slice_head: Select first or last rows

In vectra: Columnar Query Engine for Larger-than-RAM Data

Select first or last rows

Description

Select first or last rows

Usage

slice_head(.data, n = 1L)

slice_tail(.data, n = 1L)

slice_min(.data, order_by, n = 1L, with_ties = TRUE)

slice_max(.data, order_by, n = 1L, with_ties = TRUE)

Arguments

.data	A vectra_node object.
n	Number of rows to select.
order_by	Column to order by (for slice_min/slice_max).
with_ties	If TRUE (default), includes all rows that tie with the nth value. If FALSE, returns exactly n rows.

Details

When slice_min()/slice_max() follow group_by(), the n smallest/largest rows are taken within each group and the whole winning row is kept (every column, including geometry carried as a string). with_ties = FALSE returns exactly n rows per group; with_ties = TRUE keeps rows tied at the nth value via min-rank. The n = 1, with_ties = FALSE case streams: it holds only the running winner per group, so memory scales with the number of groups (the result size), not the input. Other grouped cases buffer their input.

Value

A vectra_node for slice_head(), for grouped slice_min()/slice_max(), and for ungrouped slice_min/max(..., with_ties = FALSE). A data.frame for slice_tail() and ungrouped slice_min/max(..., with_ties = TRUE) (the default), since these must materialize all rows.

Examples

f <- tempfile(fileext = ".vtr")
write_vtr(mtcars, f)
tbl(f) |> slice_head(n = 3) |> collect()
tbl(f) |> slice_min(order_by = mpg, n = 3) |> collect()
tbl(f) |> slice_max(order_by = mpg, n = 3) |> collect()
# earliest row per group, geometry/attrs preserved:
tbl(f) |> group_by(cyl) |> slice_min(mpg, n = 1, with_ties = FALSE) |> collect()
unlink(f)

vectra documentation built on July 10, 2026, 5:08 p.m.