DT[TRUE] can lead to invalid key
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Assessment
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Newbie friendliness
- 45/100
Research direction
Start by reproducing the DT[TRUE] example in the issue and read the new test 1542.08 added in PR #2313. Trace the shallow-copy and key-handling behavior involved in that case. Done means modifying DT1 no longer changes DT$id or leaves DT with an invalid key, with the regression test passing.
Written by the indexing model from the issue text.
Description
I didn't realize DT[TRUE] was a way to achieve a shallow copy. shallow copy was only intended for internal use. Thanks to @renkun-ken for highlighting this in #3214, and related #2254.
- #3214
- #2254
In v1.11.8 we see this :
DT = data.table(id = 1:5, key="id")
DT1 = DT[TRUE]
key(DT1)
[1] "id"
DT1[3, id:=6L]
key(DT1)
# NULL # correct
DT$id
# [1] 1 2 6 4 5 # should be 1:5
key(DT)
# [1] "id" # invalid key
It only occurs after DT[TRUE], iiuc, which hopefully folk have not discovered or relied on too much?! I hope the usage out there is like @renkun-ken described to add new columns to the shallow copy, not to change existing columns!
New test 1542.08 was added in PR #2313 ready for when this is fixed.
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