Difference in Git merge order?

Question

Is there any difference between

git merge c1 c2

and

git merge c2 c1

? Also, is there any difference between

git checkout c1
git merge c2

and

git checkout c2
git merge c1

?

Answer 1

The end result in terms of the file content should be the same in all cases you described.

But there will be a difference in the DAG, in the ordering of commits in the graph of all commits, for example:

Case 1: git merge c1 c2

*   dd24250 (HEAD, master) Merge branches 'c1' and 'c2'
|\  
| * 9d09bec (c2) change in c2
* | 1f5c0ca (c1) change in c1
|/  

Case 2: git merge c2 c1

*   3256c8d (HEAD, master) Merge branches 'c2' and 'c1'
|\  
| * 1f5c0ca (c1) change in c1
* | 9d09bec (c2) change in c2
|/  

Case 3: git checkout c1; git merge c2

*   111e7da (HEAD, c1) Merge branch 'c2' into c1
|\  
| * 9d09bec (c2) change in c2
* | 1f5c0ca change in c1
|/  

Case 4: git checkout c2; git merge c1

*   8ccf531 (HEAD, c2) Merge branch 'c1' into c2
|\  
| * 1f5c0ca (c1) change in c1
* | 9d09bec change in c2
|/  

Answer 2

I'd start with the second question.

The resulting tree objects in the merge commits will be identical. However the two commit objects won't be:

The order of the two parent commit objects will be different. Which may lead to subtle differences when referring to commits with the HEAD^1~5 notation.

The same applies to the first question:

1st case, 1st parent = current branch last commit, 2nd parent = c1 last commit, 3rd parent = c2 last commit.

2nd case, 1st parent = current branch last commit, 2nd parent = c2 last commit, 3rd parent = c1 last commit.