ClickHouse/docs/en/sql-reference/functions/other-functions.md

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/en/sql-reference/functions/other-functions	67	Other

Other Functions

hostName()

Returns a string with the name of the host that this function was performed on. For distributed processing, this is the name of the remote server host, if the function is performed on a remote server. If it is executed in the context of a distributed table, then it generates a normal column with values relevant to each shard. Otherwise it produces a constant value.

getMacro

Gets a named value from the macros section of the server configuration.

Syntax

getMacro(name);

Arguments

• name — Name to retrieve from the macros section. String.

Returned value

• Value of the specified macro.

Type: String.

Example

The example macros section in the server configuration file:

<macros>
    <test>Value</test>
</macros>

Query:

SELECT getMacro('test');

Result:

┌─getMacro('test')─┐
│ Value            │
└──────────────────┘

An alternative way to get the same value:

SELECT * FROM system.macros
WHERE macro = 'test';

┌─macro─┬─substitution─┐
│ test  │ Value        │
└───────┴──────────────┘

FQDN

Returns the fully qualified domain name.

Syntax

fqdn();

This function is case-insensitive.

Returned value

• String with the fully qualified domain name.

Type: String.

Example

Query:

SELECT FQDN();

Result:

┌─FQDN()──────────────────────────┐
│ clickhouse.ru-central1.internal │
└─────────────────────────────────┘

basename

Extracts the trailing part of a string after the last slash or backslash. This function if often used to extract the filename from a path.

basename( expr )

Arguments

• expr — Expression resulting in a String type value. All the backslashes must be escaped in the resulting value.

Returned Value

A string that contains:

• The trailing part of a string after the last slash or backslash.

  If the input string contains a path ending with slash or backslash, for example, `/` or `c:\`, the function returns an empty string.
  

• The original string if there are no slashes or backslashes.

Example

SELECT 'some/long/path/to/file' AS a, basename(a)

┌─a──────────────────────┬─basename('some\\long\\path\\to\\file')─┐
│ some\long\path\to\file │ file                                   │
└────────────────────────┴────────────────────────────────────────┘

SELECT 'some\\long\\path\\to\\file' AS a, basename(a)

┌─a──────────────────────┬─basename('some\\long\\path\\to\\file')─┐
│ some\long\path\to\file │ file                                   │
└────────────────────────┴────────────────────────────────────────┘

SELECT 'some-file-name' AS a, basename(a)

┌─a──────────────┬─basename('some-file-name')─┐
│ some-file-name │ some-file-name             │
└────────────────┴────────────────────────────┘

visibleWidth(x)

Calculates the approximate width when outputting values to the console in text format (tab-separated). This function is used by the system for implementing Pretty formats.

NULL is represented as a string corresponding to NULL in Pretty formats.

SELECT visibleWidth(NULL)

┌─visibleWidth(NULL)─┐
│                  4 │
└────────────────────┘

toTypeName(x)

Returns a string containing the type name of the passed argument.

If NULL is passed to the function as input, then it returns the Nullable(Nothing) type, which corresponds to an internal NULL representation in ClickHouse.

blockSize()

Gets the size of the block. In ClickHouse, queries are always run on blocks (sets of column parts). This function allows getting the size of the block that you called it for.

byteSize

Returns estimation of uncompressed byte size of its arguments in memory.

Syntax

byteSize(argument [, ...])

Arguments

• argument — Value.

Returned value

• Estimation of byte size of the arguments in memory.

Type: UInt64.

Examples

For String arguments the funtion returns the string length + 9 (terminating zero + length).

Query:

SELECT byteSize('string');

Result:

┌─byteSize('string')─┐
│                 15 │
└────────────────────┘

Query:

CREATE TABLE test
(
    `key` Int32,
    `u8` UInt8,
    `u16` UInt16,
    `u32` UInt32,
    `u64` UInt64,
    `i8` Int8,
    `i16` Int16,
    `i32` Int32,
    `i64` Int64,
    `f32` Float32,
    `f64` Float64
)
ENGINE = MergeTree
ORDER BY key;

INSERT INTO test VALUES(1, 8, 16, 32, 64,  -8, -16, -32, -64, 32.32, 64.64);

SELECT key, byteSize(u8) AS `byteSize(UInt8)`, byteSize(u16) AS `byteSize(UInt16)`, byteSize(u32) AS `byteSize(UInt32)`, byteSize(u64) AS `byteSize(UInt64)`, byteSize(i8) AS `byteSize(Int8)`, byteSize(i16) AS `byteSize(Int16)`, byteSize(i32) AS `byteSize(Int32)`, byteSize(i64) AS `byteSize(Int64)`, byteSize(f32) AS `byteSize(Float32)`, byteSize(f64) AS `byteSize(Float64)` FROM test ORDER BY key ASC FORMAT Vertical;

Result:

Row 1:
──────
key:               1
byteSize(UInt8):   1
byteSize(UInt16):  2
byteSize(UInt32):  4
byteSize(UInt64):  8
byteSize(Int8):    1
byteSize(Int16):   2
byteSize(Int32):   4
byteSize(Int64):   8
byteSize(Float32): 4
byteSize(Float64): 8

If the function takes multiple arguments, it returns their combined byte size.

Query:

SELECT byteSize(NULL, 1, 0.3, '');

Result:

┌─byteSize(NULL, 1, 0.3, '')─┐
│                         19 │
└────────────────────────────┘

materialize(x)

Turns a constant into a full column containing just one value. In ClickHouse, full columns and constants are represented differently in memory. Functions work differently for constant arguments and normal arguments (different code is executed), although the result is almost always the same. This function is for debugging this behavior.

ignore(…)

Accepts any arguments, including NULL. Always returns 0. However, the argument is still evaluated. This can be used for benchmarks.

sleep(seconds)

Sleeps ‘seconds’ seconds on each data block. You can specify an integer or a floating-point number.

sleepEachRow(seconds)

Sleeps ‘seconds’ seconds on each row. You can specify an integer or a floating-point number.

currentDatabase()

Returns the name of the current database. You can use this function in table engine parameters in a CREATE TABLE query where you need to specify the database.

currentUser()

Returns the login of current user. Login of user, that initiated query, will be returned in case distibuted query.

SELECT currentUser();

Alias: user(), USER().

Returned values

• Login of current user.
• Login of user that initiated query in case of disributed query.

Type: String.

Example

Query:

SELECT currentUser();

Result:

┌─currentUser()─┐
│ default       │
└───────────────┘

isConstant

Checks whether the argument is a constant expression.

A constant expression means an expression whose resulting value is known at the query analysis (i.e. before execution). For example, expressions over literals are constant expressions.

The function is intended for development, debugging and demonstration.

Syntax

isConstant(x)

Arguments

• x — Expression to check.

