Additional functions supported for DataFrame.jl

[bkamins]

We could consider adding support for count for AbstractDataFrame like we now support filter (also #2048 is related).

Additionally now filter is very inefficeint, we could introduce additional form:

filter(col => fun, df)

that will be much faster as it would be type stable. This is a style planned for select (the only difference is that fun is applied to each element, not a whole column).

[bkamins]

#2091 adds filter/filter! proposal. I leave count for later when we decide what to do with #2048 and similar.

[bkamins]

I would close it. I have not seen any requests for count.