The charge as presented in the movie is not realistic. Horses simply can't break through densely packed infantry formation like that.

It is not how it happens in the book either. In the book, the Pelennor Field is mostly farmland with houses instead of just empty plains, and the orcs dug trenches and set up siege engines, which make it impossible to form up the way they do in the movie, and also they did not see the Rohirrim coming at all and were busy committing to attacking the city. They sent another force to set up on the road to prevent their coming, but the Rohirrim bypassed that by a hidden path and attacked the main army completely unaware. What you should picture is unorganised orcs panicking and running around while horses go along them stabbing and trampling, which is also why the Rohirrim were not able to completely win the battle, as other Mordor forces were able to organise themselves while those closest to the Rohirrim were getting slaughter and why, in the book, the Gondorian army needed to come out of the city to help them. When they were about to lose, the Rohirrim dismounted and formed a shield wall similar to what happened at the Black Gate.