If you have a choice on how to keep your data, then the smart way is to keep it sorted. Merging the two collections is then a simple matter of streaming merge. Typically, this can be done as fast as you can do the i/o.

In reply to Re: Fastest way to merge (and de-dup) two large arrays? by Anonymous Monk
in thread Fastest way to merge (and de-dup) two large arrays? by technojosh

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