TYPO3 Core

Shortest complete answer: https://stackoverflow.com/questions/13728106/unexpectedly-hitting-php-memory-limit-with-a-single-pdo-query

But we can't switch from buffered to unbuffered read, as we do queries while iterating over the rows.

Just stumbled into this while updating a huge instance.
If I set $connectionForTable->getWrappedConnection()->setAttribute(\PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false); with driver pdo_mysql, I can step over the statement execution, but the open unbuffered connection does not allow for another query.
```
An exception occurred while executing 'SELECT COUNT FROM `sys_registry` WHERE (`entry_namespace` = ?) AND (`entry_key` = ?)' with params ["installUpdateRows", "rowUpdatePosition"]: SQLSTATE[HY000]: General error: 2014 Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll(). Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.
```
I am unsure how to solve this. The table sys_file_metadata has more than 1.5 Million entries alone.
I mean, an Idea to solve this could be, that we fetch the whole row inside of the for-loop and only fetch uids beforehand.
This would however cause a massive increase in database connections, 1.5 Million for the sys_file_metadata alone in my case.