Rebase vs merge mental model in Git - Performance Comparison

Time Complexity: Rebase vs merge mental model

O(n)

Understanding Time Complexity

When using Git, combining changes from different branches can be done by merging or rebasing. Understanding how the time to combine grows with the number of commits helps us choose the right method.

We want to know: How does the work Git does increase as the number of commits to combine grows?

Scenario Under Consideration

Analyze the time complexity of these Git commands:

git merge main

git rebase main

These commands combine changes from one branch into another, but in different ways.

Identify Repeating Operations

Look at what Git does internally when running these commands:

• Primary operation: For merge, Git compares the latest commits and creates a new commit combining changes. For rebase, Git re-applies each commit from the feature branch one by one onto the main branch.
• How many times: Merge does a single combine operation regardless of commits. Rebase repeats the apply step once per commit in the feature branch.

How Execution Grows With Input

Imagine the feature branch has different numbers of commits to combine:

Input Size (n commits)	Approx. Operations
10	Merge: 1 combine, Rebase: 10 applies
100	Merge: 1 combine, Rebase: 100 applies
1000	Merge: 1 combine, Rebase: 1000 applies

Pattern observation: Merge work stays the same no matter how many commits. Rebase work grows linearly with the number of commits.

Final Time Complexity

Time Complexity: O(n)

This means the time Git takes to rebase grows directly with the number of commits, while merge time stays mostly constant.

Common Mistake

[X] Wrong: "Rebase and merge take the same time no matter how many commits there are."

[OK] Correct: Merge combines all changes at once, but rebase applies each commit separately, so more commits mean more work for rebase.

Interview Connect

Knowing how Git commands scale with commit count shows you understand their inner workings. This helps you explain your choices clearly and confidently in real projects or interviews.

Self-Check

"What if we used interactive rebase to squash commits before rebasing? How would that affect the time complexity?"