The varianceif aggregation in APL calculates the variance of values that meet a specified condition. This is useful when you want to understand the variability of a subset of data without considering all data points. For example, you can use varianceif to compute the variance of request durations for HTTP requests that resulted in a specific status code or to track anomalies in trace durations for a particular service. You can use the varianceif aggregation when analyzing logs, telemetry data, or security events where conditions on subsets of the data are critical to your analysis.

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For users of other query languages

If you come from other query languages, this section explains how to adjust your existing queries to achieve the same results in APL.

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Usage

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Syntax

summarize varianceif(Expr, Predicate)

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Parameters

  • Expr: The expression (numeric) for which you want to calculate the variance.
  • Predicate: A boolean condition that determines which records to include in the calculation.

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Returns

Returns the variance of Expr for the records where the Predicate is true. If no records match the condition, it returns null.

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Use case examples

You can use the varianceif function to calculate the variance of HTTP request durations for requests that succeeded (status == '200').Query
['sample-http-logs']
| summarize varianceif(req_duration_ms, status == '200')
Run in PlaygroundOutput
varianceif_req_duration_ms
15.6
This query calculates the variance of request durations for all HTTP requests that returned a status code of 200 (successful requests).
  • avgif: Computes the average value of an expression for records that match a given condition. Use avgif when you want the average instead of variance.
  • sumif: Returns the sum of values that meet a specified condition. Use sumif when you’re interested in totals, not variance.
  • stdevif: Returns the standard deviation of values based on a condition. Use stdevif when you want to measure dispersion using standard deviation instead of variance.