Merge vs Squash Merge

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1️⃣ Normal Merge (git merge feat)

Logic

Git preserves the full commit history of the feature branch and creates a merge commit.

Command

git checkout main
git merge feat

Graph Before Merge

C1 -> C2 -> C3 (main)
       \
        F1 -> F2 -> F3 -> F4 -> F5 (feat)

Graph After Merge

C1 -> C2 -> C3 -------- M1 (main)
       \               /
        F1 -> F2 -> F3 -> F4 -> F5 (feat)

M1 = merge commit with two parents

parent1 = C3
parent2 = F5

What Happens

All commits appear in history:

C1
C2
C3
F1
F2
F3
F4
F5
M1

Benefit

• Preserves true development history
• Shows branching structure
• Useful for debugging large systems (like repos at Amazon)

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2️⃣ Squash Merge (git merge --squash feat)

Logic

Git combines all feature commits into one commit before adding them to main.

Command

git checkout main
git merge --squash feat
git commit -m "Add feature implementation"

Graph Before

C1 -> C2 -> C3 (main)
       \
        F1 -> F2 -> F3 -> F4 -> F5 (feat)

Graph After

C1 -> C2 -> C3 -> S1 (main)
       \
        F1 -> F2 -> F3 -> F4 -> F5 (feat)

S1 = single commit representing F1–F5

What Happens

main history becomes:

C1
C2
C3
S1

All intermediate feature commits disappear from main.

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3️⃣ Key Differences

Feature	Normal Merge	Squash Merge
History preserved	✅ Yes	❌ No
Merge commit created	✅ Yes	❌ No
Feature commits visible in main	✅ Yes	❌ No
Branch structure visible	✅ Yes	❌ No
Main branch history clean	❌ Sometimes messy	✅ Very clean

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4️⃣ Real Team Usage

Normal Merge

Used when:

• Feature branch history is meaningful
• Multiple devs collaborated
• Debugging may require seeing intermediate commits

Example in big repos like Amazon internal services.

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Squash Merge

Used when:

• Feature branch has many small commits
• Teams enforce 1 PR = 1 commit
• Keep main very clean

Typical commit message:

Add checkout discount logic

instead of:

fix bug
try again
debug
final fix
add tests

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5️⃣ One Critical Difference (Most Devs Miss)

With normal merge, Git remembers the branch was merged.

With squash merge, Git does not know the branch was merged.

So if you later run:

git merge feat

Git may try merging again because there is no merge relationship recorded.

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✅ Simple way to remember

Normal merge  → keep history
Squash merge  → compress history

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