Choosing Between Merge Join and Hash Join

Erik Darling gives us a Sophie’s Choice:

It could have chosen a Hash Join, but then the order of the Id column from the Posts table wouldn’t have been preserved on the other side.

Merge Joins are order preserving, Hash Joins aren’t. If we use a Hash Join, we’re looking at ordering the results of the join after it’s done.

But why?

Read on to learn why, as well as why a few other things are so.

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