Git rebase vs squash function and performance tests

This repo contains some simple examples to understand the differences.

You can have a general view of the repo with:

git log --graph --simplify-by-decoration --pretty=format:'%d' --all

Conflict difference between rebase and squash

There is a small difference between rebasing a branch with a list of small commits or squashing them into one and rebase the single resulting commit: squash usually reduces the information available to Git in order to apply state changes and thus, increasing the probability of a conflict.

As an example, create a long-branch-rebase branch, then rebase on another: it should perform that without any conflicts.

git checkout short-branch
git checkout long-branch
git checkout -b long-branch-rebase
git rebase short-branch

Now we create a copy of the original long-branch, then squash all commits and rebase the single commit.

git checkout long-branch
git checkout -b long-branch-squash
git rebase -i master
# rewrite 'pick' to 's' in all but the first
git rebase short-branch

You should get a CONFLICT (modify/delete): shared-file deleted in xxxxxx…​

Rebase performance

When performing a rebase, Git checks out the specified branch to rebase onto and, from that state, it will sequentially apply the commits of the local branch. This means the time to perform a rebase is proportional to the local branch and not to the one to rebase onto.

As example of that we can measure the time to rebase extremely-long-branch, that has 10k commits onto short-branch and then doing the other way around.

git checkout extremely-long-branch
git checkout -b extremely-long-branch-onto-short
time git rebase short-branch

The output should look like this one.

real	0m18,172s
user	0m5,086s
sys	    0m12,724s

While doing the reverse, namely short-branch onto extemely-long-branch is faster since it has to apply only few commits:

git branch short-branch-onto-extremely-long extremely-long-branch
git checkout short-branch
time git rebase short-branch-onto-extremely-long

real	0m0,186s
user	0m0,138s
sys	    0m0,047s