"This comparison is taking too long to generate." error on github

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I'm working on a project that has a large number of json files that are never reviewed in pull requests but occasionally need to be changed. Recently we had to make minor changes to them, and github isn't allowing me to create a pull request with those changes. Instead it gives me:

This comparison is taking too long to generate.
Unfortunately it looks like we can’t render this comparison for you right now. It might be too big, or there might be something weird with your repository.

I checked the diff locally and the actual code changes are pretty minor (maybe 200 lines changed), but there are millions of changed lines in these json files. Is there any way to tell Github to ignore them? Right now I am unable to make a PR so the changes can't go through our normal company review process.

I've tried using the .gitattributes file with *.json linguist-generated=true unfortunately that had no effect.

Edit: As suggested in the accepted answer, I contacted github support about this case. Their suggestion was to create a new branch with a small commit, create the PR, and then merge the actual branch that I want to deploy into it. This will update the PR, and while the diff still won't display, it will let me create a PR.

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1 Answers

When you compare two branches on GitHub, GitHub has to go compute the diff for those changes using Git and then render it for you. In your case, you have millions of lines of changes, and Git doesn't perform very well in this case because the algorithm that's used to compute diffs is O((N + M)D). Thus, if you have a number of differences proportional to the number of lines, the algorithm is essentially O(N²). Having a large N makes that even worse.

GitHub has a limit on long a request can take, so your large number of changes are just not going to render in the interface. It may be possible to choose the branches you want even though the diff won't render and still open the pull request. If not, you may need to resort to using the API, which won't generate the diff for rendering and therefore is likely to work a little better.

I would encourage you to let GitHub Support know about this, if you haven't been able to find a way to do it through the UI, since they can notify someone to make sure the interface is usable to create a PR even if the diff can't render. You probably aren't the first person to encounter this.

You may also want to store these files outside of Git on some sort of artifact server and pull them down to your repository based on hash, in which case you wouldn't have this pathological case.

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