CREATE DATABASE [IF NOT EXISTS] db_name [ON CLUSTER cluster] [ENGINE = engine(...)]
IF NOT EXISTS
db_namedatabase already exists, then ClickHouse doesn't create a new database and:
- Doesn't throw an exception if clause is specified.
- Throws an exception if clause isn't specified.
ClickHouse creates the
db_namedatabase on all the servers of a specified cluster.
Allows you to retrieve data from the remote MySQL server.
By default, ClickHouse uses its own database engine.
CREATE TABLE query can have several forms.
CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster] ( name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [compression_codec] [TTL expr1], name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [compression_codec] [TTL expr2], ... ) ENGINE = engine
Creates a table named 'name' in the 'db' database or the current database if 'db' is not set, with the structure specified in brackets and the 'engine' engine. The structure of the table is a list of column descriptions. If indexes are supported by the engine, they are indicated as parameters for the table engine.
A column description is
name type in the simplest case. Example:
RegionID UInt32. Expressions can also be defined for default values (see below).
CREATE TABLE [IF NOT EXISTS] [db.]table_name AS [db2.]name2 [ENGINE = engine]
Creates a table with the same structure as another table. You can specify a different engine for the table. If the engine is not specified, the same engine will be used as for the
CREATE TABLE [IF NOT EXISTS] [db.]table_name AS table_function()
Creates a table with the structure and data returned by a table function.
CREATE TABLE [IF NOT EXISTS] [db.]table_name ENGINE = engine AS SELECT ...
Creates a table with a structure like the result of the
SELECT query, with the 'engine' engine, and fills it with data from SELECT.
In all cases, if
IF NOT EXISTS is specified, the query won't return an error if the table already exists. In this case, the query won't do anything.
There can be other clauses after the
ENGINE clause in the query. See detailed documentation on how to create tables in the descriptions of table engines.
The column description can specify an expression for a default value, in one of the following ways:
ALIAS expr. Example:
URLDomain String DEFAULT domain(URL).
If an expression for the default value is not defined, the default values will be set to zeros for numbers, empty strings for strings, empty arrays for arrays, and
0000-00-00 for dates or
0000-00-00 00:00:00 for dates with time. NULLs are not supported.
If the default expression is defined, the column type is optional. If there isn't an explicitly defined type, the default expression type is used. Example:
EventDate DEFAULT toDate(EventTime) – the 'Date' type will be used for the 'EventDate' column.
If the data type and default expression are defined explicitly, this expression will be cast to the specified type using type casting functions. Example:
Hits UInt32 DEFAULT 0 means the same thing as
Hits UInt32 DEFAULT toUInt32(0).
Default expressions may be defined as an arbitrary expression from table constants and columns. When creating and changing the table structure, it checks that expressions don't contain loops. For INSERT, it checks that expressions are resolvable – that all columns they can be calculated from have been passed.
Normal default value. If the INSERT query doesn't specify the corresponding column, it will be filled in by computing the corresponding expression.
Materialized expression. Such a column can't be specified for INSERT, because it is always calculated. For an INSERT without a list of columns, these columns are not considered. In addition, this column is not substituted when using an asterisk in a SELECT query. This is to preserve the invariant that the dump obtained using
SELECT * can be inserted back into the table using INSERT without specifying the list of columns.
Synonym. Such a column isn't stored in the table at all. Its values can't be inserted in a table, and it is not substituted when using an asterisk in a SELECT query. It can be used in SELECTs if the alias is expanded during query parsing.
When using the ALTER query to add new columns, old data for these columns is not written. Instead, when reading old data that does not have values for the new columns, expressions are computed on the fly by default. However, if running the expressions requires different columns that are not indicated in the query, these columns will additionally be read, but only for the blocks of data that need it.
If you add a new column to a table but later change its default expression, the values used for old data will change (for data where values were not stored on the disk). Note that when running background merges, data for columns that are missing in one of the merging parts is written to the merged part.
It is not possible to set default values for elements in nested data structures.
Along with columns descriptions constraints could be defined:
CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster] ( name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [compression_codec] [TTL expr1], ... CONSTRAINT constraint_name_1 CHECK boolean_expr_1, ... ) ENGINE = engine
boolean_expr_1 could by any boolean expression. If constraints are defined for the table, each of them will be checked for every row in
INSERT query. If any constraint is not satisfied — server will raise an exception with constraint name and checking expression.
