Add explanation about implicit indexes

docs/en/explanation/implicit-indexes.rst

@@ -0,0 +1,58 @@
+Implicit indexes
+================
+
+Ever noticed the DBAL creating indexes you did not remember asking for,
+with names such as ``IDX_885DBAFAA76ED395``? In this document, we will
+distinguish three types of indexes:
+- indexes you did ask for, or user-defined indexes;
+- indexes you did not ask for, created on your behalf by the DBAL, or
+  DBAL-defined indexes;
+- indexes you did not ask for, created on your behalf by the RDBMS, or
+  RDBMS-defined indexes, also known as implicit indexes.
+
+Implicit indexes can be created by some database platforms when you
+create a foreign key: they will create an index on the referencing
+table, using the referencing columns.
+
+The rationale behind this is that these indexes improve performance, for
+instance for checking that a delete operation can be performed on a
+referenced table without violating the constraint in the referencing
+table.
+
+Here are some database platforms that are known to create indexes when
+creating a foreign key:
+
+- `MySQL <https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html>`_
+- `MariaDB <https://mariadb.com/kb/en/foreign-keys>`_
+
+These platforms can drop an existing implicit index once it is fulfilled
+by a newly created user-defined index.
+
+Some other will not do so, on grounds that such indexes are not always
+needed, and can be created in many different ways. They instead leave
+that responsibility to the user:
+
+- `PostgreSQL <https://stackoverflow.com/questions/970562/postgres-and-indexes-on-foreign-keys-and-primary-keys>`_
+- `SQLite <https://sqlite.org/foreignkeys.html#fk_indexes>`_
+- `SQL Server <https://stackoverflow.com/questions/836167/does-a-foreign-key-automatically-create-an-index>`_
+
+Regardless of the behavior of the platform, the DBAL will create an
+index for you and will automatically pick an index name that obeys
+string length constraints of the platform you are using. That way,
+differences between platforms are reduced because you always end up with
+an index.
+
+This is a detail, but these indexes will be prefixed with ``IDX_``, and
+typically look like this:
+
+.. code-block:: sql
+
+   CREATE INDEX IDX_885DBAFAA76ED395 ON posts (user_id)
+
+In the case of MariaDB and MySQL, the creation of that DBAL-defined
+index will result in the RDBMS-defined index being dropped.
+
+You can still explicitly create such indexes yourself, and the DBAL will
+notice when your index fulfills the indexing and constraint needs of the
+implicit index it would create, and will refrain from doing so, much
+like some platforms drop indexes that are redundant as explained above.

docs/en/sidebar.rst

@@ -20,3 +20,5 @@
     reference/caching
     reference/known-vendor-issues
     reference/upgrading
+
+    explanation/implicit-indexes

lib/Doctrine/DBAL/Schema/Table.php

@@ -554,11 +554,13 @@ protected function _addForeignKeyConstraint(ForeignKeyConstraint $constraint)
 
         $this->_fkConstraints[$name] = $constraint;
 
-        // Add an explicit index on the foreign key columns.
-        // If there is already an index that fulfils this requirements drop the request.
-        // In the case of __construct calling this method during hydration from schema-details
-        // all the explicitly added indexes lead to duplicates. This creates computation overhead in this case,
-        // however no duplicate indexes are ever added (based on columns).
+        /* Add an implicit index (defined by the DBAL) on the foreign key
+           columns. If there is already a user-defined index that fulfills these
+           requirements drop the request. In the case of __construct() calling
+           this method during hydration from schema-details, all the explicitly
+           added indexes lead to duplicates. This creates computation overhead in
+           this case, however no duplicate indexes are ever added (based on
+           columns). */
         $indexName = $this->_generateIdentifierName(
             array_merge([$this->getName()], $constraint->getColumns()),
             'idx',