Large Dataset query fails if column contains Missing

Question

In version 11.0 (Win 8.1 Pro) a query on a large Dataset for a column that contains Missing fails most unceremoniously.

With

dsTest = AssociationThread[{"a", "b"}, 
     If[Last@# == 10000, {Missing["No Data"], Last@#}, #]] & /@ 
   Transpose@{RandomChoice[Alphabet[], 22000], Range[22000]} // 
  Dataset;

One row has column "a" set to Missing["No Data"]. Then

dsTest[All, "a"]

Bug, yes?

Work-around

A work-around is to remove the Missing instances from the Select.

dsTest[Select[Head[#"a"] =!= Missing &], "a"]

More work-arounds in comments/answers below.

Confirmed Bug - [CASE:3720717]

Answer 1

Extended comment converted to a non-answer, nailing down what is required to trigger this bug:

Here's a simple Dataset with one key having all string values, another having integer values:

ds = Association @@@ ({"a" -> ToString@#, "b" -> #} & /@ Range[20]) // 
  ReplacePart[{1, 1} -> Missing[]] // Dataset

If we query the integer values, then we see that the displayed form changes based on whether the number of rows we ask for is less than 9,

{ds[;; 8, "b"], ds[;; 9, "b"], ds[All, "b"]}

It switches from a column view to a grid-view when you query more than 8 elements. This is also where you have a problem with the "a" column, the Key with String values,

{ds[;; 8, "a"], ds[;; 9, "a"]}

A workaround is to set a global preference to use the old Dataset formatting,

Dataset`$UseNewDatasetFormatting = False;
{ds[;; 8, "a"], ds[;; 9, "a"]}

Answer 2

Too long for a comment: Another example narrowed down to a curious threshold:

vars = {"Var_A", "Var_B", "Var_C"};
assoc = AssociationMap[Random[] &] /@ Array[vars[[#2]] &, {9999, 3}];
assoc2 = Prepend[<|"Var_A" -> Random[], "Var_B" -> Random[], "Var_C" -> Missing[]|>]@assoc;
Predict[assoc -> "Var_C"]
Predict[assoc2 -> "Var_C"]

works fine but now change 9999 to 10 000 and Predict fails on assoc2 apparently confused by the Missing[] in the predicted variable.

(Confirmed as a bug a few weeks back)