Aggregate Operators and Expression Functions

Window aggregates (MIN, MAX, AVG, SUMC)

Aggregate operators operate on a stream with multiple fields — typically the output stream of the @(k,w) operator (a data window). They reduce all fields of the record to a single value.

Syntax

FROM stream.aggregator

where aggregator is one of:

Keywords are accepted in both lowercase and uppercase.

Output interval

Aggregates do not change the stream’s rate — the output interval is the same as the source’s:

\[\Delta_{result} = \Delta_{stream}\]

Example: moving average

DECLARE val INTEGER STREAM src, 1 FILE 'data.txt'

-- a 5-element window shifted by 1
SELECT * STREAM win5 FROM src@(1,5)

-- average of the last 5 values
SELECT win5[0] STREAM ma5 FROM win5.avg

The ma5 stream contains, at every moment, the average of the five most recent src samples.

Example: signal filter (sumc)

An excerpt from the signal-filter implementation example:

SELECT signalRow[_] * filter[_] STREAM accRow FROM signalRow+filter
SELECT accRow[0] STREAM output FROM accRow.sumc

accRow.sumc sums all fields of the accRow record (products of signal samples and filter coefficients), producing the output of an FIR filter.

Example: MIN and MAX

DECLARE v INTEGER STREAM src, 0.1 FILE '/dev/urandom'
SELECT * STREAM win10 FROM src@(1,10)

SELECT win10[0] STREAM min10 FROM win10.min
SELECT win10[0] STREAM max10 FROM win10.max

NOTE: The functionality described here is covered by the tests: simple_max, Pattern4, described in the appendix Integration Tests.

The to_string function

The to_string function converts a numeric expression to a text string of a given width. The result goes into a field of type STRING in the output stream.

Syntax

to_string(expression : width)
to_string(expression)

The width parameter (a natural number after the colon :) specifies the output field’s width in bytes. Omitting the parameter gives a default width of 32 bytes.

ℹ️ Info

The argument separator is a colon :, not a comma ,. A comma is the SELECT list separator — using a comma in to_string(x, n) will cause a parse error.

Example

DECLARE v INTEGER STREAM src, 1 FILE 'data.txt'

SELECT to_string(src[0]:10) STREAM labels FROM src

The labels stream contains the values of src formatted as text in a 10-byte field.

Concatenation with a literal

The resulting string can be joined with a string literal using the + operator:

SELECT to_string(src[0]:8) + '_ok' STREAM tagged FROM src

Output field size: 8 (from to_string) + 3 (literal _ok) = 11 bytes.

Use cases

to_string is useful when exporting to systems that accept text data (Graphite, InfluxDB via xqry), or when creating event labels combined with DO DUMP output.

NOTE: The functionality described here is covered by the tests: issue121_isnull, issue128_numeric_to_string, issue128_string_to_numeric, described in the appendix Integration Tests.