Returned values

• 1 — x is constant.
• 0 — x is non-constant.

Type: UInt8.

Examples

Query:

SELECT isConstant(x + 1) FROM (SELECT 43 AS x)

Result:

┌─isConstant(plus(x, 1))─┐
│                      1 │
└────────────────────────┘

Query:

WITH 3.14 AS pi SELECT isConstant(cos(pi))

Result:

┌─isConstant(cos(pi))─┐
│                   1 │
└─────────────────────┘

Query:

SELECT isConstant(number) FROM numbers(1)

Result:

┌─isConstant(number)─┐
│                  0 │
└────────────────────┘

isFinite(x)

Accepts Float32 and Float64 and returns UInt8 equal to 1 if the argument is not infinite and not a NaN, otherwise 0.

isInfinite(x)

Accepts Float32 and Float64 and returns UInt8 equal to 1 if the argument is infinite, otherwise 0. Note that 0 is returned for a NaN.

ifNotFinite

Checks whether floating point value is finite.

Syntax

ifNotFinite(x,y)

Arguments

• x — Value to be checked for infinity. Type: Float*.
• y — Fallback value. Type: Float*.

Returned value

• x if x is finite.
• y if x is not finite.

Example

Query:

SELECT 1/0 as infimum, ifNotFinite(infimum,42)

Result:

┌─infimum─┬─ifNotFinite(divide(1, 0), 42)─┐
│     inf │                            42 │
└─────────┴───────────────────────────────┘

You can get similar result by using ternary operator: isFinite(x) ? x : y.

isNaN(x)

Accepts Float32 and Float64 and returns UInt8 equal to 1 if the argument is a NaN, otherwise 0.

hasColumnInTable([‘hostname’[, ‘username’[, ‘password’]],] ‘database’, ‘table’, ‘column’)

Accepts constant strings: database name, table name, and column name. Returns a UInt8 constant expression equal to 1 if there is a column, otherwise 0. If the hostname parameter is set, the test will run on a remote server. The function throws an exception if the table does not exist. For elements in a nested data structure, the function checks for the existence of a column. For the nested data structure itself, the function returns 0.

bar

Allows building a unicode-art diagram.

bar(x, min, max, width) draws a band with a width proportional to (x - min) and equal to width characters when x = max.

Arguments

• x — Size to display.
• min, max — Integer constants. The value must fit in Int64.
• width — Constant, positive integer, can be fractional.

The band is drawn with accuracy to one eighth of a symbol.

Example:

SELECT
    toHour(EventTime) AS h,
    count() AS c,
    bar(c, 0, 600000, 20) AS bar
FROM test.hits
GROUP BY h
ORDER BY h ASC

┌──h─┬──────c─┬─bar────────────────┐
│  0 │ 292907 │ █████████▋         │
│  1 │ 180563 │ ██████             │
│  2 │ 114861 │ ███▋               │
│  3 │  85069 │ ██▋                │
│  4 │  68543 │ ██▎                │
│  5 │  78116 │ ██▌                │
│  6 │ 113474 │ ███▋               │
│  7 │ 170678 │ █████▋             │
│  8 │ 278380 │ █████████▎         │
│  9 │ 391053 │ █████████████      │
│ 10 │ 457681 │ ███████████████▎   │
│ 11 │ 493667 │ ████████████████▍  │
│ 12 │ 509641 │ ████████████████▊  │
│ 13 │ 522947 │ █████████████████▍ │
│ 14 │ 539954 │ █████████████████▊ │
│ 15 │ 528460 │ █████████████████▌ │
│ 16 │ 539201 │ █████████████████▊ │
│ 17 │ 523539 │ █████████████████▍ │
│ 18 │ 506467 │ ████████████████▊  │
│ 19 │ 520915 │ █████████████████▎ │
│ 20 │ 521665 │ █████████████████▍ │
│ 21 │ 542078 │ ██████████████████ │
│ 22 │ 493642 │ ████████████████▍  │
│ 23 │ 400397 │ █████████████▎     │
└────┴────────┴────────────────────┘

transform

Transforms a value according to the explicitly defined mapping of some elements to other ones. There are two variations of this function:

transform(x, array_from, array_to, default)

x – What to transform.

array_from – Constant array of values for converting.

array_to – Constant array of values to convert the values in ‘from’ to.

default – Which value to use if ‘x’ is not equal to any of the values in ‘from’.

array_from and array_to – Arrays of the same size.

Types:

transform(T, Array(T), Array(U), U) -> U

T and U can be numeric, string, or Date or DateTime types. Where the same letter is indicated (T or U), for numeric types these might not be matching types, but types that have a common type. For example, the first argument can have the Int64 type, while the second has the Array(UInt16) type.

If the ‘x’ value is equal to one of the elements in the ‘array_from’ array, it returns the existing element (that is numbered the same) from the ‘array_to’ array. Otherwise, it returns ‘default’. If there are multiple matching elements in ‘array_from’, it returns one of the matches.

Example:

SELECT
    transform(SearchEngineID, [2, 3], ['Yandex', 'Google'], 'Other') AS title,
    count() AS c
FROM test.hits
WHERE SearchEngineID != 0
GROUP BY title
ORDER BY c DESC

┌─title─────┬──────c─┐
│ Yandex    │ 498635 │
│ Google    │ 229872 │
│ Other     │ 104472 │
└───────────┴────────┘

transform(x, array_from, array_to)

Differs from the first variation in that the ‘default’ argument is omitted. If the ‘x’ value is equal to one of the elements in the ‘array_from’ array, it returns the matching element (that is numbered the same) from the ‘array_to’ array. Otherwise, it returns ‘x’.

Types:

transform(T, Array(T), Array(T)) -> T

Example:

SELECT
    transform(domain(Referer), ['yandex.ru', 'google.ru', 'vkontakte.ru'], ['www.yandex', 'example.com', 'vk.com']) AS s,
    count() AS c
FROM test.hits
GROUP BY domain(Referer)
ORDER BY count() DESC
LIMIT 10

┌─s──────────────┬───────c─┐
│                │ 2906259 │
│ www.yandex     │  867767 │
│ ███████.ru     │  313599 │
│ mail.yandex.ru │  107147 │
│ ██████.ru      │  100355 │
│ █████████.ru   │   65040 │
│ news.yandex.ru │   64515 │
│ ██████.net     │   59141 │
│ example.com    │   57316 │
└────────────────┴─────────┘

formatReadableDecimalSize(x)

Accepts the size (number of bytes). Returns a rounded size with a suffix (KB, MB, etc.) as a string.

