Magento2 Indexer ATTRIBUTES_CHUNK_SIZE

Question

In Magento\Catalog\Model\Indexer\Product\Flat\Action\Indexer there is one constant:

/**
 * Maximum size of attributes chunk
 */
const ATTRIBUTES_CHUNK_SIZE = 59;

reference: https://github.com/magento/magento2/blob/2.0/app/code/Magento/Catalog/Model/Indexer/Product/Flat/Action/Indexer.php#L26

It serves for splitting attributes into chunks during Flat table indexation process.

The question is: Why 59?

Answer 1

Mysql has a limit of 63 joins per query. The 4 remaining are reserved for things like website and category associations.