lemon meringue pie…yes fresh pies …from store.
But yet there is video on cutting the pies where they show a serrated knife being used and also a chef knife on ( fruit pie) one video is at How to Cut Pie Without Making a Crumbly Mess at… purewow…then go to mrs. smith pie cutting video …How to cut, garnish and serve Mrs. Smith Pie matt horn …yes i know what he says why he uses a serrated knife. but if what he says is true than why does purewow and even american test kitchen…at how to serve the perfect fruit pie video show a chef knife cutting pie. any help appr.
Here’s my experienced-based WAG (40+ years cooking pies). Different cooks like different tools. They will teach you to use the tools that they like. There may or may not be a good reason for why. If there is, they should tell you.
Using a Chef’s blade for example, you will want a sharp Chef’s blade. A dull one will tend to just smash the crust. A sharp one should cut through. Start with the point first, then press down along the rest of the blade to make the cut. You’ll need something called a pie server/pastry server or cake server to lift it out. A knife is not a good tool for this.
A good source for this type of basic information is Alton Brown. He had a show called Good Eats, and some books. He was very good on the details of why things worked the way they did and why you should use the tool he suggested.
I hope this helps.
You could always just sit down with the pie on your lap and dig in with a fork. 
Early recipes for pie generally call for the contents to be completely enclosed by the crust; in some cases the entire point of baking food into a pie was that the crust would seal it in and preserve it for storage. The crust wasn’t necessarily eaten, and in fact might not be particularly edible (although that is also true for the original proto-pizzas).
I prefer a thin bladed, unserrated knife.
A serrated knife is for sawing through a hard crust without crushing a softer inside part. That’s why they’re good for tomatoes and bread, and some pies.
A pie is supposed to be made using pastry rather than dough, completely encloses the filling, and they’re all baked together.
Pizza is a totally different thing; the two items have almost nothing in common.
What about Shepherd’s Pie, or Cottage Pie?
mmmm. pie
Shepherd’s/Cottage Pie gets a pass because it’s got mashed potato baked onto the top and in all other respects it’s the same as a meat pie.
Hot pie, sharp knife. Cold pie, serrated knife. Not in a pan, though, you may as well just spoon or fork those.
Blackbirds.
What do you mean by “pastry, not dough”? Just what do you think pastry is made out of, anyway?
And shepard’s pie is a lot more like stew than it is like pie, and much less pie-like than pizza is.
Your definition rules out all fried pies, pumpkin pies, chocolate pies, pecan pies, lattice- or meringue- or whipped cream-topped pies, and a lot of pot pies. Also, other things I would have considered to be pies, like quiche and tarts.
Furthermore, it does not rule out corn dogs, so long as they’re the baked kind. Nor does it rule out Beef Wellington.
I think he means “short bread”, which is a distinction I just recently learned. Yeast bread (like a baguette or pizza) is kneaded and chewey when cooked. It may or may not be made with fat or oil. Short bread uses shortening and baking soda/powder and is simply combined instead of kneaded. It is crumbly before and after cooking.
By “Pastry”, I mean “Puff Pastry” or “Flaky Pastry”. Pizza has a bread base, not a pastry one.
I don’t think we’re talking about the same foods here. Every Shepherd’s Pie I’ve ever had has been a conventional pie with a minced meat filling (sometimes with vegetables) and the top covered with mashed spud and baked together.
The toppings on a standard pizza are not covered so they’re not a pie.
No pastry in my cottage/shepherd’s pies - the minced meat/vegetables/gravy sits in the dish, and mashed potato is layered on top, with cheddar cheese grated over.
Similarly with a beef 'n ale pie - beef casserole in a dish with a puff pastry top.
And some pies do not have puff pastry crusts - pork pies, for example, have a hot water crust pastry.
Pie, I rather suspect, is in the eye of the consumer.
One thing we do agree on - pizza is not pie.
this is really getting off the original question.
Welcome to the SDMB. 
Also, I believe your question was answered back in post #23. 
Some pizzas are covered, and some things widely called pies (like pumpkin pie) aren’t. Which is not to say that I’d necessarily call pizza a pie, but it’s at least closer to pie than shepherd’s pie is: I’ve never seen a shepherd’s pie with a bottom crust, and even if it does have one, there’s still a world of difference between mashed potatoes and a top crust. You might as well say that cheese forms the top crust of a pizza.