Example:

SELECT
    arrayJoin([1, 1024, 1024*1024, 192851925]) AS filesize_bytes,
    formatReadableDecimalSize(filesize_bytes) AS filesize

┌─filesize_bytes─┬─filesize───┐
│              1 │ 1.00 B     │
│           1024 │ 1.02 KB   │
│        1048576 │ 1.05 MB   │
│      192851925 │ 192.85 MB │
└────────────────┴────────────┘

formatReadableSize(x)

Accepts the size (number of bytes). Returns a rounded size with a suffix (KiB, MiB, etc.) as a string.

Example:

SELECT
    arrayJoin([1, 1024, 1024*1024, 192851925]) AS filesize_bytes,
    formatReadableSize(filesize_bytes) AS filesize

┌─filesize_bytes─┬─filesize───┐
│              1 │ 1.00 B     │
│           1024 │ 1.00 KiB   │
│        1048576 │ 1.00 MiB   │
│      192851925 │ 183.92 MiB │
└────────────────┴────────────┘

formatReadableQuantity(x)

Accepts the number. Returns a rounded number with a suffix (thousand, million, billion, etc.) as a string.

It is useful for reading big numbers by human.

Example:

SELECT
    arrayJoin([1024, 1234 * 1000, (4567 * 1000) * 1000, 98765432101234]) AS number,
    formatReadableQuantity(number) AS number_for_humans

┌─────────number─┬─number_for_humans─┐
│           1024 │ 1.02 thousand     │
│        1234000 │ 1.23 million      │
│     4567000000 │ 4.57 billion      │
│ 98765432101234 │ 98.77 trillion    │
└────────────────┴───────────────────┘

formatReadableTimeDelta

Accepts the time delta in seconds. Returns a time delta with (year, month, day, hour, minute, second) as a string.

Syntax

formatReadableTimeDelta(column[, maximum_unit])

Arguments

• column — A column with numeric time delta.
• maximum_unit — Optional. Maximum unit to show. Acceptable values seconds, minutes, hours, days, months, years.

Example:

SELECT
    arrayJoin([100, 12345, 432546534]) AS elapsed,
    formatReadableTimeDelta(elapsed) AS time_delta

┌────elapsed─┬─time_delta ─────────────────────────────────────────────────────┐
│        100 │ 1 minute and 40 seconds                                         │
│      12345 │ 3 hours, 25 minutes and 45 seconds                              │
│  432546534 │ 13 years, 8 months, 17 days, 7 hours, 48 minutes and 54 seconds │
└────────────┴─────────────────────────────────────────────────────────────────┘

SELECT
    arrayJoin([100, 12345, 432546534]) AS elapsed,
    formatReadableTimeDelta(elapsed, 'minutes') AS time_delta

┌────elapsed─┬─time_delta ─────────────────────────────────────────────────────┐
│        100 │ 1 minute and 40 seconds                                         │
│      12345 │ 205 minutes and 45 seconds                                      │
│  432546534 │ 7209108 minutes and 54 seconds                                  │
└────────────┴─────────────────────────────────────────────────────────────────┘

parseTimeDelta

Parse a sequence of numbers followed by something resembling a time unit.

Syntax

parseTimeDelta(timestr)

Arguments

• timestr — A sequence of numbers followed by something resembling a time unit.

Returned value

• A floating-point number with the number of seconds.

Example

SELECT parseTimeDelta('11s+22min')

┌─parseTimeDelta('11s+22min')─┐
│                        1331 │
└─────────────────────────────┘

SELECT parseTimeDelta('1yr2mo')

┌─parseTimeDelta('1yr2mo')─┐
│                 36806400 │
└──────────────────────────┘

least(a, b)

Returns the smallest value from a and b.

greatest(a, b)

Returns the largest value of a and b.

uptime()

Returns the server’s uptime in seconds. If it is executed in the context of a distributed table, then it generates a normal column with values relevant to each shard. Otherwise it produces a constant value.

version()

Returns the version of the server as a string. If it is executed in the context of a distributed table, then it generates a normal column with values relevant to each shard. Otherwise it produces a constant value.

buildId()

Returns the build ID generated by a compiler for the running ClickHouse server binary. If it is executed in the context of a distributed table, then it generates a normal column with values relevant to each shard. Otherwise it produces a constant value.

blockNumber

Returns the sequence number of the data block where the row is located.

rowNumberInBlock

Returns the ordinal number of the row in the data block. Different data blocks are always recalculated.

rowNumberInAllBlocks()

Returns the ordinal number of the row in the data block. This function only considers the affected data blocks.

neighbor

The window function that provides access to a row at a specified offset which comes before or after the current row of a given column.

Syntax

neighbor(column, offset[, default_value])

The result of the function depends on the affected data blocks and the order of data in the block.

:::warning
It can reach the neighbor rows only inside the currently processed data block. :::

The rows order used during the calculation of neighbor can differ from the order of rows returned to the user. To prevent that you can make a subquery with ORDER BY and call the function from outside the subquery.

Arguments

• column — A column name or scalar expression.
• offset — The number of rows forwards or backwards from the current row of column. Int64.
• default_value — Optional. The value to be returned if offset goes beyond the scope of the block. Type of data blocks affected.

Returned values

• Value for column in offset distance from current row if offset value is not outside block bounds.
• Default value for column if offset value is outside block bounds. If default_value is given, then it will be used.

Type: type of data blocks affected or default value type.

Example

Query:

SELECT number, neighbor(number, 2) FROM system.numbers LIMIT 10;

Result:

┌─number─┬─neighbor(number, 2)─┐
│      0 │                   2 │
│      1 │                   3 │
│      2 │                   4 │
│      3 │                   5 │
│      4 │                   6 │
│      5 │                   7 │
│      6 │                   8 │
│      7 │                   9 │
│      8 │                   0 │
│      9 │                   0 │
└────────┴─────────────────────┘

Query:

SELECT number, neighbor(number, 2, 999) FROM system.numbers LIMIT 10;

Result:

┌─number─┬─neighbor(number, 2, 999)─┐
│      0 │                        2 │
│      1 │                        3 │
│      2 │                        4 │
│      3 │                        5 │
│      4 │                        6 │
│      5 │                        7 │
│      6 │                        8 │
│      7 │                        9 │
│      8 │                      999 │
│      9 │                      999 │
└────────┴──────────────────────────┘

This function can be used to compute year-over-year metric value:

Query:

WITH toDate('2018-01-01') AS start_date
SELECT
    toStartOfMonth(start_date + (number * 32)) AS month,
    toInt32(month) % 100 AS money,
    neighbor(money, -12) AS prev_year,
    round(prev_year / money, 2) AS year_over_year
FROM numbers(16)

Result:

┌──────month─┬─money─┬─prev_year─┬─year_over_year─┐
│ 2018-01-01 │    32 │         0 │              0 │
│ 2018-02-01 │    63 │         0 │              0 │
│ 2018-03-01 │    91 │         0 │              0 │
│ 2018-04-01 │    22 │         0 │              0 │
│ 2018-05-01 │    52 │         0 │              0 │
│ 2018-06-01 │    83 │         0 │              0 │
│ 2018-07-01 │    13 │         0 │              0 │
│ 2018-08-01 │    44 │         0 │              0 │
│ 2018-09-01 │    75 │         0 │              0 │
│ 2018-10-01 │     5 │         0 │              0 │
│ 2018-11-01 │    36 │         0 │              0 │
│ 2018-12-01 │    66 │         0 │              0 │
│ 2019-01-01 │    97 │        32 │           0.33 │
│ 2019-02-01 │    28 │        63 │           2.25 │
│ 2019-03-01 │    56 │        91 │           1.62 │
│ 2019-04-01 │    87 │        22 │           0.25 │
└────────────┴───────┴───────────┴────────────────┘

runningDifference(x)

Calculates the difference between successive row values ​​in the data block. Returns 0 for the first row and the difference from the previous row for each subsequent row.

