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External Merge Sort

When data exceeds memory, the engine sorts memory-sized chunks into sorted runs on disk, then merges the runs in one or more passes. This is how databases implement large ORDER BY and sort-based joins, and why a sort that fits in memory is dramatically faster than one that spills.

Questions this Concept answers

Why is a sort that spills so much slower than one that fits in memory?

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External Merge Sort for Sorting Data Larger Than Available RAM

External merge sort (external sort) orders a dataset larger than available main memory by decomposing the problem into two phases: a splitting phase that divides the data into chunks each small enoug…