I worked on a large site that used MongoDB. We hundreds of thousands of rows in one specific collection that were used as a log. We couldn't even query it witho

I worked on a large site that used MongoDB. We hundreds of thousands of rows in one specific collection that were used as a log. We couldn't even query it without the whole database crashing. We moved that specific collection to MySQL, no problem and there was virtually no change in the data structure. Both were of course indexed.