:::warning
It can reach the previous row only inside the currently processed data block. :::

The result of the function depends on the affected data blocks and the order of data in the block.

The rows order used during the calculation of runningDifference can differ from the order of rows returned to the user. To prevent that you can make a subquery with ORDER BY and call the function from outside the subquery.

Example:

SELECT
    EventID,
    EventTime,
    runningDifference(EventTime) AS delta
FROM
(
    SELECT
        EventID,
        EventTime
    FROM events
    WHERE EventDate = '2016-11-24'
    ORDER BY EventTime ASC
    LIMIT 5
)

┌─EventID─┬───────────EventTime─┬─delta─┐
│    1106 │ 2016-11-24 00:00:04 │     0 │
│    1107 │ 2016-11-24 00:00:05 │     1 │
│    1108 │ 2016-11-24 00:00:05 │     0 │
│    1109 │ 2016-11-24 00:00:09 │     4 │
│    1110 │ 2016-11-24 00:00:10 │     1 │
└─────────┴─────────────────────┴───────┘

Please note - block size affects the result. With each new block, the runningDifference state is reset.

SELECT
    number,
    runningDifference(number + 1) AS diff
FROM numbers(100000)
WHERE diff != 1

┌─number─┬─diff─┐
│      0 │    0 │
└────────┴──────┘
┌─number─┬─diff─┐
│  65536 │    0 │
└────────┴──────┘

set max_block_size=100000 -- default value is 65536!

SELECT
    number,
    runningDifference(number + 1) AS diff
FROM numbers(100000)
WHERE diff != 1

┌─number─┬─diff─┐
│      0 │    0 │
└────────┴──────┘

runningDifferenceStartingWithFirstValue

Same as for runningDifference, the difference is the value of the first row, returned the value of the first row, and each subsequent row returns the difference from the previous row.

runningConcurrency

Calculates the number of concurrent events. Each event has a start time and an end time. The start time is included in the event, while the end time is excluded. Columns with a start time and an end time must be of the same data type. The function calculates the total number of active (concurrent) events for each event start time.

:::warning
Events must be ordered by the start time in ascending order. If this requirement is violated the function raises an exception. Every data block is processed separately. If events from different data blocks overlap then they can not be processed correctly. :::

Syntax

runningConcurrency(start, end)

Arguments

• start — A column with the start time of events. Date, DateTime, or DateTime64.
• end — A column with the end time of events. Date, DateTime, or DateTime64.

Returned values

• The number of concurrent events at each event start time.

Type: UInt32

Example

Consider the table:

┌──────start─┬────────end─┐
│ 2021-03-03 │ 2021-03-11 │
│ 2021-03-06 │ 2021-03-12 │
│ 2021-03-07 │ 2021-03-08 │
│ 2021-03-11 │ 2021-03-12 │
└────────────┴────────────┘

Query:

SELECT start, runningConcurrency(start, end) FROM example_table;

Result:

┌──────start─┬─runningConcurrency(start, end)─┐
│ 2021-03-03 │                              1 │
│ 2021-03-06 │                              2 │
│ 2021-03-07 │                              3 │
│ 2021-03-11 │                              2 │
└────────────┴────────────────────────────────┘

MACNumToString(num)

Accepts a UInt64 number. Interprets it as a MAC address in big endian. Returns a string containing the corresponding MAC address in the format AA:BB:CC:DD:EE:FF (colon-separated numbers in hexadecimal form).

MACStringToNum(s)

The inverse function of MACNumToString. If the MAC address has an invalid format, it returns 0.

MACStringToOUI(s)

Accepts a MAC address in the format AA:BB:CC:DD:EE:FF (colon-separated numbers in hexadecimal form). Returns the first three octets as a UInt64 number. If the MAC address has an invalid format, it returns 0.

getSizeOfEnumType

Returns the number of fields in Enum.

getSizeOfEnumType(value)

Arguments:

• value — Value of type Enum.

Returned values

• The number of fields with Enum input values.
• An exception is thrown if the type is not Enum.

Example

SELECT getSizeOfEnumType( CAST('a' AS Enum8('a' = 1, 'b' = 2) ) ) AS x

┌─x─┐
│ 2 │
└───┘

blockSerializedSize

Returns size on disk (without taking into account compression).

blockSerializedSize(value[, value[, ...]])

Arguments

• value — Any value.

Returned values

• The number of bytes that will be written to disk for block of values (without compression).

Example

Query:

SELECT blockSerializedSize(maxState(1)) as x

Result:

┌─x─┐
│ 2 │
└───┘

toColumnTypeName

Returns the name of the class that represents the data type of the column in RAM.

toColumnTypeName(value)

Arguments:

• value — Any type of value.

Returned values

• A string with the name of the class that is used for representing the value data type in RAM.

Example of the difference betweentoTypeName ' and ' toColumnTypeName

SELECT toTypeName(CAST('2018-01-01 01:02:03' AS DateTime))

┌─toTypeName(CAST('2018-01-01 01:02:03', 'DateTime'))─┐
│ DateTime                                            │
└─────────────────────────────────────────────────────┘

SELECT toColumnTypeName(CAST('2018-01-01 01:02:03' AS DateTime))

┌─toColumnTypeName(CAST('2018-01-01 01:02:03', 'DateTime'))─┐
│ Const(UInt32)                                             │
└───────────────────────────────────────────────────────────┘

The example shows that the DateTime data type is stored in memory as Const(UInt32).

dumpColumnStructure

Outputs a detailed description of data structures in RAM

dumpColumnStructure(value)

Arguments:

• value — Any type of value.

Returned values

• A string describing the structure that is used for representing the value data type in RAM.

