non equi join for character columns

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#2,308 6 comments 3 reactions 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
28/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
databases

Research direction

Start with src/bmerge.c around the linked error and reproduce the two R data.table examples from the issue. Trace how non-equi joins handle character columns and define completion as supporting the requested character comparisons without the current error, with regression coverage for the shown cases.

Written by the indexing model from the issue text.

Description

enhancement IDate/ITime non-equi joins

I noticed this error message:

library(data.table)
data.table(id = LETTERS[1:3])[.(id = "B"), on=.(id > id)]
# Error in bmerge(i, x, leftcols, rightcols, io, xo, roll, rollends, nomatch,  : 
#   Only '==' operator is supported for columns of type character.

I couldn't find any open issue filed for adding this feature (since #1452 is done already). Is it not planned?

I ran into it looking at an SO question:

# goal: use bmerge for this subset condition: letter != '' & date < as.IDate('2014-10-01')
library(data.table)
dt <- data.table(letter = rep(c('',letters[1:4]), each = 4),
  date = as.IDate(c('2014-09-29','2014-09-30','2014-10-01','2014-10-02')))

# attempt
dt[.(L = '', D = as.IDate('2014-10-01')), on=.(letter > L, date < D), verbose = TRUE]
# Non-equi join operators detected ... 
#   forder took ... 0.02 secs
#   Generating non-equi group ids ... done in 0 secs
#   Recomputing forder with non-equi ids ... done in 0 secs
#   Found 4 non-equi group(s) ...
# Starting bmerge ...Error in bmerge(i, x, leftcols, rightcols, io, xo, roll, rollends, nomatch,  : 
#   Only '==' operator is supported for columns of type character.

(I figured this would work for letter != '' so long as "" is the minimum character.)

Anyway, not a big deal for me, but figured I'd ask.

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