diff --git a/core/lib/Drupal/Core/Database/Query/PagerSelectExtender.php b/core/lib/Drupal/Core/Database/Query/PagerSelectExtender.php
index e92689c345..f04e918adc 100644
--- a/core/lib/Drupal/Core/Database/Query/PagerSelectExtender.php
+++ b/core/lib/Drupal/Core/Database/Query/PagerSelectExtender.php
@@ -15,7 +15,7 @@
  * PagerSelectExtender last, so that its range and count are based on the full
  * query.
  */
-class PagerSelectExtender extends SelectExtender {
+class PagerSelectExtender extends SelectExtender implements PagerSelectExtenderInterface {
 
   /**
    * The number of elements per page to allow.
@@ -61,10 +61,7 @@ public function __construct(
   }
 
   /**
-   * Override the execute method.
-   *
-   * Before we run the query, we need to add pager-based range() instructions
-   * to it.
+   * {@inheritdoc}
    */
   public function execute() {
     // By calling preExecute() here, we force it to preprocess the extender
@@ -101,27 +98,14 @@ protected function ensureElement() {
   }
 
   /**
-   * Specify the count query object to use for this pager.
-   *
-   * You will rarely need to specify a count query directly.  If not specified,
-   * one is generated off of the pager query itself.
-   *
-   * @param \Drupal\Core\Database\Query\SelectInterface $query
-   *   The count query object.  It must return a single row with a single column,
-   *   which is the total number of records.
+   * {@inheritdoc}
    */
   public function setCountQuery(SelectInterface $query) {
     $this->customCountQuery = $query;
   }
 
   /**
-   * Retrieve the count query for this pager.
-   *
-   * The count query may be specified manually or, by default, taken from the
-   * query we are extending.
-   *
-   * @return \Drupal\Core\Database\Query\SelectInterface
-   *   A count query object.
+   * {@inheritdoc}
    */
   public function getCountQuery() {
     if ($this->customCountQuery) {
@@ -133,13 +117,7 @@ public function getCountQuery() {
   }
 
   /**
-   * Specify the maximum number of elements per page for this query.
-   *
-   * The default if not specified is 10 items per page.
-   *
-   * @param int|false $limit
-   *   An integer specifying the number of elements per page. If passed a false
-   *   value (FALSE, 0, NULL), the pager is disabled.
+   * {@inheritdoc}
    */
   public function limit($limit = 10) {
     $this->limit = $limit;
@@ -147,20 +125,7 @@ public function limit($limit = 10) {
   }
 
   /**
-   * Specify the element ID for this pager query.
-   *
-   * The element is used to differentiate different pager queries on the same
-   * page so that they may be operated independently.  If you do not specify an
-   * element, every pager query on the page will get a unique element.  If for
-   * whatever reason you want to explicitly define an element for a given query,
-   * you may do so here.
-   *
-   * Note that no collision detection is done when setting an element ID
-   * explicitly, so it is possible for two pagers to end up using the same ID
-   * if both are set explicitly.
-   *
-   * @param $element
-   *   Element ID that is used to differentiate different pager queries.
+   * {@inheritdoc}
    */
   public function element($element) {
     $this->element = $element;
@@ -169,14 +134,7 @@ public function element($element) {
   }
 
   /**
-   * Gets the element ID for this pager query.
-   *
-   * The element is used to differentiate different pager queries on the same
-   * page so that they may be operated independently.
-   *
-   * @return int
-   *   Element ID that is used to differentiate between different pager
-   *   queries.
+   * {@inheritdoc}
    */
   public function getElement(): int {
     $this->ensureElement();
diff --git a/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderFactory.php b/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderFactory.php
index dc8aac6069..e6757e5bde 100644
--- a/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderFactory.php
+++ b/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderFactory.php
@@ -8,7 +8,7 @@
 /**
  * Select extender factory for pager queries.
  */
-class PagerSelectExtenderFactory {
+class PagerSelectExtenderFactory implements PagerSelectExtenderFactoryInterface {
 