Example

SELECT dumpColumnStructure(CAST('2018-01-01 01:02:03', 'DateTime'))

┌─dumpColumnStructure(CAST('2018-01-01 01:02:03', 'DateTime'))─┐
│ DateTime, Const(size = 1, UInt32(size = 1))                  │
└──────────────────────────────────────────────────────────────┘

defaultValueOfArgumentType

Outputs the default value for the data type.

Does not include default values for custom columns set by the user.

defaultValueOfArgumentType(expression)

Arguments:

• expression — Arbitrary type of value or an expression that results in a value of an arbitrary type.

Returned values

• 0 for numbers.
• Empty string for strings.
• ᴺᵁᴸᴸ for Nullable.

Example

SELECT defaultValueOfArgumentType( CAST(1 AS Int8) )

┌─defaultValueOfArgumentType(CAST(1, 'Int8'))─┐
│                                           0 │
└─────────────────────────────────────────────┘

SELECT defaultValueOfArgumentType( CAST(1 AS Nullable(Int8) ) )

┌─defaultValueOfArgumentType(CAST(1, 'Nullable(Int8)'))─┐
│                                                  ᴺᵁᴸᴸ │
└───────────────────────────────────────────────────────┘

defaultValueOfTypeName

Outputs the default value for given type name.

Does not include default values for custom columns set by the user.

defaultValueOfTypeName(type)

Arguments:

• type — A string representing a type name.

Returned values

• 0 for numbers.
• Empty string for strings.
• ᴺᵁᴸᴸ for Nullable.

Example

SELECT defaultValueOfTypeName('Int8')

┌─defaultValueOfTypeName('Int8')─┐
│                              0 │
└────────────────────────────────┘

SELECT defaultValueOfTypeName('Nullable(Int8)')

┌─defaultValueOfTypeName('Nullable(Int8)')─┐
│                                     ᴺᵁᴸᴸ │
└──────────────────────────────────────────┘

indexHint

The function is intended for debugging and introspection purposes. The function ignores it's argument and always returns 1. Arguments are not even evaluated.

But for the purpose of index analysis, the argument of this function is analyzed as if it was present directly without being wrapped inside indexHint function. This allows to select data in index ranges by the corresponding condition but without further filtering by this condition. The index in ClickHouse is sparse and using indexHint will yield more data than specifying the same condition directly.

Syntax

SELECT * FROM table WHERE indexHint(<expression>)

Returned value

1. Type: Uint8.

Example

Here is the example of test data from the table ontime.

Input table:

SELECT count() FROM ontime

┌─count()─┐
│ 4276457 │
└─────────┘

The table has indexes on the fields (FlightDate, (Year, FlightDate)).

Create a query, where the index is not used.

Query:

SELECT FlightDate AS k, count() FROM ontime GROUP BY k ORDER BY k

ClickHouse processed the entire table (Processed 4.28 million rows).

Result:

┌──────────k─┬─count()─┐
│ 2017-01-01 │   13970 │
│ 2017-01-02 │   15882 │
........................
│ 2017-09-28 │   16411 │
│ 2017-09-29 │   16384 │
│ 2017-09-30 │   12520 │
└────────────┴─────────┘

To apply the index, select a specific date.

Query:

SELECT FlightDate AS k, count() FROM ontime WHERE k = '2017-09-15' GROUP BY k ORDER BY k

By using the index, ClickHouse processed a significantly smaller number of rows (Processed 32.74 thousand rows).

Result:

┌──────────k─┬─count()─┐
│ 2017-09-15 │   16428 │
└────────────┴─────────┘

Now wrap the expression k = '2017-09-15' into indexHint function.

Query:

SELECT
    FlightDate AS k,
    count()
FROM ontime
WHERE indexHint(k = '2017-09-15')
GROUP BY k
ORDER BY k ASC

ClickHouse used the index in the same way as the previous time (Processed 32.74 thousand rows). The expression k = '2017-09-15' was not used when generating the result. In examle the indexHint function allows to see adjacent dates.

Result:

┌──────────k─┬─count()─┐
│ 2017-09-14 │    7071 │
│ 2017-09-15 │   16428 │
│ 2017-09-16 │    1077 │
│ 2017-09-30 │    8167 │
└────────────┴─────────┘

replicate

Creates an array with a single value.

Used for internal implementation of arrayJoin.

SELECT replicate(x, arr);

Arguments:

• arr — Original array. ClickHouse creates a new array of the same length as the original and fills it with the value x.
• x — The value that the resulting array will be filled with.

Returned value

An array filled with the value x.

Type: Array.

Example

Query:

SELECT replicate(1, ['a', 'b', 'c'])

Result:

┌─replicate(1, ['a', 'b', 'c'])─┐
│ [1,1,1]                       │
└───────────────────────────────┘

filesystemAvailable

Returns amount of remaining space on the filesystem where the files of the databases located. It is always smaller than total free space (filesystemFree) because some space is reserved for OS.

Syntax

filesystemAvailable()

Returned value

• The amount of remaining space available in bytes.

Type: UInt64.

Example

Query:

SELECT formatReadableSize(filesystemAvailable()) AS "Available space", toTypeName(filesystemAvailable()) AS "Type";

Result:

┌─Available space─┬─Type───┐
│ 30.75 GiB       │ UInt64 │
└─────────────────┴────────┘

filesystemFree

Returns total amount of the free space on the filesystem where the files of the databases located. See also filesystemAvailable

Syntax

filesystemFree()

Returned value

• Amount of free space in bytes.

Type: UInt64.

Example

Query:

SELECT formatReadableSize(filesystemFree()) AS "Free space", toTypeName(filesystemFree()) AS "Type";

Result:

┌─Free space─┬─Type───┐
│ 32.39 GiB  │ UInt64 │
└────────────┴────────┘

filesystemCapacity

Returns the capacity of the filesystem in bytes. For evaluation, the path to the data directory must be configured.

Syntax

filesystemCapacity()

Returned value

• Capacity information of the filesystem in bytes.

Type: UInt64.

Example

Query:

SELECT formatReadableSize(filesystemCapacity()) AS "Capacity", toTypeName(filesystemCapacity()) AS "Type"

Result:

┌─Capacity──┬─Type───┐
│ 39.32 GiB │ UInt64 │
└───────────┴────────┘

initializeAggregation

Calculates result of aggregate function based on single value. It is intended to use this function to initialize aggregate functions with combinator -State. You can create states of aggregate functions and insert them to columns of type AggregateFunction or use initialized aggregates as default values.

Syntax

initializeAggregation (aggregate_function, arg1, arg2, ..., argN)

Arguments

• aggregate_function — Name of the aggregation function to initialize. String.
• arg — Arguments of aggregate function.

Returned value(s)

• Result of aggregation for every row passed to the function.

The return type is the same as the return type of function, that initializeAgregation takes as first argument.

