docs: Migration guide for subquery limit (#16519)

Adds a migration guide for Druid 30 to help users understand the new byte-based subquery limit property maxSubqueryBytes

docs/release-info/migr-subquery-limit.md

@@ -0,0 +1,60 @@
+---
+id: migr-subquery-limit
+title: "Migration guide: Subquery limit"
+sidebar_label: Subquery limit
+---
+
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+
+Druid now allows you to set a byte-based limit on subquery size, to prevent brokers from running out of memory when handling large subqueries. 
+Druid uses subqueries as joins as well as in common table expressions, such as WITH.
+
+The byte-based subquery limit overrides Druid's row-based subquery limit.
+
+:::info
+We recommend that you move towards using byte-based limits starting in Druid 30.0.
+:::
+
+For queries that generate a large number of rows (5 million or more), we recommend that you don't use `maxSubqueryBytes` from the outset. 
+You can increase `maxSubqueryRows` and then configure the byte-based limit if you find that Druid needs it to process the query.
+
+## Row-based subquery limit
+
+Druid uses the `maxSubqueryRows` property to limit the number of rows Druid returns in a subquery. 
+Because this is a row-based limit, it doesn't restrict the overall size of the returned data.
+
+The `maxSubqueryRows` property is set to 100,000 by default.
+
+## Enable a byte-based subquery limit
+
+Set the optional property `maxSubqueryBytes` to set a maximum number of returned bytes. 
+This property takes precedence over `maxSubqueryRows`.
+
+## Usage considerations
+
+You can set both `maxSubqueryRows` and `maxSubqueryBytes` at cluster level and override them in individual queries. 
+See [Overriding default query context values](../configuration#overriding-default-query-context-values) for more information.
+
+## Learn more
+
+See the following topics for more information:
+
+- [Query context](../querying/query-context.md) for information on setting query context parameters.
+- [Broker configuration reference](../configuration#guardrails-for-materialization-of-subqueries) for more information on `maxSubqueryRows` and `maxSubqueryBytes`.

website/sidebars.json

@@ -363,6 +363,7 @@
         "id": "release-info/migration-guide"
       },
       "items": [
+        "release-info/migr-subquery-limit"
       ]
     }
   ]},