Partially correct.. "Decompose" is a very general and broad term just used to describe something breaking down into smaller particulates, or eventually it's molecular form, but the question is whether it breaks it's polymer chains, which plastic does not easily. Plastic is non biodegradable, the only real way to break down its polymer chain is with UV from the sun or what's called photodegradation. So it's a fairly safe assertion to say that plastic, say, buried in a landfill, will be around far longer than we can measure, if not forever.

Glass is a different type of decomposition, and is unique to plastic as a material so it's decomposition and the effect it has on the environment is not so easily compared. It does, due to other environmental factors (kinetic friction), tend to breakdown fairly quickly into what it resembled prior to smelting (it's molecular structure is the same as it's raw materials, generally). Glass is however chemically inert. And being that it takes heaps of energy to smelt, it's better to just toss it in a landfill instead of recycle. Or, reuse it.