Rich-T-kid commented on code in PR #11145:
URL: https://github.com/apache/arrow-rs/pull/11145#discussion_r4074522361
##########
arrow-array/src/array/map_array.rs:
##########
@@ -32,6 +32,26 @@ use std::sync::Arc;
/// [`MapArray`] is physically a [`ListArray`] of key values pairs stored as
an `entries`
/// [`StructArray`] with 2 child fields.
///
+/// # Slicing
+///
+/// Slicing a `MapArray` via [`Self::slice`] creates a new `MapArray` without
+/// copying any data. The sliced array shares the same `entries` child array
and
+/// only narrows its offsets and validity, which means:
+///
+/// 1. [`Self::entries`], [`Self::keys`] and [`Self::values`] are unchanged and
+/// may contain entries both before and after the slice.
+/// 2. [`Self::offsets`] do not necessarily start at `0`, nor cover all
entries.
+///
+/// For example, given a `MapArray` holding the three maps `{a: 1, b: 2}`,
+/// `{c: 3}` and `{d: 4, e: 5}`, the `entries` array holds five key-value pairs
+/// and the offsets are `[0, 2, 3, 5]`. Calling `slice(1, 1)` yields a
`MapArray`
+/// holding the single map `{c: 3}`, but [`Self::keys`] still returns all five
+/// keys `a, b, c, d, e` and [`Self::offsets`] is `[2, 3]`. Use the offsets, or
+/// [`Self::value`], to find the entries belonging to each map.
+///
+/// The same applies to any `MapArray` constructed with offsets that do not
start
+/// at `0` or do not extend to the end of `entries`.
Review Comment:
nice I like that you provided an example
##########
arrow-array/src/array/map_array.rs:
##########
@@ -210,22 +230,40 @@ impl MapArray {
///
/// Unlike [`Self::value_offsets`] this returns the [`OffsetBuffer`]
/// allowing for zero-copy cloning
+ ///
+ /// Note: The offsets may not start at `0` and may not cover all entries in
+ /// [`Self::entries`]. This can happen when the map array was sliced via
+ /// [`Self::slice`]. See documentation for [`Self`] for more details.
Review Comment:
👍
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