Drupal entity_id integer and flag_id__uid index

entity_id_integer_and_index.patch

diff --git a/flag.install b/flag.install
index ec4794b..42ba3ba 100644
--- a/flag.install
+++ b/flag.install
@@ -30,9 +30,10 @@ function flag_schema() {
       ],
       'entity_id' => [
         'description' => 'The unique ID of the flagged entity, for example the uid, cid, or nid.',
-        'type' => 'varchar_ascii',
-        'length' => ConfigEntityStorage::MAX_ID_LENGTH,
+        'type' => 'int',
+        'unsigned' => TRUE,
         'not null' => TRUE,
+        'default' => 0,
       ],
       'count' => [
         'description' => 'The number of times this object has been flagged for this flag.',
@@ -103,3 +104,19 @@ function flag_requirements($phase) {
   */
   return $requirements;
 }
+
+/**
+ * Change {flag_counts}.entity_id to an unsigned int.
+ *
+ * @link https://www.drupal.org/project/flag/issues/2921864
+ */
+function flag_update_8001() {
+  $schema = \Drupal::database()->schema();
+  $schema->changeField('flag_counts', 'entity_id', 'entity_id', [
+    'description' => 'The unique ID of the flagged entity, for example the uid, cid, or nid.',
+    'type' => 'int',
+    'unsigned' => TRUE,
+    'not null' => TRUE,
+    'default' => 0,
+  ]);
+}
diff --git a/src/Entity/Flagging.php b/src/Entity/Flagging.php
index 7d43c29..8086c00 100644
--- a/src/Entity/Flagging.php
+++ b/src/Entity/Flagging.php
@@ -117,9 +117,14 @@ class Flagging extends ContentEntityBase implements FlaggingInterface {
       ->setLabel(t('Entity Type'))
       ->setDescription(t('The Entity Type.'));
 
-    $fields['entity_id'] = BaseFieldDefinition::create('string')
+    // Setting to 'integer' type can make the 'flag_id__uid__entity_id' index
+    // work when executing the query 'select uid from flagging where flag_id=
+    // 'follower' and entity_id=1'. See EXPLAIN for more information.
+    // see https://www.drupal.org/project/flag/issues/2921864
+    $fields['entity_id'] = BaseFieldDefinition::create('integer')
       ->setLabel(t('Entity ID'))
       ->setRequired(TRUE)
+      ->setSetting('unsigned', TRUE)
       ->setDescription(t('The Entity ID.'));
 
     $fields['flagged_entity'] = BaseFieldDefinition::create('entity_reference')
diff --git a/src/Entity/Storage/FlaggingStorageSchema.php b/src/Entity/Storage/FlaggingStorageSchema.php
index e16e6ed..cffbe04 100644
--- a/src/Entity/Storage/FlaggingStorageSchema.php
+++ b/src/Entity/Storage/FlaggingStorageSchema.php
@@ -4,6 +4,7 @@ namespace Drupal\flag\Entity\Storage;
 
 use Drupal\Core\Entity\ContentEntityTypeInterface;
 use Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
 
 /**
  * Defines the flag schema handler.
@@ -19,6 +20,28 @@ class FlaggingStorageSchema extends SqlContentEntityStorageSchema {
     $schema['flagging']['indexes'] += [
       'entity_id__uid' => ['entity_id', 'uid'],
     ];
+    $schema['flagging']['indexes'] += [
+      'flag_id__uid__entity_id' => ['flag_id', 'uid', 'entity_id'],
+    ];
+
+    return $schema;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $storage_definition, $table_name, array $column_mapping) {
+    $schema = parent::getSharedTableFieldSchema($storage_definition, $table_name, $column_mapping);
+    $field_name = $storage_definition->getName();
+
+    if ($table_name == 'flagging') {
+      switch ($field_name) {
+        case 'entity_id':
+          // Improves the performance of the indexes defined in getEntitySchema().
+          $schema['fields'][$field_name]['not null'] = TRUE;
+          break;
+      }
+    }
 
     return $schema;
   }