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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) {
needs_dummy_order <- is.null(op_sort(.data))
if (needs_dummy_order) {
dummy_order_vars <- colnames(.data)[[1]]
.data <- .data %>% window_order(!!sym(dummy_order_vars))
}
.data <- .data %>%
group_by(..., .add = TRUE) %>%
filter(row_number() == 1L) %>%
group_by(!!!grps)
if (needs_dummy_order) {
.data <- .data %>% window_order()
}
return(.data)
}
if (empty_dots) {
dots <- quos(!!!syms(colnames(.data)))
} else {
dots <- partial_eval_dots(.data, ..., .named = FALSE)
dots <- quos(!!!dots)
}
prep <- distinct_prepare_compat(.data, dots, group_vars = group_vars(.data))
out <- dplyr::select(prep$data, prep$keep)
out$lazy_query <- add_distinct(out)
out
}
# copied from dplyr with minor changes (names -> colnames)
# https://github.com/tidyverse/dplyr/blob/main/R/distinct.R
distinct_prepare_compat <- function(.data,
vars,
group_vars = character(),
.keep_all = FALSE,
caller_env = caller_env(2),
error_call = caller_env()
) {
stopifnot(is_quosures(vars), is.character(group_vars))
# If no input, keep all variables
if (length(vars) == 0) {
return(list(
data = .data,
vars = seq_along(.data),
keep = seq_along(.data)
))
}
# If any calls, use mutate to add new columns, then distinct on those
computed_columns <- add_computed_columns(.data, vars, error_call = error_call)
.data <- computed_columns$data
distinct_vars <- computed_columns$added_names
# Once we've done the mutate, we no longer need lazy objects, and
# can instead just use their names
missing_vars <- setdiff(distinct_vars, colnames(.data))
if (length(missing_vars) > 0) {
bullets <- c(
"Must use existing variables.",
set_names(glue("`{missing_vars}` not found in `.data`."), rep("x", length(missing_vars)))
)
abort(bullets, call = error_call)
}
# Only keep unique vars
distinct_vars <- unique(distinct_vars)
# Missing grouping variables are added to the front
new_vars <- c(setdiff(group_vars, distinct_vars), distinct_vars)
if (.keep_all) {
keep <- seq_along(.data)
} else {
keep <- new_vars
}
list(data = .data, vars = new_vars, keep = keep)
}
# copied from dplyr
# https://github.com/tidyverse/dplyr/blob/main/R/group-by.R#L243
add_computed_columns <- function(.data,
vars,
error_call = caller_env()) {
is_symbol <- purrr::map_lgl(vars, quo_is_variable_reference)
needs_mutate <- have_name(vars) | !is_symbol
if (any(needs_mutate)) {
out <- mutate(.data, !!!vars)
col_names <- names(exprs_auto_name(vars))
} else {
out <- .data
col_names <- names(exprs_auto_name(vars))
}
list(data = out, added_names = col_names)
}
# copied from dplyr
# https://github.com/tidyverse/dplyr/blob/main/R/group-by.R#L276
quo_is_variable_reference <- function(quo) {
if (quo_is_symbol(quo)) {
return(TRUE)
}
if (quo_is_call(quo, n = 2)) {
expr <- quo_get_expr(quo)
if (is_call(expr, c("$", "[["))) {
if (!identical(expr[[2]], sym(".data"))) {
return(FALSE)
}
param <- expr[[3]]
if (is_symbol(param) || is_string(param)) {
return(TRUE)
}
}
}
FALSE
}
add_distinct <- function(.data) {
lazy_query <- .data$lazy_query
out <- lazy_select_query(
x = lazy_query,
distinct = TRUE
)
# TODO this could also work for joins
if (!is_lazy_select_query(lazy_query)) {
return(out)
}
# Optimisation overview
# * `distinct()` adds the `DISTINCT` clause to `SELECT`
# * `WHERE`, `GROUP BY`, and `HAVING` are executed before `SELECT`
# => they do not matter
# * `ORDER BY`
# => but `arrange()` should not have an influence on `distinct()` so it
# should not matter
# * `LIMIT` are executed after `SELECT`
# => needs a subquery
if (!is_null(lazy_query$limit)) {
return(out)
}
lazy_query$distinct <- TRUE
lazy_query
}
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