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Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer
Alters the query acceleration policy properties of a specific external delta table.
Tip
Use .alter-merge to add or update properties without replacing the whole policy. Array properties are merged (new elements are added, existing values preserved).
Use .alter to fully replace the policy, including overwriting arrays.
For limitations, see Limitations.
Permissions
You must have at least Database Admin permissions to run this command.
Syntax
.alter-merge 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. This property is required if no query acceleration policy is defined on the external table. | |
| Hot | Timespan |
The hot period defined in the query acceleration policy. Minimum value = 1 d. This property is required if no query acceleration policy is defined on the external table. | |
| HotWindows | DateTime |
One or more optional time windows. Delta data files created within these time windows are accelerated. | |
| MaxAge | Timespan |
The external table returns accelerated data if the last index refresh time is greater than @now - MaxAge. Otherwise, external table operates in nonaccelerated mode. Default is 5 minutes. Minimum is 1 minute. | |
| HotDateTimeColumn | String |
Name of a datetime column used to determine hot-cache eligibility. A data file will be cached if its min/max stats for this column are within the configured Hot period and/or HotWindows. If not specified, eligibility is determined using the file modificationTime. Eligibility is computed using delta log metadata only - setting this property does not add query-time overhead |
Example
In case the external table has query acceleration policy defined:
{ "HotDateTimeColumn": "Col1" }
In case the external table doesn't have query acceleration policy defined:
{ "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
In case the external table has query acceleration policy defined:
.alter-merge external table MyExternalTable policy query_acceleration '{"Hot": "1.00:00:00", "MaxAge" : "00:05:00"}'
In case the external table doesn't have query acceleration policy defined:
.alter-merge external table MyExternalTable policy query_acceleration '{"IsEnabled": true, "Hot": "1.00:00:00", "MaxAge" : "00:05:00"}'