Support ORDER BY in subqueries for MSSQL

[krlmlr]

MSSQL is sensitive to subqueries that have an ORDER BY clause -- they are not allowed. A suggested workaround is to use TOP 100 PERCENT, but I believe we can also solve this by more accurate optimization.

This is an actual problem if window functions are involved, in this case it's not easy to move the arrange() as noted in #94 (comment).

library(tidyverse)
library(dbplyr)

df <- lazy_frame(x = 1:4, g = rep(c(1, 2), each = 2), con = simulate_mssql())

df %>%
  group_by(g) %>%
  arrange(x) %>%
  mutate(r = cumsum(x))
#> <SQL>
#> SELECT `x`, `g`, SUM(`x`) OVER (PARTITION BY `g` ORDER BY `x` ROWS UNBOUNDED PRECEDING) AS `r`
#> FROM (SELECT *
#> FROM `df`
#> ORDER BY `x`) `dbplyr_001`

^{Created on 2019-04-09 by the reprex package (v0.2.1.9000)}