# Push! is very slow on Vector{Union{...}}

**URL:** https://discourse.julialang.org/t/push-is-very-slow-on-vector-union/12329
**Category:** Internals & Design
**Created:** [July 12, 2018, 12:20pm UTC](https://discourse.julialang.org/t/push-is-very-slow-on-vector-union/12329 "2018-07-12T12:20:59Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![jamii](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jamii/32/1373_2.png) [@jamii](https://discourse.julialang.org/u/jamii)
#### Post date: [July 12, 2018, 12:21pm UTC](https://discourse.julialang.org/t/push-is-very-slow-on-vector-union/12329/1 "2018-07-12T12:21:00Z")

</div>

I’ve only found the one mention of this below and I couldn’t find any followup.

> [@Append!() function extremely slow in DataFrames + CSV](https://discourse.julialang.org/t/append-function-extremely-slow-in-dataframes-csv/8396/12):
>
> Looks like some major optimizations are missing here. If you want to help making progress, it would be useful to check whether you can reproduce this without DataFrames, i.e. by calling `append!` directly on vectors, comparing `Vector{T}` , `DataVector` {T} with `Vector{Union{T, Missing}}` . Then it would be worth filing an issue in Julia.

So here is an independent repro:

```julia
julia> versioninfo()
Julia Version 0.7.0-beta.0
Commit f41b1ecaec (2018-06-24 01:32 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Xeon(R) CPU E3-1505M v5 @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.0 (ORCJIT, skylake)

julia> function silly_copy(xs)
           ys = empty(xs)
           for x in xs
               push!(ys, x)
           end
           ys
       end
silly_copy (generic function with 1 method)

julia> using BenchmarkTools

julia> @benchmark silly_copy($(Vector{Int64}(1:1000000)))
BenchmarkTools.Trial: 
  memory estimate: 9.00 MiB
  allocs estimate: 20
  --------------
  minimum time: 5.203 ms (0.00% GC)
  median time: 5.740 ms (0.00% GC)
  mean time: 5.801 ms (3.63% GC)
  maximum time: 7.617 ms (15.03% GC)
  --------------
  samples: 831
  evals/sample: 1

julia> @benchmark silly_copy($(Vector{Union{Int64, Missing}}(1:1000000)))
BenchmarkTools.Trial: 
  memory estimate: 10.13 MiB
  allocs estimate: 20
  --------------
  minimum time: 12.880 s (0.00% GC)
  median time: 12.880 s (0.00% GC)
  mean time: 12.880 s (0.00% GC)
  maximum time: 12.880 s (0.00% GC)
  --------------
  samples: 1
  evals/sample: 1

```

Note the time units - the 2nd version is 2000x slower!

I believe this is also responsible for the massive performance regressions I’m seeing for DataFrames.join on 0.7

---

<div class="post-metadata">

### Author: ![jamii](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jamii/32/1373_2.png) [@jamii](https://discourse.julialang.org/u/jamii)
#### Post date: [July 12, 2018, 12:30pm UTC](https://discourse.julialang.org/t/push-is-very-slow-on-vector-union/12329/2 "2018-07-12T12:30:08Z")

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Actually, I’m not sure why I posted this here 🙂

I went ahead and made an issue instead - [https://github.com/JuliaLang/julia/issues/28076](https://github.com/JuliaLang/julia/issues/28076)

---

<div class="post-metadata">

### Author: ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [July 12, 2018, 12:30pm UTC](https://discourse.julialang.org/t/push-is-very-slow-on-vector-union/12329/3 "2018-07-12T12:30:59Z")

</div>

In 0.6 the Union benchmarks at 50ms vs 6ms for the non-union eltype, using a slightly modified test function:

```julia
function silly_copy(xs::Vector{T}) where T
                  ys = T[]
                  for x in xs
                      push!(ys, x)
                  end
                  ys
              end  

```