External ordering is a generic term for ordering algorithms that can handle large amounts of information. External ordering is required when the information that has to be ordered does not fit in the main memory of a computer (typically RAM) and a slower type of memory (typically a hard disk) has to be used in the process.
An example of an external ordering is the algorithm of sorting by mixing. Suppose that 900 MB of information must be ordered using only 100 MB of RAM.
Another example is the ordering algorithm by balanced mixing, which is an optimization of the previous one.
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