Data.table: Mit Index verknüpfen liefert unerwartete Ergebnisse, wenn der Name der indizierten Spalte ein Präfix des Namens der Verknüpfungsspalte ist

Dieses Problem basiert auf meiner Frage zum Stackoverflow .

Bei einer bestimmten Einrichtung von zwei data.tables liefert ein Join nicht die erwarteten Ergebnisse.

library(data.table)

# In the code below the join does not deliver the result I would expect
DT1 <- data.table(colname=c("test1","test2","test2","test3"), colname_with_suffix=c("other","test","includes test within","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[colname_with_suffix == "not found", ]  # automatically creates index on colname_with_suffix
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]
# PLEASE NOTE: same result with slightly different syntax: DT1[DT2, lookup_result := i.lookup_result, on=c(colname="lookup")][]
# colname  colname_with_suffix lookup_result
# 1:   test1                other         NA
# 2:   test2                 test         NA
# 3:   test2 includes test within         NA
# 4:   test3                other          3


# Expected result:
 # colname  colname_with_suffix lookup_result
# 1:   test1                other          1
# 2:   test2                 test          2
# 3:   test2 includes test within          2
# 4:   test3                other          3

Für die folgenden Variationen funktioniert die Verknüpfung wie erwartet. Das oben beschriebene unerwartete Verhalten scheint nur dann aufzutreten, wenn ein Index für eine Spalte vorhanden ist, deren Spaltenname das Präfix des Join-Spaltennamens ist und beide einen ähnlichen Textinhalt haben.

# For all following alternatives the join delivers the correct result

# (a) Same data tables as above, but no index
DT1 <- data.table(colname=c("test1","test2","test2","test3"), colname_with_suffix=c("other","test","includes test within","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]

# (b) Index on DT2, but completely different values in indexed column than in join column
DT1 <- data.table(colname=c("test1","test2","test2","test3"), colname_with_suffix=c("other","other","other","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[colname_with_suffix == "not found", ]  # automatically creates index on colname_with_suffix
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]

# (c) Index on DT2, similar values in indexed column, but indexed column name is not a prefix of join column name
DT1 <- data.table(colname=c("test1","test2","test2","test3"), x.colname_with_suffix=c("other","test","includes test within","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[x.colname_with_suffix == "not found", ]  # automatically creates index on x.colname_with_suffix
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]

SessionInfo:

# R version 3.3.2 (2016-10-31)
# Platform: x86_64-w64-mingw32/x64 (64-bit)
# Running under: Windows 7 x64 (build 7601) Service Pack 1
# 
# locale:
#     [1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252    LC_MONETARY=German_Germany.1252 LC_NUMERIC=C                    LC_TIME=German_Germany.1252    
# 
# attached base packages:
#     [1] stats     graphics  grDevices utils     datasets  methods   base     
# 
# other attached packages:
#     [1] data.table_1.10.0
# 
# loaded via a namespace (and not attached):
#     [1] tools_3.3.2

Bitte beachten Sie, dass das gleiche Verhalten für data.table 1.10.4 und R.Version 3.4.2 unter Windows und auch unter Ubuntu Linux 14.04 auftritt.