Solving reindex issues with Product Flat Data in Magento

Check your exception.log file And find :

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Product Flat Data index process unknown error: exception 'PDOException' with message 'SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`DATABASE_NAME`.<result 2 when explaining filename '#sql-1803_1aa01'>, CONSTRAINT `FK_CAT_PRD_FLAT_2_ENTT_ID_CAT_PRD_ENTT_ENTT_ID` FOREIGN KEY (`entity_id`) REFERENCES `catalog_product_entity` ()' in /app/magento_oneline/lib/Zend/Db/Statement/Pdo.php:228 Stack trace: #0 /app/magento_oneline/lib/Zend/Db/Statement/Pdo.php(228): PDOStatement->execute(Array) #1 /app/magento_oneline/lib/Varien/Db/Statement/Pdo/Mysql.php(110): Zend_Db_Statement_Pdo->_execute(Array) #2 /app/magento_oneline/lib/Zend/Db/Statement.php(300): Varien_Db_Statement_Pdo_Mysql->_execute(Array) #3 /app/magento_oneline/lib/Zend/Db/Adapter/Abstract.php(479): Zend_Db_Statement->execute(Array) That error for  The key is here: ? 1 FK_CAT_PRD_FLAT_2_ENTT_ID_CAT_PRD_ENTT_ENTT_ID That is telling you which table is failing. This exception tells that the referential integrity between: catalog_product_flat_2.entity_id and catalog_product_entity.entity_id is corrupted. This usually means some record(s) contain(s) null in the field catalog_product_entity.entity_id, where they are expected to contain existing entity_ids. Fixing this In order to fix this issue, you need to find what the corrupted entries are. This is easy. In this case, you’ll need to create a query to get the empty registers: ? 1 SELECT a.entity_id FROM catalog_product_flat_2 AS a LEFT JOIN catalog_product_entity AS b ON a.entity_id = b.entity_id WHERE ISNULL(b.entity_id); This will display the corrupted entities. You only need to delete them and that’s all. ? 1 2 3 4 5 6 7 +-----------+ | entity_id | +-----------+ |     35427 | |     35428 | +-----------+ 2 rows in set (0.04 sec) As example: ? 1 DELETE FROM catalog_product_flat_2 where entity_id = '35427';