   /**
    * Constructs a PagerSelectExtenderFactory object.
@@ -29,10 +29,10 @@ public function __construct(
    * @param \Drupal\Core\Database\Connection $connection
    *   Database connection object.
    *
-   * @return \Drupal\Core\Database\Query\PagerSelectExtender
+   * @return \Drupal\Core\Database\Query\PagerSelectExtenderInterface
    *   A query extender for pager queries.
    */
-  public function get(SelectInterface $query, Connection $connection): PagerSelectExtender {
+  public function get(SelectInterface $query, Connection $connection): PagerSelectExtenderInterface {
     return new PagerSelectExtender($query, $connection, $this->pagerManager);
   }
 
diff --git a/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderFactoryInterface.php b/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderFactoryInterface.php
new file mode 100644
index 0000000000..0f07690ca7
--- /dev/null
+++ b/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderFactoryInterface.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace Drupal\Core\Database\Query;
+
+use Drupal\Core\Database\Connection;
+
+/**
+ * Select extender factory for pager queries.
+ */
+interface PagerSelectExtenderFactoryInterface {
+
+  /**
+   * Returns a query extender for pager queries.
+   *
+   * @param \Drupal\Core\Database\Query\SelectInterface $query
+   *   Select query object.
+   * @param \Drupal\Core\Database\Connection $connection
+   *   Database connection object.
+   *
+   * @return \Drupal\Core\Database\Query\PagerSelectExtenderInterface
+   *   A query extender for pager queries.
+   */
+  public function get(SelectInterface $query, Connection $connection): PagerSelectExtenderInterface;
+
+}
diff --git a/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderInterface.php b/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderInterface.php
new file mode 100644
index 0000000000..1796bc3346
--- /dev/null
+++ b/core/lib/Drupal/Core/Database/Query/PagerSelectExtenderInterface.php
@@ -0,0 +1,82 @@
+<?php
+
+namespace Drupal\Core\Database\Query;
+
+/**
+ * Interface for pager queries extenders.
+ */
+interface PagerSelectExtenderInterface {
+
+  /**
+   * Override the execute method.
+   *
+   * Before we run the query, we need to add pager-based range() instructions
+   * to it.
+   */
+  public function execute();
+
+  /**
+   * Specify the count query object to use for this pager.
+   *
+   * You will rarely need to specify a count query directly.  If not specified,
+   * one is generated off of the pager query itself.
+   *
+   * @param \Drupal\Core\Database\Query\SelectInterface $query
+   *   The count query object.  It must return a single row with a single column,
+   *   which is the total number of records.
+   */
+  public function setCountQuery(SelectInterface $query);
+
+  /**
+   * Retrieve the count query for this pager.
+   *
+   * The count query may be specified manually or, by default, taken from the
+   * query we are extending.
+   *
+   * @return \Drupal\Core\Database\Query\SelectInterface
+   *   A count query object.
+   */
+  public function getCountQuery();
+
+  /**
+   * Specify the maximum number of elements per page for this query.
+   *
+   * The default if not specified is 10 items per page.
+   *
+   * @param int|false $limit
+   *   An integer specifying the number of elements per page. If passed a false
+   *   value (FALSE, 0, NULL), the pager is disabled.
+   */
+  public function limit($limit = 10);
+
+  /**
+   * Specify the element ID for this pager query.
+   *
+   * The element is used to differentiate different pager queries on the same
+   * page so that they may be operated independently.  If you do not specify an
+   * element, every pager query on the page will get a unique element.  If for
+   * whatever reason you want to explicitly define an element for a given query,
+   * you may do so here.
+   *
+   * Note that no collision detection is done when setting an element ID
+   * explicitly, so it is possible for two pagers to end up using the same ID
+   * if both are set explicitly.
+   *
+   * @param $element
+   *   Element ID that is used to differentiate different pager queries.
+   */
+  public function element($element);
+
+  /**
+   * Gets the element ID for this pager query.
+   *
+   * The element is used to differentiate different pager queries on the same
+   * page so that they may be operated independently.
+   *
+   * @return int
+   *   Element ID that is used to differentiate between different pager
+   *   queries.
+   */
+  public function getElement(): int;
+
+}