Adding large amount of constraints can negatively affect performance of big
Defines storage time for values. Can be specified only for MergeTree-family tables. For the detailed description, see TTL for columns and tables.
Column Compression Codecs¶
By default, ClickHouse applies the compression method, defined in server settings, to columns. You can also define the compression method for each individual column in the
CREATE TABLE query.
CREATE TABLE codec_example ( dt Date CODEC(ZSTD), ts DateTime CODEC(LZ4HC), float_value Float32 CODEC(NONE), double_value Float64 CODEC(LZ4HC(9)) value Float32 CODEC(Delta, ZSTD) ) ENGINE = <Engine> ...
If a codec is specified, the default codec doesn't apply. Codecs can be combined in a pipeline, for example,
CODEC(Delta, ZSTD). To select the best codec combination for you project, pass benchmarks similar to described in the Altinity New Encodings to Improve ClickHouse Efficiency article.
You can't decompress ClickHouse database files with external utilities like
lz4. Instead, use the special clickhouse-compressor utility.
Compression is supported for the following table engines:
ClickHouse supports common purpose codecs and specialized codecs.
These codecs are designed to make compression more effective by using specific features of data. Some of these codecs don't compress data themself. Instead, they prepare the data for a common purpose codec, which compresses it better than without this preparation.
Delta(delta_bytes)— Compression approach in which raw values are replaced by the difference of two neighboring values, except for the first value that stays unchanged. Up to
delta_bytesare used for storing delta values, so
delta_bytesis the maximum size of raw values. Possible
delta_bytesvalues: 1, 2, 4, 8. The default value for
sizeof(type)if equal to 1, 2, 4, or 8. In all other cases, it's 1.
DoubleDelta— Calculates delta of deltas and writes it in compact binary form. Optimal compression rates are achieved for monotonic sequences with a constant stride, such as time series data. Can be used with any fixed-width type. Implements the algorithm used in Gorilla TSDB, extending it to support 64-bit types. Uses 1 extra bit for 32-byte deltas: 5-bit prefixes instead of 4-bit prefixes. For additional information, see Compressing Time Stamps in Gorilla: A Fast, Scalable, In-Memory Time Series Database.
Gorilla— Calculates XOR between current and previous value and writes it in compact binary form. Efficient when storing a series of floating point values that change slowly, because the best compression rate is achieved when neighboring values are binary equal. Implements the algorithm used in Gorilla TSDB, extending it to support 64-bit types. For additional information, see Compressing Values in Gorilla: A Fast, Scalable, In-Memory Time Series Database.
T64— Compression approach that crops unused high bits of values in integer data types (including
DateTime). At each step of its algorithm, codec takes a block of 64 values, puts them into 64x64 bit matrix, transposes it, crops the unused bits of values and returns the rest as a sequence. Unused bits are the bits, that don't differ between maximum and minimum values in the whole data part for which the compression is used.
Gorilla codecs are used in Gorilla TSDB as the components of its compressing algorithm. Gorilla approach is effective in scenarios when there is a sequence of slowly changing values with their timestamps. Timestamps are effectively compressed by the
DoubleDelta codec, and values are effectively compressed by the
Gorilla codec. For example, to get an effectively stored table, you can create it in the following configuration:
CREATE TABLE codec_example ( timestamp DateTime CODEC(DoubleDelta), slow_values Float32 CODEC(Gorilla) ) ENGINE = MergeTree()
Common purpose codecs¶
NONE— No compression.
LZ4— Lossless data compression algorithm used by default. Applies LZ4 fast compression.
LZ4HC[(level)]— LZ4 HC (high compression) algorithm with configurable level. Default level: 9. Setting
level <= 0applies the default level. Possible levels: [1, 12]. Recommended level range: [4, 9].
ZSTD[(level)]— ZSTD compression algorithm with configurable
level. Possible levels: [1, 22]. Default value: 1.
High compression levels are useful for asymmetric scenarios, like compress once, decompress repeatedly. Higher levels mean better compression and higher CPU usage.
ClickHouse supports temporary tables which have the following characteristics:
- Temporary tables disappear when the session ends, including if the connection is lost.
- A temporary table uses the Memory engine only.