Example

Query:

SELECT uniqMerge(state) FROM (SELECT initializeAggregation('uniqState', number % 3) AS state FROM numbers(10000));

Result:

┌─uniqMerge(state)─┐
│                3 │
└──────────────────┘

Query:

SELECT finalizeAggregation(state), toTypeName(state) FROM (SELECT initializeAggregation('sumState', number % 3) AS state FROM numbers(5));

Result:

┌─finalizeAggregation(state)─┬─toTypeName(state)─────────────┐
│                          0 │ AggregateFunction(sum, UInt8) │
│                          1 │ AggregateFunction(sum, UInt8) │
│                          2 │ AggregateFunction(sum, UInt8) │
│                          0 │ AggregateFunction(sum, UInt8) │
│                          1 │ AggregateFunction(sum, UInt8) │
└────────────────────────────┴───────────────────────────────┘

Example with AggregatingMergeTree table engine and AggregateFunction column:

CREATE TABLE metrics
(
    key UInt64,
    value AggregateFunction(sum, UInt64) DEFAULT initializeAggregation('sumState', toUInt64(0))
)
ENGINE = AggregatingMergeTree
ORDER BY key

INSERT INTO metrics VALUES (0, initializeAggregation('sumState', toUInt64(42)))

See Also

• arrayReduce

finalizeAggregation

Takes state of aggregate function. Returns result of aggregation (or finalized state when using-State combinator).

Syntax

finalizeAggregation(state)

Arguments

• state — State of aggregation. AggregateFunction.

Returned value(s)

• Value/values that was aggregated.

Type: Value of any types that was aggregated.

Examples

Query:

SELECT finalizeAggregation(( SELECT countState(number) FROM numbers(10)));

Result:

┌─finalizeAggregation(_subquery16)─┐
│                               10 │
└──────────────────────────────────┘

Query:

SELECT finalizeAggregation(( SELECT sumState(number) FROM numbers(10)));

Result:

┌─finalizeAggregation(_subquery20)─┐
│                               45 │
└──────────────────────────────────┘

Note that NULL values are ignored.

Query:

SELECT finalizeAggregation(arrayReduce('anyState', [NULL, 2, 3]));

Result:

┌─finalizeAggregation(arrayReduce('anyState', [NULL, 2, 3]))─┐
│                                                          2 │
└────────────────────────────────────────────────────────────┘

Combined example:

Query:

WITH initializeAggregation('sumState', number) AS one_row_sum_state
SELECT
    number,
    finalizeAggregation(one_row_sum_state) AS one_row_sum,
    runningAccumulate(one_row_sum_state) AS cumulative_sum
FROM numbers(10);

Result:

┌─number─┬─one_row_sum─┬─cumulative_sum─┐
│      0 │           0 │              0 │
│      1 │           1 │              1 │
│      2 │           2 │              3 │
│      3 │           3 │              6 │
│      4 │           4 │             10 │
│      5 │           5 │             15 │
│      6 │           6 │             21 │
│      7 │           7 │             28 │
│      8 │           8 │             36 │
│      9 │           9 │             45 │
└────────┴─────────────┴────────────────┘

See Also

• arrayReduce
• initializeAggregation

runningAccumulate

Accumulates states of an aggregate function for each row of a data block.

:::warning
The state is reset for each new data block. :::

Syntax

runningAccumulate(agg_state[, grouping]);

Arguments

• agg_state — State of the aggregate function. AggregateFunction.
• grouping — Grouping key. Optional. The state of the function is reset if the grouping value is changed. It can be any of the supported data types for which the equality operator is defined.

Returned value

• Each resulting row contains a result of the aggregate function, accumulated for all the input rows from 0 to the current position. runningAccumulate resets states for each new data block or when the grouping value changes.

Type depends on the aggregate function used.

Examples

Consider how you can use runningAccumulate to find the cumulative sum of numbers without and with grouping.

Query:

SELECT k, runningAccumulate(sum_k) AS res FROM (SELECT number as k, sumState(k) AS sum_k FROM numbers(10) GROUP BY k ORDER BY k);

Result:

┌─k─┬─res─┐
│ 0 │   0 │
│ 1 │   1 │
│ 2 │   3 │
│ 3 │   6 │
│ 4 │  10 │
│ 5 │  15 │
│ 6 │  21 │
│ 7 │  28 │
│ 8 │  36 │
│ 9 │  45 │
└───┴─────┘

The subquery generates sumState for every number from 0 to 9. sumState returns the state of the sum function that contains the sum of a single number.

The whole query does the following:

1. For the first row, runningAccumulate takes sumState(0) and returns 0.
2. For the second row, the function merges sumState(0) and sumState(1) resulting in sumState(0 + 1), and returns 1 as a result.
3. For the third row, the function merges sumState(0 + 1) and sumState(2) resulting in sumState(0 + 1 + 2), and returns 3 as a result.
4. The actions are repeated until the block ends.

The following example shows the groupping parameter usage:

Query:

SELECT
    grouping,
    item,
    runningAccumulate(state, grouping) AS res
FROM
(
    SELECT
        toInt8(number / 4) AS grouping,
        number AS item,
        sumState(number) AS state
    FROM numbers(15)
    GROUP BY item
    ORDER BY item ASC
);

Result:

┌─grouping─┬─item─┬─res─┐
│        0 │    0 │   0 │
│        0 │    1 │   1 │
│        0 │    2 │   3 │
│        0 │    3 │   6 │
│        1 │    4 │   4 │
│        1 │    5 │   9 │
│        1 │    6 │  15 │
│        1 │    7 │  22 │
│        2 │    8 │   8 │
│        2 │    9 │  17 │
│        2 │   10 │  27 │
│        2 │   11 │  38 │
│        3 │   12 │  12 │
│        3 │   13 │  25 │
│        3 │   14 │  39 │
└──────────┴──────┴─────┘

As you can see, runningAccumulate merges states for each group of rows separately.

joinGet

The function lets you extract data from the table the same way as from a dictionary.

Gets data from Join tables using the specified join key.

Only supports tables created with the ENGINE = Join(ANY, LEFT, <join_keys>) statement.

Syntax

joinGet(join_storage_table_name, `value_column`, join_keys)

Arguments

• join_storage_table_name — an identifier indicates where search is performed. The identifier is searched in the default database (see parameter default_database in the config file). To override the default database, use the USE db_name or specify the database and the table through the separator db_name.db_table, see the example.
• value_column — name of the column of the table that contains required data.
• join_keys — list of keys.

Returned value

Returns list of values corresponded to list of keys.

If certain does not exist in source table then 0 or null will be returned based on join_use_nulls setting.

More info about join_use_nulls in Join operation.

