If you re-index your data and you see this error:

Product Attributes 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 (`your_database_name`.`catalog_product_index_eav`, CONSTRAINT `FK_CAT_PRD_IDX_EAV_ENTT_ID_CAT_PRD_ENTT_ENTT_ID` FOREIGN KEY (`entity_id`) REFERENCES `catalog_product_entity` (`entity_id`) ON DELETE CASCADE)’ in . ..\lib\Zend\Db\Statement\Pdo.php:228

It causes by some products no longer exists in the main product table’, it would cause constraint problems when indexing anyway.

Its sort of like having arms and legs floating around without any body. 

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Here is the solution.

You can execute these queries:

delete FROM `catalog_product_entity_datetime` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_decimal` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_gallery` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_group_price` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_int` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_media_gallery` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_text` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_tier_price` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_varchar` where entity_id not in (select entity_id from catalog_product_entity);
#originals with formatting fixed:
delete from `catalog_category_product` WHERE product_id not in(select entity_id from catalog_product_entity);
delete from `catalog_category_product` WHERE category_id not in(select entity_id from catalog_category_entity); 
delete from `catalog_product_website` WHERE product_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_index_eav_idx` WHERE entity_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_index_eav` WHERE entity_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_link` WHERE product_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_relation` WHERE parent_id not in(select entity_id from catalog_product_entity);


After that you need to re-index data again. The issue should disappear.

Good luck ^^