- The DB can't be specified for a temporary table. It is created outside of databases.
- Impossible to create a temporary table with distributed DDL query on all cluster servers (by using
ON CLUSTER): this table exists only in the current session.
- If a temporary table has the same name as another one and a query specifies the table name without specifying the DB, the temporary table will be used.
- For distributed query processing, temporary tables used in a query are passed to remote servers.
To create a temporary table, use the following syntax:
CREATE TEMPORARY TABLE [IF NOT EXISTS] table_name ( name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1], name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2], ... )
In most cases, temporary tables are not created manually, but when using external data for a query, or for distributed
(GLOBAL) IN. For more information, see the appropriate sections
It's possible to use tables with ENGINE = Memory instead of temporary tables.
Distributed DDL queries (ON CLUSTER clause)¶
RENAME queries support distributed execution on a cluster. For example, the following query creates the
Distributed table on each host in
CREATE TABLE IF NOT EXISTS all_hits ON CLUSTER cluster (p Date, i Int32) ENGINE = Distributed(cluster, default, hits)
In order to run these queries correctly, each host must have the same cluster definition (to simplify syncing configs, you can use substitutions from ZooKeeper). They must also connect to the ZooKeeper servers. The local version of the query will eventually be implemented on each host in the cluster, even if some hosts are currently not available. The order for executing queries within a single host is guaranteed.
CREATE [MATERIALIZED] VIEW [IF NOT EXISTS] [db.]table_name [TO[db.]name] [ENGINE = engine] [POPULATE] AS SELECT ...
Creates a view. There are two types of views: normal and MATERIALIZED.
Normal views don't store any data, but just perform a read from another table. In other words, a normal view is nothing more than a saved query. When reading from a view, this saved query is used as a subquery in the FROM clause.
As an example, assume you've created a view:
CREATE VIEW view AS SELECT ...
and written a query:
SELECT a, b, c FROM view
This query is fully equivalent to using the subquery:
SELECT a, b, c FROM (SELECT ...)
Materialized views store data transformed by the corresponding SELECT query.
When creating a materialized view without
TO [db].[table], you must specify ENGINE – the table engine for storing data.
When creating a materialized view with
TO [db].[table], you must not use
A materialized view is arranged as follows: when inserting data to the table specified in SELECT, part of the inserted data is converted by this SELECT query, and the result is inserted in the view.
If you specify POPULATE, the existing table data is inserted in the view when creating it, as if making a
CREATE TABLE ... AS SELECT ... . Otherwise, the query contains only the data inserted in the table after creating the view. We don't recommend using POPULATE, since data inserted in the table during the view creation will not be inserted in it.
SELECT query can contain
LIMIT... Note that the corresponding conversions are performed independently on each block of inserted data. For example, if
GROUP BY is set, data is aggregated during insertion, but only within a single packet of inserted data. The data won't be further aggregated. The exception is when using an ENGINE that independently performs data aggregation, such as
The execution of
ALTER queries on materialized views has not been fully developed, so they might be inconvenient. If the materialized view uses the construction
TO [db.]name, you can
DETACH the view, run
ALTER for the target table, and then
ATTACH the previously detached (
Views look the same as normal tables. For example, they are listed in the result of the
SHOW TABLES query.
There isn't a separate query for deleting views. To delete a view, use
CREATE DICTIONARY [IF NOT EXISTS] [db.]dictionary_name ( key1 type1 [DEFAULT|EXPRESSION expr1] [HIERARCHICAL|INJECTIVE|IS_OBJECT_ID], key2 type2 [DEFAULT|EXPRESSION expr2] [HIERARCHICAL|INJECTIVE|IS_OBJECT_ID], attr1 type2 [DEFAULT|EXPRESSION expr3], attr2 type2 [DEFAULT|EXPRESSION expr4] ) PRIMARY KEY key1, key2 SOURCE(SOURCE_NAME([param1 value1 ... paramN valueN])) LAYOUT(LAYOUT_NAME([param_name param_value])) LIFETIME([MIN val1] MAX val2)
External dictionary structure consists of attributes. Dictionary attributes are specified similarly to table columns. The only required attribute property is its type, all other properties may have default values.
Depending on dictionary layout one or more attributes can be specified as dictionary keys.
For more information, see External Dictionaries section.