Impact of secondary indices on ordering after subset in i #2400

library(data.table)

DT <- data.table(x = c(3, 3, 2, 2, 1, 1),
                 y = c(2, 1, 2, 1, 2, 1))

DTix <- copy(DT)
setindex(DTix, x)

DTixy <- copy(DT)
setindex(DTixy, x, y)

## with reduced index after assign
DTir <- copy(DTixy)
DTir[1, y := 2]

i <- data.table(x = c(2, 2, 1, 1),
                y = c(2, 1, 2, 1))

iix <- copy(i)
setindex(iix, x)

iixy <- copy(i)
setindex(iixy, x, y)

iixr <- copy(i)
setindex(iixr, x, y)
iixr[1, y := 2]

## function to determine, which columns have been reordered in a data.table with x and y columns
isSortedTwoCol <- function(DT){
  if(identical(DT$x, sort(DT$x))){
    if(identical(setorder(copy(DT), x, y), DT)) {
      sorted <- "x & y" 
    } else {
      sorted <- "x"
    }
  } else if(identical(setorder(copy(DT), y), DT)){
    sorted <- "y"
  } else {
    sorted <- " none"
  }
  sorted
}

## function to determine, which columns have been reordered in a data.table with x, y, and i.y columns
isSortedThreeCol <- function(DT){
  if(identical(DT$x, sort(DT$x))){
    if(identical(setorder(copy(DT), x, y), DT)) {
      if(identical(setorder(copy(DT), x, y, i.y), DT)){
        sorted <- "x & y & i.y"
      } else {
        sorted <- "x & y"
      }
    } else {
      sorted <- "x"
    }
  } else if(identical(setorder(copy(DT), y), DT)){
    if(identical(setorder(copy(DT), y, i.y), DT)){
      sorted <- "y & i.y"
    } else {
      sorted <- "y"
    }
  } else if(identical(setorder(copy(DT), i.y), DT)){
    sorted <- " i.y"
  } else {
    sorted <- "none"
  }
  sorted
}

cat("\n\nsubsets with lists")
cat(paste0("\nDT[1,, on = 'x']: ", isSortedTwoCol(DT[.(c(1, 2)),, on = 'x'])))
cat(paste0("\nDTix[1,, on = 'x']: ", isSortedTwoCol(DTix[.(c(1, 2)),, on = 'x'])))
cat(paste0("\nDTixy[1,, on = 'x']: ", isSortedTwoCol(DTixy[.(c(1, 2)),, on = 'x'])))

cat("\n\nsubsets with calls")
cat(paste0("\nDT[x %in% c(1,2)]: ", isSortedTwoCol(DT[x %in% (c(1, 2)),])))
cat(paste0("\nDTix[x %in% c(1,2)]: ", isSortedTwoCol(DTix[x %in% (c(1, 2)),])))
cat(paste0("\nDTixy[x %in% c(1,2)]: ", isSortedTwoCol(DTixy[x %in% (c(1, 2)),])))


cat("\n\nsubsets with data.tables")
cat(paste0("\nDT[i,, on = 'x']: ", isSortedThreeCol(DT[i,, on = 'x'])))
cat(paste0("\nDT[iix,, on = 'x']: ", isSortedThreeCol(DT[iix,, on = 'x'])))
cat(paste0("\nDTix[i,, on = 'x']: ", isSortedThreeCol(DTix[i,, on = 'x'])))
cat(paste0("\nDTix[iix,, on = 'x']: ", isSortedThreeCol(DTix[iix,, on = 'x'])))

cat(paste0("\nDT[i,, on = c('x', 'y')]: ", isSortedTwoCol(DT[i,, on = c('x', 'y')])))

cat(paste0("\nDT[iixy,, on = c('x', 'y')]: ", isSortedTwoCol(DT[iixy,, on = c('x', 'y')])))
cat(paste0("\nDTixy[i,, on = c('x', 'y')]: ", isSortedTwoCol(DTixy[i,, on = c('x', 'y')])))
cat(paste0("\nDTixy[iixy,, on = c('x', 'y')]: ", isSortedTwoCol(DTixy[iixy,, on = c('x', 'y')])))