Update default eager loading strategy to subquery

Document the change from 'select' to 'subquery' as the default eager
loading strategy for HasMany and BelongsToMany associations.

Refs cakephp/cakephp#19345

Author: dereuromark

docs/en/appendices/5-4-migration-guide.md

@@ -16,7 +16,21 @@ bin/cake upgrade rector --rules cakephp54 <path/to/app/src>
 
 ## Behavior Changes
 
-- WIP
+### ORM
+
+The default eager loading strategy for `HasMany` and `BelongsToMany` associations
+has changed from ``select`` to ``subquery``. The ``subquery`` strategy performs
+better for larger datasets as it avoids packet size limits from large ``WHERE IN``
+clauses and reduces PHP memory usage by keeping IDs in the database.
+
+If you need the previous behavior, you can explicitly set the strategy when
+defining associations:
+
+```php
+$this->hasMany('Comments', [
+    'strategy' => 'select',
+]);
+```
 
 ## Deprecations
 

docs/en/orm/associations.md

@@ -462,9 +462,9 @@ Possible keys for hasMany association arrays include:
 - **propertyName**: The property name that should be filled with data from the
   associated table into the source table results. By default, this is the
   underscored & plural name of the association so `comments` in our example.
-- **strategy**: Defines the query strategy to use. Defaults to 'select'. The
-  other valid value is 'subquery', which replaces the `IN` list with an
-  equivalent subquery.
+- **strategy**: Defines the query strategy to use. Defaults to 'subquery'. The
+  other valid value is 'select', which uses the `IN` list of parent keys
+  directly instead of a subquery.
 - **saveStrategy**: Either `append` or `replace`. Defaults to `append`. When `append` the current
   records are appended to any records in the database. When `replace` associated
   records not in the current set will be removed. If the foreign key is a nullable
@@ -487,15 +487,15 @@ The above would output SQL similar to:
 
 ```sql
 SELECT * FROM articles;
-SELECT * FROM comments WHERE article_id IN (1, 2, 3, 4, 5);
+SELECT * FROM comments WHERE article_id IN (SELECT id FROM articles);
 ```
 
-When the subquery strategy is used, SQL similar to the following will be
+When the select strategy is used, SQL similar to the following will be
 generated:
 
 ```sql
 SELECT * FROM articles;
-SELECT * FROM comments WHERE article_id IN (SELECT id FROM articles);
+SELECT * FROM comments WHERE article_id IN (1, 2, 3, 4, 5);
 ```
 
 You may want to cache the counts for your hasMany associations. This is useful
@@ -616,9 +616,9 @@ Possible keys for belongsToMany association arrays include:
 - **propertyName**: The property name that should be filled with data from the
   associated table into the source table results. By default, this is the
   underscored & plural name of the association, so `tags` in our example.
-- **strategy**: Defines the query strategy to use. Defaults to 'select'. The
-  other valid value is 'subquery', which replaces the `IN` list with an
-  equivalent subquery.
+- **strategy**: Defines the query strategy to use. Defaults to 'subquery'. The
+  other valid value is 'select', which uses the `IN` list of parent keys
+  directly instead of a subquery.
 - **saveStrategy**: Either `append` or `replace`. Defaults to `replace`.
   Indicates the mode to be used for saving associated entities. The former will
   only create new links between both side of the relation and the latter will
@@ -645,19 +645,19 @@ SELECT * FROM articles;
 SELECT * FROM tags
 INNER JOIN articles_tags ON (
   tags.id = article_tags.tag_id
-  AND article_id IN (1, 2, 3, 4, 5)
+  AND article_id IN (SELECT id FROM articles)
 );
 ```
 
-When the subquery strategy is used, SQL similar to the following will be
+When the select strategy is used, SQL similar to the following will be
 generated:
 
 ```sql
 SELECT * FROM articles;
 SELECT * FROM tags
 INNER JOIN articles_tags ON (
   tags.id = article_tags.tag_id
-  AND article_id IN (SELECT id FROM articles)
+  AND article_id IN (1, 2, 3, 4, 5)
 );
 ```