Magento 2: What is the difference between the row_id and entity_id in the %_entity tables?

Question

In Magento 1, there wasn't a column called row_id in the entity tables, you just had the entity_id:

CREATE TABLE `catalog_category_entity` (
  `entity_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Entity ID',
  `entity_type_id` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Entity Type ID',
  `attribute_set_id` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Attriute Set ID',
  `parent_id` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'Parent Category ID',
  `created_at` timestamp NULL DEFAULT NULL COMMENT 'Creation Time',
  `updated_at` timestamp NULL DEFAULT NULL COMMENT 'Update Time',
  `path` varchar(255) NOT NULL COMMENT 'Tree Path',
  `position` int(11) NOT NULL COMMENT 'Position',
  `level` int(11) NOT NULL DEFAULT '0' COMMENT 'Tree Level',
  `children_count` int(11) NOT NULL COMMENT 'Child Count',
  PRIMARY KEY (`entity_id`),
  KEY `IDX_CATALOG_CATEGORY_ENTITY_LEVEL` (`level`),
  KEY `IDX_CATALOG_CATEGORY_ENTITY_PATH_ENTITY_ID` (`path`,`entity_id`)
) ENGINE=InnoDB AUTO_INCREMENT=943 DEFAULT CHARSET=utf8 COMMENT='Catalog Category Table'

But Magento 2 introduces a concept of row_id which becomes the primary key for an entity, it seems to have something to do with versioning from the comment:

CREATE TABLE `catalog_category_entity` (
  `row_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Version Id',
  `entity_id` int(10) unsigned NOT NULL COMMENT 'Entity Id',
  `created_in` bigint(20) unsigned NOT NULL COMMENT 'Update Id',
  `updated_in` bigint(20) unsigned NOT NULL COMMENT 'Next Update Id',
  `attribute_set_id` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Attriute Set ID',
  `parent_id` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'Parent Category ID',
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT 'Creation Time',
  `updated_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT 'Update Time',
  `path` varchar(255) NOT NULL COMMENT 'Tree Path',
  `position` int(11) NOT NULL COMMENT 'Position',
  `level` int(11) NOT NULL DEFAULT '0' COMMENT 'Tree Level',
  `children_count` int(11) NOT NULL COMMENT 'Child Count',
  PRIMARY KEY (`row_id`),
  KEY `CATALOG_CATEGORY_ENTITY_LEVEL` (`level`),
  KEY `CATALOG_CATEGORY_ENTITY_CREATED_IN` (`created_in`),
  KEY `CATALOG_CATEGORY_ENTITY_UPDATED_IN` (`updated_in`),
  KEY `CAT_CTGR_ENTT_ENTT_ID_SEQUENCE_CAT_CTGR_SEQUENCE_VAL` (`entity_id`),
  CONSTRAINT `CAT_CTGR_ENTT_ENTT_ID_SEQUENCE_CAT_CTGR_SEQUENCE_VAL` FOREIGN KEY (`entity_id`) REFERENCES `sequence_catalog_category` (`sequence_value`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=956 DEFAULT CHARSET=utf8 COMMENT='Catalog Category Table'

This is causing me some uncertainty when wanting to join eav fields as now the types refer to the row_id instead of the entity_id:

CREATE TABLE `catalog_category_entity_varchar` (
  `value_id` int(11) NOT NULL AUTO_INCREMENT COMMENT 'Value ID',
  `attribute_id` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Attribute ID',
  `store_id` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Store ID',
  `row_id` int(10) unsigned NOT NULL COMMENT 'Version Id',
  `value` varchar(255) DEFAULT NULL COMMENT 'Value',
  PRIMARY KEY (`value_id`),
  UNIQUE KEY `CATALOG_CATEGORY_ENTITY_VARCHAR_ENTITY_ID_ATTRIBUTE_ID_STORE_ID` (`row_id`,`attribute_id`,`store_id`),
  KEY `CATALOG_CATEGORY_ENTITY_VARCHAR_ENTITY_ID` (`row_id`),
  KEY `CATALOG_CATEGORY_ENTITY_VARCHAR_ATTRIBUTE_ID` (`attribute_id`),
  KEY `CATALOG_CATEGORY_ENTITY_VARCHAR_STORE_ID` (`store_id`),
  CONSTRAINT `CATALOG_CATEGORY_ENTITY_VARCHAR_STORE_ID_STORE_STORE_ID` FOREIGN KEY (`store_id`) REFERENCES `store` (`store_id`) ON DELETE CASCADE,
  CONSTRAINT `CAT_CTGR_ENTT_VCHR_ATTR_ID_EAV_ATTR_ATTR_ID` FOREIGN KEY (`attribute_id`) REFERENCES `eav_attribute` (`attribute_id`) ON DELETE CASCADE,
  CONSTRAINT `CAT_CTGR_ENTT_VCHR_ROW_ID_CAT_CTGR_ENTT_ROW_ID` FOREIGN KEY (`row_id`) REFERENCES `catalog_category_entity` (`row_id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=266383 DEFAULT CHARSET=utf8 COMMENT='Catalog Category Varchar Attribute Backend Table'

Currently in the entity table row_id and entity_id are the same:

select row_id, entity_id from catalog_category_entity limit 3;        
+--------+-----------+
| row_id | entity_id |
+--------+-----------+
|      1 |         1 |
|      2 |         2 |
|      3 |         3 |
+--------+-----------+
3 rows in set (0.00 sec)

I am unsure the circumstances where row_id will change and what impact this will have on the Magento system, I have looked for documentation on this feature but cannot find any.

I want to know the following:

• Can the same entity have multiple row_ids?
  • If so how is the active entity defined?
  • If not what is the mechanism Magento uses to update these.
• What is the core use of this feature and is there any documentation?
  • http://devdocs.magento.com/guides/v2.1/extension-dev-guide/staging/Staging.html

Answer 1

From what I have found on this one, row_id is the identifier for the row, while entity_id is the actual entity.

There are situations where a Scheduled Update is set to run for a fixed period and in this case a whole new row is created to associate with it's variant eav_attribute(s) and it can retain the same entity_id while being issued a new row_id Then the row_id is changed to the new temporary entity and then back to the original once the schedule completes.

There may be other use cases where they make use of this, this was one which I witnessed and illuminates how it can be used.

Answer 2

The entity_id is used in the Magento 2 EE Version only. It is used as an Index defined in catalog_category_entity pointing to sequence_catalog_category.

The sequence tables, in general, are used for the database splitting feature.

Answer 3

I would say that the purpose is to keep compatibility with Magento 1. When you need to migrate data from m1 to m2 you will need that relationship.