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43 lines
1.9 KiB
Markdown
43 lines
1.9 KiB
Markdown
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#Log Engine Family
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These engines were developed for scenarios when you need to write many tables with the small amount of data (less than 1 million rows).
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Engines of the family:
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- [StripeLog](stripelog.md)
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- [Log](log.md)
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- [TinyLog](tinylog.md)
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## Common properties
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Engines:
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- Store data on a disk.
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- Append data to the end of file when writing.
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- Do not support [mutation](../../query_language/alter.md#alter-mutations) operations.
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- Do not support indexes.
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This means that `SELECT` queries for ranges of data are not efficient.
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- Do not write data atomically.
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You can get a table with corrupted data if something breaks the write operation, for example, abnormal server shutdown.
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## Differences
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The `Log` and `StripeLog` engines support:
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- Locks for concurrent data access.
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During `INSERT` query the table is locked, and other queries for reading and writing data both wait for unlocking. If there are no writing data queries, any number of reading data queries can be performed concurrently.
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- Parallel reading of data.
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When reading data ClickHouse uses multiple threads. Each thread processes separated data block.
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The `Log` engine uses the separate file for each column of the table. The `StripeLog` stores all the data in one file. Thus the `StripeLog` engine uses fewer descriptors in the operating system, but the `Log` engine provides a more efficient reading of the data.
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The `TinyLog` engine is the simplest in the family and provides the poorest functionality and lowest efficiency. The `TinyLog` engine does not support a parallel reading and concurrent access and stores each column in a separate file. It reads the data slower than both other engines with parallel reading, and it uses almost as many descriptors as the `Log` engine. You can use it in simple low-load scenarios.
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[Original article](https://clickhouse.yandex/docs/en/operations/table_engines/log_family/) <!--hide-->
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