerID'),
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summing_cols=('Shows', 'Clicks', 'Cost'))
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For a `ReplacingMergeTree` you can optionally specify the version column:
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engine = engines.ReplacingMergeTree('EventDate', ('OrderID', 'EventDate', 'BannerID'), ver_col='Version')
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### Custom partitioning
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ClickHouse supports [custom partitioning](https://clickhouse.yandex/docs/en/table_engines/custom_partitioning_key/) expressions since version 1.1.54310
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You can use custom partitioning with any `MergeTree` family engine.
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To set custom partitioning:
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* Instead of specifying the `date_col` (first) constructor parameter, pass a tuple of field names or expressions in the `order_by` (second) constructor parameter.
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* Add `partition_key` parameter. It should be a tuple of expressions, by which partitions are built.
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Standard monthly partitioning by date column can be specified using the `toYYYYMM(date)` function.
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Example:
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engine = engines.ReplacingMergeTree(order_by=('OrderID', 'EventDate', 'BannerID'), ver_col='Version',
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partition_key=('toYYYYMM(EventDate)', 'BannerID'))
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### Data Replication
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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`:
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engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'),
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replica_table_path='/clickhouse/tables/{layer}-{shard}/hits',
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replica_name='{replica}')
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Buffer Engine
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-------------
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A `Buffer` engine is only used in conjunction with a `BufferModel`.
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The model should be a subclass of both `models.BufferModel` and the main model.
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The main model is also passed to the engine:
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class PersonBuffer(models.BufferModel, Person):
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engine = engines.Buffer(Person)
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Additional buffer parameters can optionally be specified:
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engine = engines.Buffer(Person, num_layers=16, min_time=10,
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max_time=100, min_rows=10000, max_rows=1000000,
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min_bytes=10000000, max_bytes=100000000)
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Then you can insert objects into Buffer model and they will be handled by ClickHouse properly:
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db.create_table(PersonBuffer)
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suzy = PersonBuffer(first_name='Suzy', last_name='Jones')
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dan = PersonBuffer(first_name='Dan', last_name='Schwartz')
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db.insert([dan, suzy])
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Merge Engine
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-------------
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[ClickHouse docs](https://clickhouse.yandex/docs/en/table_engines/merge/)
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A `Merge` engine is only used in conjunction with a `MergeModel`.
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This table does not store data itself, but allows reading from any number of other tables simultaneously. So you can't insert in it.
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Engine parameter specifies re2 (similar to PCRE) regular expression, from which data is selected.
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class MergeTable(models.MergeModel):
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engine = engines.Merge('^table_prefix')
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---
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[<< Field Types](field_types.md) | [Table of Contents](toc.md) | [Schema Migrations >>](schema_migrations.md) |