Why does this SQL ORDER BY RAND() melt the database?

Why does this SQL ORDER BY RAND() melt the database. The SQL code uses ORDER BY RAND() for random rows. In MySQL that sorts the entire table. In SQL one query can take down the server.

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https://www.youtube.com/watch?v=2G3tRvhnyJM

Why does this SQL ORDER BY RAND melt the database? #sqlperformance

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