Example

Input table:

CREATE DATABASE db_test
CREATE TABLE db_test.id_val(`id` UInt32, `val` UInt32) ENGINE = Join(ANY, LEFT, id) SETTINGS join_use_nulls = 1
INSERT INTO db_test.id_val VALUES (1,11)(2,12)(4,13)

┌─id─┬─val─┐
│  4 │  13 │
│  2 │  12 │
│  1 │  11 │
└────┴─────┘

Query:

SELECT joinGet(db_test.id_val,'val',toUInt32(number)) from numbers(4) SETTINGS join_use_nulls = 1

Result:

┌─joinGet(db_test.id_val, 'val', toUInt32(number))─┐
│                                                0 │
│                                               11 │
│                                               12 │
│                                                0 │
└──────────────────────────────────────────────────┘

catboostEvaluate(path_to_model, feature_1, feature_2, …, feature_n)

:::note This function is not available in ClickHouse Cloud. :::

Evaluate external catboost model. CatBoost is an open-source gradient boosting library developed by Yandex for machine learing. Accepts a path to a catboost model and model arguments (features). Returns Float64.

SELECT feat1, ..., feat_n, catboostEvaluate('/path/to/model.bin', feat_1, ..., feat_n) AS prediction
FROM data_table

Prerequisites

1. Build the catboost evaluation library

Before evaluating catboost models, the libcatboostmodel.<so|dylib> library must be made available. See CatBoost documentation how to compile it.

Next, specify the path to libcatboostmodel.<so|dylib> in the clickhouse configuration:

<clickhouse>
...
    <catboost_lib_path>/path/to/libcatboostmodel.so</catboost_lib_path>
...
</clickhouse>

For security and isolation reasons, the model evaluation does not run in the server process but in the clickhouse-library-bridge process. At the first execution of catboostEvaluate(), the server starts the library bridge process if it is not running already. Both processes communicate using a HTTP interface. By default, port 9012 is used. A different port can be specified as follows - this is useful if port 9012 is already assigned to a different service.

<library_bridge>
    <port>9019</port>
</library_bridge>

2. Train a catboost model using libcatboost

See Training and applying models for how to train catboost models from a training data set.

throwIf(x[, message[, error_code]])

Throw an exception if the argument is non zero. message - is an optional parameter: a constant string providing a custom error message error_code - is an optional parameter: a constant integer providing a custom error code

To use the error_code argument, configuration parameter allow_custom_error_code_in_throwif must be enabled.

SELECT throwIf(number = 3, 'Too many') FROM numbers(10);

↙ Progress: 0.00 rows, 0.00 B (0.00 rows/s., 0.00 B/s.) Received exception from server (version 19.14.1):
Code: 395. DB::Exception: Received from localhost:9000. DB::Exception: Too many.

identity

Returns the same value that was used as its argument. Used for debugging and testing, allows to cancel using index, and get the query performance of a full scan. When query is analyzed for possible use of index, the analyzer does not look inside identity functions. Also constant folding is not applied too.

Syntax

identity(x)

Example

Query:

SELECT identity(42)

Result:

┌─identity(42)─┐
│           42 │
└──────────────┘

randomPrintableASCII

Generates a string with a random set of ASCII printable characters.

Syntax

randomPrintableASCII(length)

Arguments

• length — Resulting string length. Positive integer.

  If you pass `length < 0`, behavior of the function is undefined.
  

Returned value

• String with a random set of ASCII printable characters.

Type: String

Example

SELECT number, randomPrintableASCII(30) as str, length(str) FROM system.numbers LIMIT 3

┌─number─┬─str────────────────────────────┬─length(randomPrintableASCII(30))─┐
│      0 │ SuiCOSTvC0csfABSw=UcSzp2.`rv8x │                               30 │
│      1 │ 1Ag NlJ &RCN:*>HVPG;PE-nO"SUFD │                               30 │
│      2 │ /"+<"wUTh:=LjJ Vm!c&hI*m#XTfzz │                               30 │
└────────┴────────────────────────────────┴──────────────────────────────────┘

randomString

Generates a binary string of the specified length filled with random bytes (including zero bytes).

Syntax

randomString(length)

Arguments

• length — String length. Positive integer.

Returned value

• String filled with random bytes.

Type: String.

Example

Query:

SELECT randomString(30) AS str, length(str) AS len FROM numbers(2) FORMAT Vertical;

Result:

Row 1:
──────
str: 3 G  :   pT ?w тi  k aV f6
len: 30

Row 2:
──────
str: 9 ,]    ^   )  ]??  8
len: 30

See Also

• generateRandom
• randomPrintableASCII

randomFixedString

Generates a binary string of the specified length filled with random bytes (including zero bytes).

Syntax

randomFixedString(length);

Arguments

• length — String length in bytes. UInt64.

Returned value(s)

• String filled with random bytes.

Type: FixedString.

Example

Query:

SELECT randomFixedString(13) as rnd, toTypeName(rnd)

Result:

┌─rnd──────┬─toTypeName(randomFixedString(13))─┐
│ j▒h㋖HɨZ'▒ │ FixedString(13)                 │
└──────────┴───────────────────────────────────┘

randomStringUTF8

Generates a random string of a specified length. Result string contains valid UTF-8 code points. The value of code points may be outside of the range of assigned Unicode.

Syntax

randomStringUTF8(length);

Arguments

• length — Required length of the resulting string in code points. UInt64.

Returned value(s)

• UTF-8 random string.

Type: String.

Example

Query:

SELECT randomStringUTF8(13)

Result:

┌─randomStringUTF8(13)─┐
│ 𘤗𙉝д兠庇󡅴󱱎󦐪􂕌𔊹𓰛   │
└──────────────────────┘

getSetting

Returns the current value of a custom setting.

Syntax

getSetting('custom_setting');

Parameter

• custom_setting — The setting name. String.

Returned value

• The setting current value.

Example

SET custom_a = 123;
SELECT getSetting('custom_a');

Result

123

See Also

• Custom Settings

isDecimalOverflow

Checks whether the Decimal value is out of its (or specified) precision.

Syntax

isDecimalOverflow(d, [p])

Arguments

• d — value. Decimal.
• p — precision. Optional. If omitted, the initial precision of the first argument is used. Using of this paratemer could be helpful for data extraction to another DBMS or file. UInt8.

Returned values

• 1 — Decimal value has more digits then it's precision allow,
• 0 — Decimal value satisfies the specified precision.

Example

Query:

SELECT isDecimalOverflow(toDecimal32(1000000000, 0), 9),
       isDecimalOverflow(toDecimal32(1000000000, 0)),
       isDecimalOverflow(toDecimal32(-1000000000, 0), 9),
       isDecimalOverflow(toDecimal32(-1000000000, 0));

Result:

1	1	1	1

countDigits

Returns number of decimal digits you need to represent the value.

Syntax

countDigits(x)

Arguments

• x — Int or Decimal value.

Returned value

Number of digits.

Type: UInt8.

