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Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer
Alters the query acceleration policy of a specific external delta table.
For limitations, see Limitations.
Permissions
You must have at least Database Admin permissions to run this command.
Syntax
.alter external table ExternalTableName policy query_acceleration 'JSON-serialized policy'
Parameters
| Name | Type | Required | Description |
|---|---|---|---|
| ExternalTableName | string |
✔️ | The name of the external delta table. |
| JSON-serialized policy | string |
✔️ | String literal holding a JSON property bag. |
JSON property bag
| Property | Type | Required | Description |
|---|---|---|---|
| IsEnabled | Boolean |
✔️ | Indicates whether the policy is enabled. |
| Hot | Timespan |
✔️ | The hot period defined in the query acceleration policy. Minimum value = 1 d. |
| HotWindows | DateTime |
One or more optional time windows. Delta data files created within these time windows are accelerated. | |
| MaxAge | Timespan |
The external table will return accelerated data if the last index refresh time is greater than @now - MaxAge. Otherwise, external table will operate in non-accelerated mode. Default is 5 minutes. Minimum is 1 minute. |
Note
Query acceleration is applied to data within a specific time period, defined as timespan, starting from the modificationTime as stated for each file in the delta log.
Example
{ "IsEnabled": true, "Hot": "1.00:00:00" }
Returns
The command returns a table with one record that includes the modified policy object.
| Column | Type | Description |
|---|---|---|
| PolicyName | string |
The name of the policy - QueryAcceleration |
| EntityName | string |
The fully qualified name of the entity: [DatabaseName].[ExternalTableName] |
| Policy | string |
A JSON-serialization of the query acceleration policy that is set on the external delta table |
| ChildEntities | string |
The child entities this policy affects - null |
| EntityType | string |
The type of the entity the policy applies to - ExternalTable |
Example
.alter external table MyExternalTable policy query_acceleration '{"IsEnabled": true, "Hot": "1.00:00:00", "HotWindows":[{"MinValue":"2025-07-06 07:53:55.0192810","MaxValue":"2025-07-06 07:53:55.0192814"}], "MaxAge" : "00:05:00"}'