Magento 2 reindex bloccato o Allowed memory size of 134217728 bytes exhausted

Si è verificato l’errore di impossibilità di reindicizzazione che rimaneva appesa, questo da back-end di Magento 2
da linea di comando eseguendo il comando
php bin/magento indexer:reindex
si riceve

Design Config Grid index has been rebuilt successfully in 00:00:00
Customer Grid index is locked by another reindex process. Skipping.
Category Products index has been rebuilt successfully in 00:00:01
Product Categories index has been rebuilt successfully in 00:00:00
Product Price index has been rebuilt successfully in 00:00:08
Product EAV index has been rebuilt successfully in 00:00:00
Stock index has been rebuilt successfully in 00:00:01
Catalog Rule Product index has been rebuilt successfully in 00:00:00
Catalog Product Rule index has been rebuilt successfully in 00:00:00
Catalog Search index has been rebuilt successfully in 00:00:03

eseguendo
php bin/magento indexer:reset
si riceve

Design Config Grid indexer has been invalidated.
Customer Grid indexer has been invalidated.
Category Products indexer has been invalidated.
Product Categories indexer has been invalidated.
Product Price indexer has been invalidated.
Product EAV indexer has been invalidated.
Stock indexer has been invalidated.
Catalog Rule Product indexer has been invalidated.
Catalog Product Rule indexer has been invalidated.
Catalog Search indexer has been invalidated.

eseguendo
php bin/magento indexer:reindex
si riceve
Allowed memory size of 134217728 bytes exhausted (tried to
allocate 667648 bytes) in
/var/....../httpdocs/vendor/magento/zendframework1/library/Zend/Db/Statement/Pdo.php
on line 228

per risolvere il problema
eseguire
php bin/magento indexer:reset
si riceve

Design Config Grid indexer has been invalidated.
Customer Grid indexer has been invalidated.
Category Products indexer has been invalidated.
Product Categories indexer has been invalidated.
Product Price indexer has been invalidated.
Product EAV indexer has been invalidated.
Stock indexer has been invalidated.
Catalog Rule Product indexer has been invalidated.
Catalog Product Rule indexer has been invalidated.
Catalog Search indexer has been invalidated.

e poi
php -d memory_limit=-1 bin/magento indexer:reindex

dove memory_limit=-1 elimina il massimo della memoria eseguibile e dunque si riesce a portare a termine l’esecuzione del comando.