Data.table: La combinación con índice ofrece resultados inesperados si el nombre de la columna indexada es un prefijo del nombre de la columna de combinación

Este problema se basa en mi pregunta sobre stackoverflow .

Para una configuración específica de dos tablas de datos, una combinación no ofrece los resultados que esperaba.

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

Para las siguientes variaciones, la combinación funciona como se esperaba. El comportamiento inesperado anterior parece estar ocurriendo solo, si existe un índice en una columna que tiene un nombre de columna que es el prefijo del nombre de la columna de unión y ambos tienen contenido de texto similar.

# 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

Tenga en cuenta que el mismo comportamiento ocurre para data.table 1.10.4 y R.Version 3.4.2 en Windows y también en Ubuntu Linux 14.04.