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- Glossary
- Release Notes
- Product Introduction
- Purchase Guide
- Getting Started
- Operation Guide
- Development Guide
- Service Level Agreement
- CDWCH Policy
- FAQs
- Contact Us
- Glossary
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 Types
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. For more information, see MergeTree.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 engines
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 Columns
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 them.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 a table virtual column, the virtual column becomes inaccessible. To avoid such conflicts, virtual column names are usually prefixed with an underscore.
Yes
No
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