# Is Union{Missing, Float64} a concrete type?

**URL:** <https://discourse.julialang.org/t/is-union-missing-float64-a-concrete-type/65901>\
**Category:** Performance\
**Created:** [August 5, 2021, 5:37pm UTC](https://discourse.julialang.org/t/is-union-missing-float64-a-concrete-type/65901 "2021-08-05T17:37:00Z")\
**Posts on this page:** 1\
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**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [August 5, 2021, 5:57pm UTC](https://discourse.julialang.org/t/is-union-missing-float64-a-concrete-type/65901/2 "2021-08-05T17:57:31Z")

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Should not be an issue:

> [@Performance tips for Union Types](https://discourse.julialang.org/t/performance-tips-for-union-types/13928):
>
> As I’ve summed up from the scattered discourse posts, the Julia compiler has been optimised for the small Union Types. Yet, it seems, there is no information about it in the Julia docs (I’ve searched in the Performance Tips section). As I understand, output Union Types break type stability, yet it is the mechanism used in the new iteration protocol and it clearly contradicts with what is written in Performance Tips. Looks like the docs need updating. It would be great to know what are the li…

but as always benchmark your concrete use case to see the impact, if there’s a problem we can see whether there’s ways around it.

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