Table engines

The table engine (type of table) determines:

• How and where data is stored, where to write it to, and where to read it from.
• Which queries are supported, and how.
• Concurrent data access.
• Use of indexes, if present.
• Whether multithread request execution is possible.
• Data replication parameters.

Engine families

MergeTree

The most universal and functional table engines for high-load tasks. The property shared by these engines is quick data insertion with subsequent background data processing. MergeTree family engines support data replication (with Replicated* versions of engines), partitioning, secondary data-skipping indexes, and other features not supported in other engines.

Engines in the family:

MergeTree Engines
MergeTree
ReplacingMergeTree
SummingMergeTree
AggregatingMergeTree
CollapsingMergeTree
VersionedCollapsingMergeTree
GraphiteMergeTree
CoalescingMergeTree

Log

Lightweight engines with minimum functionality. They're the most effective when you need to quickly write many small tables (up to approximately 1 million rows) and read them later as a whole.

Engines in the family:

Log Engines
TinyLog
StripeLog
Log

Integration engines

Engines for communicating with other data storage and processing systems.

Engines in the family:

Integration Engines
ODBC
JDBC
MySQL
MongoDB
Redis
HDFS
S3
Kafka
EmbeddedRocksDB
RabbitMQ
PostgreSQL
S3Queue
TimeSeries

Special engines

Engines in the family:

Special Engines
Distributed
Dictionary
Merge
Executable
File
Null
Set
Join
URL
View
Memory
Buffer
External Data
GenerateRandom
KeeperMap
FileLog

Virtual columns

A virtual column is an integral table engine attribute that is defined in the engine source code.

You shouldn't specify virtual columns in the CREATE TABLE query, and you can't 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 values 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. We do not recommend doing this. To help avoid conflicts, virtual column names are usually prefixed with an underscore.

• _table — Contains the name of the table from which data was read. Type: String.

  Regardless of the table engine being used, each table includes a universal virtual column named _table.

  When querying a table with the merge table engine, you can set the constant conditions on _table in the WHERE/PREWHERE clause (for example, WHERE _table='xyz'). In this case the read operation is performed only for that tables where the condition on _table is satisfied, so the _table column acts as an index.

  When using queries formatted like SELECT ... FROM (... UNION ALL ...), we can determine which actual table the returned rows originate from by specifying the _table column.