unnest

[jariji]

Extract a table-valued column into top-level columns. Code by Bogumił Kamiński on Slack.

unnest(df, col) = flatten(
    select(df, Not(col), col => ByRow(Tables.columntable) => AsTable), 
    names(df, Not(col))
)

This might come with its opposite, nest.

See also tidyr - nest and unnest.

[bkamins]

OK, let us decide on these + #2767 and #2890 in 1.5 release.

This requires a careful design, especially, handling the case when unnested data frames do not have matching column names.

[bkamins]

x-ref: #3005

[bkamins]

Do we want unnest to support only data frames, or also other Tables.jl tables? For row-like objects I think that #3005 solution with Tables.dictrowtable is good enough.

[bkamins]

We could not make a decision if we want it or no, so I keep this open.
When working on it:

• make sure to check requirements in a new method of the flatten function in DataFrames  #2890
• look at previous PR Add nest, unnest, extract and extract! #3258 that was planned to address it
• a related request is feature: cols=:union argument (or something like it) for combine with AsTable #3005 (maybe if we improve AsTable then unnest will not be needed)