:::note
For Decimal values takes into account their scales: calculates result over underlying integer type which is (value * scale). For example: countDigits(42) = 2, countDigits(42.000) = 5, countDigits(0.04200) = 4. I.e. you may check decimal overflow for Decimal64 with countDecimal(x) > 18. It's a slow variant of isDecimalOverflow. :::

Example

Query:

SELECT countDigits(toDecimal32(1, 9)), countDigits(toDecimal32(-1, 9)),
       countDigits(toDecimal64(1, 18)), countDigits(toDecimal64(-1, 18)),
       countDigits(toDecimal128(1, 38)), countDigits(toDecimal128(-1, 38));

Result:

10	10	19	19	39	39

errorCodeToName

Returned value

• Variable name for the error code.

Type: LowCardinality(String).

Syntax

errorCodeToName(1)

Result:

UNSUPPORTED_METHOD

tcpPort

Returns native interface TCP port number listened by this server. If it is executed in the context of a distributed table, then it generates a normal column, otherwise it produces a constant value.

Syntax

tcpPort()

Arguments

• None.

Returned value

• The TCP port number.

Type: UInt16.

Example

Query:

SELECT tcpPort();

Result:

┌─tcpPort()─┐
│      9000 │
└───────────┘

See Also

• tcp_port

currentProfiles

Returns a list of the current settings profiles for the current user.

The command SET PROFILE could be used to change the current setting profile. If the command SET PROFILE was not used the function returns the profiles specified at the current user's definition (see CREATE USER).

Syntax

currentProfiles()

Returned value

• List of the current user settings profiles.

Type: Array(String).

enabledProfiles

Returns settings profiles, assigned to the current user both explicitly and implicitly. Explicitly assigned profiles are the same as returned by the currentProfiles function. Implicitly assigned profiles include parent profiles of other assigned profiles, profiles assigned via granted roles, profiles assigned via their own settings, and the main default profile (see the default_profile section in the main server configuration file).

Syntax

enabledProfiles()

Returned value

• List of the enabled settings profiles.

Type: Array(String).

defaultProfiles

Returns all the profiles specified at the current user's definition (see CREATE USER statement).

Syntax

defaultProfiles()

Returned value

• List of the default settings profiles.

Type: Array(String).

currentRoles

Returns the names of the roles which are current for the current user. The current roles can be changed by the SET ROLE statement. If the SET ROLE statement was not used, the function currentRoles returns the same as defaultRoles.

Syntax

currentRoles()

Returned value

• List of the current roles for the current user.

Type: Array(String).

enabledRoles

Returns the names of the current roles and the roles, granted to some of the current roles.

Syntax

enabledRoles()

Returned value

• List of the enabled roles for the current user.

Type: Array(String).

defaultRoles

Returns the names of the roles which are enabled by default for the current user when he logins. Initially these are all roles granted to the current user (see GRANT), but that can be changed with the SET DEFAULT ROLE statement.

Syntax

defaultRoles()

Returned value

• List of the default roles for the current user.

Type: Array(String).

getServerPort

Returns the number of the server port. When the port is not used by the server, throws an exception.

Syntax

getServerPort(port_name)

Arguments

• port_name — The name of the server port. String. Possible values:

  • 'tcp_port'
  • 'tcp_port_secure'
  • 'http_port'
  • 'https_port'
  • 'interserver_http_port'
  • 'interserver_https_port'
  • 'mysql_port'
  • 'postgresql_port'
  • 'grpc_port'
  • 'prometheus.port'

Returned value

• The number of the server port.

Type: UInt16.

Example

Query:

SELECT getServerPort('tcp_port');

Result:

┌─getServerPort('tcp_port')─┐
│ 9000                      │
└───────────────────────────┘

queryID

Returns the ID of the current query. Other parameters of a query can be extracted from the system.query_log table via query_id.

In contrast to initialQueryID function, queryID can return different results on different shards (see example).

Syntax

queryID()

Returned value

• The ID of the current query.

Type: String

Example

Query:

CREATE TABLE tmp (str String) ENGINE = Log;
INSERT INTO tmp (*) VALUES ('a');
SELECT count(DISTINCT t) FROM (SELECT queryID() AS t FROM remote('127.0.0.{1..3}', currentDatabase(), 'tmp') GROUP BY queryID());

Result:

┌─count()─┐
│ 3       │
└─────────┘

initialQueryID

Returns the ID of the initial current query. Other parameters of a query can be extracted from the system.query_log table via initial_query_id.

In contrast to queryID function, initialQueryID returns the same results on different shards (see example).

Syntax

initialQueryID()

Returned value

• The ID of the initial current query.

Type: String

Example

Query:

CREATE TABLE tmp (str String) ENGINE = Log;
INSERT INTO tmp (*) VALUES ('a');
SELECT count(DISTINCT t) FROM (SELECT initialQueryID() AS t FROM remote('127.0.0.{1..3}', currentDatabase(), 'tmp') GROUP BY queryID());

Result:

┌─count()─┐
│ 1       │
└─────────┘

shardNum

Returns the index of a shard which processes a part of data for a distributed query. Indices are started from 1. If a query is not distributed then constant value 0 is returned.

Syntax

shardNum()

Returned value

• Shard index or constant 0.

Type: UInt32.

Example

In the following example a configuration with two shards is used. The query is executed on the system.one table on every shard.

Query:

CREATE TABLE shard_num_example (dummy UInt8) 
    ENGINE=Distributed(test_cluster_two_shards_localhost, system, one, dummy);
SELECT dummy, shardNum(), shardCount() FROM shard_num_example;

Result:

┌─dummy─┬─shardNum()─┬─shardCount()─┐
│     0 │          2 │            2 │
│     0 │          1 │            2 │
└───────┴────────────┴──────────────┘

See Also

• Distributed Table Engine

shardCount

Returns the total number of shards for a distributed query. If a query is not distributed then constant value 0 is returned.

Syntax

shardCount()

Returned value

• Total number of shards or 0.

Type: UInt32.

See Also

• shardNum() function example also contains shardCount() function call.

getOSKernelVersion

Returns a string with the current OS kernel version.

Syntax

getOSKernelVersion()

Arguments

• None.

Returned value

• The current OS kernel version.

Type: String.

Example

Query:

SELECT getOSKernelVersion();

Result:

┌─getOSKernelVersion()────┐
│ Linux 4.15.0-55-generic │
└─────────────────────────┘

zookeeperSessionUptime

Returns the uptime of the current ZooKeeper session in seconds.

Syntax

zookeeperSessionUptime()

Arguments

• None.

Returned value

• Uptime of the current ZooKeeper session in seconds.

Type: UInt32.

Example

Query:

SELECT zookeeperSessionUptime();

Result:

┌─zookeeperSessionUptime()─┐
│                      286 │
└──────────────────────────┘