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docs: add maxSubqueryBytes limit to migration guide landing page (#16547) 

Co-authored-by: 317brian <53799971+317brian@users.noreply.github.com>
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Jill Osborne 2024-06-04 20:52:06 +01:00 committed by GitHub
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docs/release-info/migration-guide.md
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@ -33,11 +33,12 @@ The guides in this section outline breaking changes introduced in Druid 25 and l
Druid now supports SQL-compliant array types. Whenever possible, you should use the array type over multi-value dimensions. See []()>.
## Migrate to `maxSubqueryBytes` from `maxSubqueryRows`
`maxSubqueryBytes` and `maxSubqueryRows` are guardrails to limit the amount of subquery data stored in the Java heap. `maxSubqueryBytes` is a better alternative to maxSubqueryRows because row-based limits ignore the size of the individual rows. The values for `maxSubqueryRows` also doesn't take into account the size of the cluster, which is available with the `maxSubqueryBytes` automatic configuration. See []().
-->
## Migrate to front-coded dictionary encoding
Druid encodes string columns into dictionaries for better compression. Front-coded dictionary encoding reduces storage and improves performance by optimizing for strings that share similar beginning substrings. See [Migration guide: front-coded dictionaries](migr-front-coded-dict.md) for more information.
## Migrate to `maxSubqueryBytes` from `maxSubqueryRows`
Druid allows you to set a byte-based limit on subquery size to prevent Brokers from running out of memory when handling large subqueries. The byte-based subquery limit overrides Druid's row-based subquery limit. We recommend that you move towards using byte-based limits starting in Druid 30.0. See [Migration guide: subquery limit](migr-subquery-limit.md) for more information.