More of a guess given your lack of details, but:

  1. Make 1 pass of the first file building a hash, keyed by the common field with the values being the file position of the start of the record containing that key.
  2. Make 1 pass through the second file, adding the file position(s) of the records containing the common keys to the hash.
  3. Process the hash, reading in the records for each key from both files, join them as appropriate, and write them to the new file.

You'll never have more than one record from each file in memory at any given time, so beyond the size of the hash required to hold the keys, the size of the files doesn't matter.

An O(3N) process should be substantially quicker than 2 x O(NlogN) sorts + an O(N) merge if coded properly.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re: Working on huge (GB sized) files by BrowserUk
in thread Working on huge (GB sized) files by vasavi

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