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#' Subset distinct/unique rows
#'
#' This is a method for the dplyr [distinct()] generic. It adds the
#' `DISTINCT` clause to the SQL query.
#'
#' @inheritParams arrange.tbl_lazy
#' @inheritParams dplyr::distinct
#' @inherit arrange.tbl_lazy return
#' @export
#' @importFrom dplyr distinct
#' @examples
#' library(dplyr, warn.conflicts = FALSE)
#'
#' db <- memdb_frame(x = c(1, 1, 2, 2), y = c(1, 2, 1, 1))
#' db %>% distinct() %>% show_query()
#' db %>% distinct(x) %>% show_query()
distinct.tbl_lazy <- function(.data, ..., .keep_all = FALSE) {
grps <- syms(op_grps(.data))
empty_dots <- dots_n(...) == 0
can_use_distinct <- !.keep_all || (empty_dots && is_empty(grps))
if (!can_use_distinct) {
.data <- .data %>%
group_by(..., .add = TRUE) %>%
filter(row_number() == 1L) %>%
group_by(!!!grps)
return(.data)
}
if (empty_dots) {
dots <- quos(!!!syms(colnames(.data)))
} else {
dots <- enquos(...)
}
prep <- dplyr::distinct_prepare(.data, dots, group_vars = group_vars(.data))
out <- dplyr::select(prep$data, prep$keep)
out$lazy_query <- add_distinct(out)
out
}
add_distinct <- function(.data) {
lazy_query <- .data$lazy_query
out <- lazy_select_query(
x = lazy_query,
distinct = TRUE
)
# TODO this could also work for joins
if (!inherits(lazy_query, "lazy_select_query")) {
return(out)
}
# Optimisation overview
# * `distinct()` adds the `DISTINCT` clause to `SELECT`
# * `WHERE`, `GROUP BY`, and `HAVING` are executed before `SELECT`
# => they do not matter
# * `ORDER BY`
# => but `arrange()` should not have an influence on `distinct()` so it
# should not matter
# * `LIMIT` are executed after `SELECT`
# => needs a subquery
if (!is_null(lazy_query$limit)) {
return(out)
}
lazy_query$distinct <- TRUE
lazy_query
}
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