Tutorial: Replace custom fields in Log Analytics workspace with KQL-based custom columns

Custom fields is a feature of Azure Monitor that allows you to extract into a separate column data from a different text column of the same table. Creation of new custom fields will be disabled starting March 31, 2023. Custom fields functionality will be deprecated and existing custom fields will stop functioning on March 31, 2026.

There are several advantages to using DCR-based ingestion-time transformations to accomplish the same result:

  • You can apply full set of string functions to shape your custom columns.
  • You can apply multiple operations to the same data. For example, extract a portion of a value to a separate column and remove the original column.
  • You can use ingestion-time transformations in your ARM templates to deploy custom columns at scale.

With the introduction of data collection rules (DCR), KQL-based transformations are the standard method of table customization, replacing legacy custom fields.

In this tutorial, you learn how to:

  • Locate custom fields that require replacement
  • Understand the content of the custom fields
  • Setup ingestion-time transformation to replace custom fields within the table


  • Log Analytics workspace with a table containing custom fields
  • Sufficient account privilege to create and modify data collection rules (DCR)

Locate custom fields for replacement

Start by locating custom fields to replace. If you already know the custom fields you plan to replace, proceed to the next step.

  1. Navigate to the Log Analytics workspace where the table with custom fields is located.

  2. In the side menu, select Tables. Select Manage table from the context menu for the table.

    Screenshot showing the manage table option for a table in a Log Analytics workspace

  3. Note if any data collection rules (DCRs) are associated with given table.

    • If any DCRs are present in corresponding section, it means that any pre-existing custom fields were either already implemented within these DCRs, or abandoned upon DCR creation. You're going to examine the content of custom fields on the next step of this tutorial and determine whether more updates to DCRs needed.
    • If there are no data collection rules associated with the table, then all columns in given table with names ending with "_CF" will be custom fields subject to replacement.

    Screenshot showing the properties of a table including data collection rules associated with the table

  4. Close the table properties dialog and select Edit schema from the table context menu. Scroll to the bottom of page where custom columns are listed. These columns end with _CF.

    Screenshot showing the column listing for a table including any custom columns

  5. Note the names of these columns since you'll determine their content in the next step.

Understand custom field content

Since there is no way to examine the custom field definition directly, you need to query the table to determine the custom field formula.

  1. Select Logs in the side menu and run a query to get a sample of data from the table.

    Screenshot of Log Analytics with query returning sample data

  2. Locate the columns noted in the previous step and examine their content.

    • If the column is not empty and there are DCRs associated with the table, then custom field logic has been already implemented with transformation. No action is required
    • If the column is empty (or not present in query results) and there are DCRs associated with the table, the custom field logic wasn't implemented with the DCR. Add a transformation to the dataflow in the existing DCR.
    • If the column isn't empty and there are no DCRs associated with the table, the custom field logic needs to implemented as a transformation in the workspace DCR.
  3. Examine the content of the custom field and determine the logic how it's being calculated. Custom fields usually calculate substrings of other columns in the same table. Determine which column the data comes from and the portion of the string it extracts.

Create transformation

You're now ready to create the required KQL snippet and add it to a DCR. This logic is applied to each record as it's ingested into the workspace.

  1. Modify the query for the table using KQL to replicate the custom field logic. If you have multiple custom fields to replace, you may combine their calculation logic into a single statement.

    Screenshot of Log Analytics with query returning data using transformation query

  2. Determine where your new KQL definition of the custom column needs to be placed.

Frequently Asked Questions

How do I migrate custom fields for a text log collected with legacy Log Analytics agent (MMA)?

Consider migrating to Azure Monitor Agent (AMA). Log Analytics agent is approaching its end of support, and you should migrate to Azure Monitor Agent (AMA). Text logs collected with AMA use log parsing logic defined in form of KQL transformations from the start. Custom fields aren't required and not supported in text logs collected by Azure Monitor Agent.

Is migration of custom fields to KQL mandatory?

No, you need to migrate your custom fields only if you still want your custom columns populated. If you don't migrate your custom fields, corresponding columns will stop being populated when support of custom fields is ended. Data that has been already processed and stored in the table won't be affected and will remain usable.

Will I lose my existing data in corresponding columns if I don't migrate my custom fields in time?

No, custom fields are calculated at the time of data ingestion. Deleting the field definition or not migrating them in time won't affect any data previously ingested.

Next steps