Relational Data - Aggregate Functions - Reference - Quantiletdigest

Computes an approximate quantile of a numeric data sequence using the t-digest algorithm.

Memory consumption is log(n), where n is a number of values. The result depends on the order of running the query, and is nondeterministic.

The performance of the function is lower than performance of quantile or quantileTiming. In terms of the ratio of State size to precision, this function is much better than quantile.

When using multiple quantile* functions with different levels in a query, the internal states are not combined (that is, the query works less efficiently than it could). In this case, use the quantiles function.

Syntax

quantileTDigest(level)(expr)

Alias: medianTDigest.

Arguments

• level — Level of quantile. Optional parameter. Constant floating-point number from 0 to 1. We recommend using a level value in the range of [0.01, 0.99]. Default value: 0.5. At level=0.5 the function calculates median.
• expr — Expression over the column values resulting in numeric data types, Date or DateTime.

Returned value

• Approximate quantile of the specified level.

Type:

• Float64 for numeric data type input.
• Date if input values have the Date type.
• DateTime if input values have the DateTime type.

Example

Query:

SELECT quantileTDigest(number) FROM numbers(10)

Result:

┌─quantileTDigest(number)─┐
│                     4.5 │
└─────────────────────────┘

See Also

• median
• quantiles