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Table Engine

Last updated: 2022-03-02 12:15:03

    The table engine (i.e., table type) determines the following:

    • Data storage method and location, and where to write the data to or read the data from
    • Supported queries and how they are supported
    • Concurrent data access
    • Use of indexes (if any)
    • Whether multi-threaded requests can be executed
    • Data replication parameters

    Engine Type

    MergeTree

    MergeTree engines are the most universal and powerful table engines for high-load tasks. A common feature among them is quick data insertion with subsequent backend data processing. They support data replication (with Replicated* versions of engines), partitioning, and some features not supported by other engines.

    Engines in this family:

    • MergeTree
    • ReplacingMergeTree
    • SummingMergeTree
    • AggregatingMergeTree
    • CollapsingMergeTree
    • VersionedCollapsingMergeTree
    • GraphiteMergeTree

    Log

    Log engines are lightweight engines with minimum functionality. They are most effective when you need to quickly write many small tables (up to around 1 million rows) and read them later as a whole.

    Engines in this family:

    • TinyLog
    • StripeLog
    • Log

    Integration engine

    Integration engines are for integration with other data storage and processing systems.

    Engines in this family:

    • Kafka
    • MySQL
    • ODBC
    • JDBC
    • HDFS

    Special engines

    Engines in this family:

    • Distributed
    • MaterializedView
    • Dictionary
    • Merge
    • File
    • Null
    • Set
    • Join
    • URL
    • View
    • Memory
    • Buffer

    Virtual Column

    • Virtual column is an integral table engine attribute that is defined in the engine source code.
    • You should not specify virtual columns in the CREATE TABLE query and you cannot see them in SHOW CREATE TABLE and DESCRIBE TABLE query results. Virtual columns are also read-only, so you can't insert data into virtual columns.
    • To select data from a virtual column, you must specify its name in the SELECT query. SELECT * does not return data from virtual columns.
    • If you create a table with a column that has the same name as one of the table virtual columns, the virtual column becomes inaccessible. To help avoid conflicts, virtual column names are usually prefixed with an underscore.
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