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The OP message could have better said that 50TB databases are common these days when single metal or 24xl I3en or I4* instance on AWS can hold 60T raw.

it's more than big enough to cause big problems / risk days of downtime to change, yea. 50GB is not big. 50TB is at least touching big - you can do it on one physical machine if needed, but it's the sort of scale that benefits from bigger-system architecture. 50PB would be world-class big, hitting exciting new problems every time they do something.

With 50TB, and if you were doing a full text search, wouldn't the entirety of the index have to be held in memory?

No. Full-text indexes exist.

You can also do an incremental/streaming search. Lots of ways to avoid loading it all into memory at once, yeah.