bigrquery中的percentile_cont

Question

我想在bigrquery中使用dplyr语法在大查询中获得列的20％，但我不断遇到以下错误。这是一个可重现的示例：

library(bigrquery)
library(dplyr)
library(DBI)
billing <- YOUR_BILLING_INFO

con <- dbConnect(
  bigrquery::bigquery(),
  project = "publicdata",
  dataset = "samples",
  billing = billing
)

natality <- tbl(con, "natality")

natality %>%
  filter(year %in% c(1969, 1970)) %>%
  group_by(year) %>%
  summarise(percentile_20 = percentile_cont(weight_pounds, 0.2))

我收到以下错误：

Error: Analytic function PERCENTILE_CONT cannot be called without an OVER clause at [1:16] [invalidQuery]

但是，目前尚不清楚如何在此处包括OVER子句。如何使用dplyr语法获得第20个百分位数？