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Composite indexes aka multi-column indexes are about 10x faster than index merges. In Postgres, the gap is larger than in MySQL because Postgres doesn’t support index-only scans for queries that involve index merges.
Just wondering how many transactions or writes per second MySQL can handle? While it depends on many factors, fundamentally, about as many transactions as MySQL can commit to disk per second. A modern disk can do ~1000 fsyncs per second , but MySQL will group multiple writes with each fsync. An okay rule-of-thumb…