For many implementations, much emphasis has been placed on some kind of readahead to boost performance. But if it is used blindly, it can have adverse effect.
Cache maintenance is not free, and is definitely not unlimited. If the DB is allowed to perform prefetch too aggressively, you might end up flooding the cache with useless data pages which will be flushed out anyway.
Remember that the key to good performance is to let the system do as little work as possible.
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