Are Ecto Transactions concurrency safe

I believe this question arose because of this topic on using Ecto.Multi vs prepare_changes for an incrementing order column.

Important to note is in that case, the order should not be incremented for each new column, but rather, the order only makes sense for siblings within a belongs_to relationship (i.e. the order of photos in album #3, or the order of songs in playlist #500, etc.).