Cross-cluster join
A cross-cluster join involves joining data from datasets that reside in different clusters.
In a cross-cluster join, the query can be executed in three possible locations, each with a specific designation for reference throughout this document:
- Local cluster: The cluster to which the request is sent, which is also known as the cluster hosting the database in context.
- Left cluster: The cluster hosting the data on the left side of the join operation.
- Right cluster: The cluster hosting the data on the right side of the join operation.
The cluster that runs the query fetches the data from the other cluster.
Note
If the data on the left and right sides of a join operation is hosted in the same cluster, it isn't considered a cross-cluster join, even if the data is hosted outside of the local cluster.
Syntax
[ cluster(ClusterName).database(DatabaseName).]LeftTable | ...
| join [ hint.remote=Strategy ] (
[ cluster(ClusterName).database(DatabaseName).]RightTable | ...
) on Conditions
Learn more about syntax conventions.
Parameters
| Name | Type | Required | Description |
|---|---|---|---|
| LeftTable | string |
✔️ | The left table or tabular expression whose rows are to be merged. Denoted as $left. |
| Strategy | string |
Determines the cluster on which to execute the join. Supported values are: left, right, local, and auto. For more information, see Strategies. |
|
| ClusterName | string |
If the data for the join resides outside of the local cluster, use the cluster() function to specify the cluster. | |
| DatabaseName | string |
If the data for the join resides outside of the local database context, use the database() function to specify the database. | |
| RightTable | string |
✔️ | The right table or tabular expression whose rows are to be merged. Denoted as $right. |
| Conditions | string |
✔️ | Determines how rows from LeftTable are matched with rows from RightTable. If the columns you want to match have the same name in both tables, use the syntax ON ColumnName. Otherwise, use the syntax ON $left.LeftColumn == $right.RightColumn. To specify multiple conditions, you can either use the "and" keyword or separate them with commas. If you use commas, the conditions are evaluated using the "and" logical operator. |
Strategies
The following list explains the supported values for the Strategy parameter:
left: Execute join on the cluster of the left table, or left cluster.right: Execute join on the cluster of the right table, or right cluster.local: Execute join on the cluster of the current cluster, or local cluster.auto: (Default) Kusto makes the remoting decision.
Note
The join remoting hint is ignored if the hinted strategy isn't applicable to the join operation.
How the auto strategy works
By default, the auto strategy determines where the cross-cluster join should be executed based on the following rules:
- If one of the tables is hosted in the local cluster, then the join is performed on the local cluster.
- If both tables are hosted outside of the local cluster, then join is performed on the right cluster.
Consider the following examples:
// Example 1
T | ... | join (cluster("B").database("DB").T2 | ...) on Col1
// Example 2
cluster("B").database("DB").T | ... | join (cluster("C").database("DB2").T2 | ...) on Col1
With the auto strategy, "Example 1" would be executed on the local cluster. In "Example 2", assuming neither cluster is the local cluster, the join would be executed on the right cluster.
Performance considerations
For optimal performance, we recommend running the query on the cluster that contains the largest table.
Let's consider the following examples again:
// Example 1
T | ... | join (cluster("B").database("DB").T2 | ...) on Col1
// Example 2
cluster("B").database("DB").T | ... | join (cluster("C").database("DB2").T2 | ...) on Col1
"Example 1" is set to run on the local cluster, but if the dataset produced by T | ... is smaller than one produced by cluster("B").database("DB").T2 | ... then it would be more efficient to execute the join operation on cluster B, in this case the right cluster, instead of on the local cluster.
The following query does this by using the right strategy. With the right strategy, the join operation is performed on the right cluster, even if left table is in the local cluster.
T | ... | join hint.remote=right (cluster("B").database("DB").T2 | ...) on Col1
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