我已阅读常见问题解答，但现在仍然清楚在相当大的 data.table 的串联列表中使用键与使用该键的含义是什么？
从我的实验中，我只看到性能，但不确定是否还有其他东西。
# install.packages(c("data.table", "stringi"), dependencies = TRUE)
library(data.table)
library(stringi)
download.file("https://www.ssa.gov/oact/babynames/state/namesbystate.zip", dest="namesbystate.zip", mode="wb")
unzip("namesbystate.zip", exdir=".")
# Read the list of all text files in variable "filelist"
filelist = list.files(path=".",pattern = ".*.TXT")
colnamelist=c("State","gender","year","name","frequency")
#Read the CSV from all the text files into a data.frame
babynames =lapply(filelist, FUN=read.csv, header=FALSE,col.name=colnamelist);
nametable = rbindlist(babynames,use.names = FALSE,fill = FALSE)
DT = data.table(nametable)
dim(DT) #[1] 5647426 5
setkey(DT,NULL)
system.time(head(DT[,( stri_length(name)),by=c("name", "year")]))
# user system elapsed
# 156.47 0.03 157.64
setkey(DT,year)
system.time(head(DT[,( stri_length(name)),by=name]))
# user system elapsed
# 8.90 0.00 8.99
两种情况下的输出相同
name year V1
1: Anna 1910 4
2: Annie 1910 5
3: Dorothy 1910 7
4: Elsie 1910 5
5: Helen 1910 5
6: Lucy 1910 4