by = .EACHI key fix 4603 (#4917)

Author: ColeMiller1

NEWS.md

@@ -12,6 +12,8 @@
 
 ## BUG FIXES
 
+1. `by=.EACHI` when `i` is keyed but `on=` different columns than `i`'s key could create an invalidly keyed result, [#4603](https://github.com/Rdatatable/data.table/issues/4603) [#4911](https://github.com/Rdatatable/data.table/issues/4911). Thanks to @myoung3 and @adamaltmejd for reporting, and @ColeMiller1 for the PR. An invalid key is where a `data.table` is marked as sorted by the key columns but the data is not sorted by those columns, leading to incorrect results from subsequent queries.
+
 ## NOTES
 
 1. New feature 29 in v1.12.4 (Oct 2019) introduced zero-copy coercion. Our thinking is that requiring you to get the type right in the case of `0` (type double) vs `0L` (type integer) is too inconvenient for you the user. So such coercions happen in `data.table` automatically without warning. Thanks to zero-copy coercion there is no speed penalty, even when calling `set()` many times in a loop, so there's no speed penalty to warn you about either. However, we believe that assigning a character value such as `"2"` into an integer column is more likely to be a user mistake that you would like to be warned about. The type difference (character vs integer) may be the only clue that you have selected the wrong column, or typed the wrong variable to be assigned to that column. For this reason we view character to numeric-like coercion differently and will warn about it. If it is correct, then the warning is intended to nudge you to wrap the RHS with `as.<type>()` so that it is clear to readers of your code that a coercion from character to that type is intended. For example :

R/data.table.R

@@ -1385,7 +1385,8 @@ replace_dot_alias = function(e) {
     byval = i
     bynames = if (missing(on)) head(key(x),length(leftcols)) else names(on)
     allbyvars = NULL
-    bysameorder = haskey(i) || (is.sorted(f__) && ((roll == FALSE) || length(f__) == 1L)) # Fix for #1010
+    bysameorder = (haskey(i) && identical(leftcols, chmatch(head(key(i),length(leftcols)), names(i)))) || # leftcols leading subset of key(i); see #4917
+                  (roll==FALSE && is.sorted(f__)) # roll==FALSE is fix for #1010
     ##  'av' correct here ??  *** TO DO ***
     xjisvars = intersect(av, names_x[rightcols])  # no "x." for xvars.
     # if 'get' is in 'av' use all cols in 'i', fix for bug #34

inst/tests/tests.Rraw

@@ -17273,3 +17273,25 @@ if (test_bit64) {
   test(2164.3, d[, mean(b, na.rm=TRUE), by=a], data.table(a=INT(1,2), V1=c(2.5, 4)))
 }
 
+# invalid key when by=.EACHI, haskey(i) but on= non-leading-subset of i's key, #4603 #4911
+X = data.table(id = c(6456372L, 6456372L, 6456372L, 6456372L,6456372L, 6456372L, 6456372L, 6456372L, 6456372L, 6456372L, 6456372L, 6456372L, 6456372L, 6456372L),
+               id_round = c(197801L, 199405L, 199501L, 197901L, 197905L, 198001L, 198005L, 198101L, 198105L, 198201L, 198205L, 198301L, 198305L, 198401L), 
+               field = c(NA, NA, NA, "medicine", "medicine", "medicine", "medicine", "medicine", "medicine", "medicine", "medicine", "medicine", "medicine", "medicine"),
+               key = "id")
+Y = data.table(id = c(6456372L, 6456345L, 6456356L), 
+               id_round = c(197705L, 197905L, 201705L),
+               field = c("medicine", "teaching", "health"), 
+               prio = c(6L, 1L, 10L),
+               key = c("id_round", "id", "prio", "field" ))
+test(2165.1, X[Y, on = .(id, id_round > id_round, field), .(x.id_round[1], i.id_round[1]), by=.EACHI][id==6456372L],
+             data.table(id=6456372L, id_round=197705L, field='medicine', V1=197901L, V2=197705L))
+# Y$id_round happens to be sorted, so in 2165.2 we test Y$field which is not sorted 
+test(2165.2, X[Y, on="field", .(x.id_round[1]), by=.EACHI][field=="health"],
+             data.table(field="health", V1=NA_integer_))
+# a minimal example too ...
+X = data.table(A=c(4L,2L,3L), B=1:3, key="A")
+Y = data.table(A=2:1, B=2:3, key=c("B","A"))
+test(2165.3, X[Y],                          data.table(A=2:3, B=2:3, i.A=2:1, key="A"))  # keyed
+test(2165.4, X[Y, on=.(A)],                 data.table(A=2:1, B=c(2L,NA), i.B=2:3))      # no key
+test(2165.5, X[Y, on=.(A), x.B, by=.EACHI], data.table(A=2:1, x.B=c(2L,NA)))             # no key
+