Empty data.table produced with .SD when grouping by all columns

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by reproducing the two .SD examples in the issue, grouping by all columns and by A+B. Trace the grouping and .SD entry points involved in those expressions. Done means both expressions return the grouped rows and columns rather than an empty data.table, with regression coverage for the reported cases.

Written by the indexing model from the issue text.

Description

consistency 
> DT = data.table(A=c(1,2,1,2,1,2), B=c(1,2,1,1,2,2))
> DT[, .SD, by=.(A)]  # As expected
   A B
1: 1 1
2: 1 1
3: 1 2
4: 2 2
5: 2 1
6: 2 2
> DT[, .SD, by=.(A, B)]  # not expected
Empty data.table (0 rows) of 2 cols: A,B

Likewise,

> DT[, .SD, by=.(A+B)]  # not expected
Empty data.table (0 rows) of 1 col: A
Dominant language
R
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