docs(common): add more docs for DataChunk (risingwavelabs#8736)

Author: kwannoel

src/common/src/array/column.rs

@@ -20,8 +20,18 @@ use risingwave_pb::data::PbColumn;
 use super::{Array, ArrayError, ArrayResult, I64Array};
 use crate::array::{ArrayImpl, ArrayRef};
 
-/// Column is owned by `DataChunk`. It consists of logic data type and physical array
-/// implementation.
+/// A [`Column`] consists of its logical data type
+/// and its corresponding physical array implementation,
+/// The array contains all the datums bound to this [`Column`].
+/// [`Column`] is owned by [`DataChunk`].
+///
+/// For instance, in this [`DataChunk`],
+/// for column `v1`, [`ArrayRef`] will contain: [1,1,1]
+/// | v1 | v2 |
+/// |----|----|
+/// | 1 |  a |
+/// | 1 |  b |
+/// | 1 |  c |
 #[derive(Clone, Debug, PartialEq)]
 pub struct Column {
     array: ArrayRef,

src/common/src/array/data_chunk.rs

@@ -34,7 +34,23 @@ use crate::util::hash_util::finalize_hashers;
 use crate::util::iter_util::{ZipEqDebug, ZipEqFast};
 use crate::util::value_encoding::{serialize_datum_into, ValueRowSerializer};
 
-/// `DataChunk` is a collection of arrays with visibility mask.
+/// [`DataChunk`] is a collection of Columns,
+/// a with visibility mask for each row.
+/// For instance, we could have a [`DataChunk`] of this format.
+/// | v1 | v2 | v3 |
+/// |----|----|----|
+/// | 1  | a  | t  |
+/// | 2  | b  | f  |
+/// | 3  | c  | t  |
+/// | 4  | d  | f  |
+///
+/// Our columns are v1, v2, v3.
+/// Then, if the Visibility Mask hides rows 2 and 4,
+/// We will only have these rows visible:
+/// | v1 | v2 | v3 |
+/// |----|----|----|
+/// | 1  | a  | t  |
+/// | 3  | c  | t  |
 #[derive(Clone, PartialEq)]
 #[must_use]
 pub struct DataChunk {
@@ -170,7 +186,18 @@ impl DataChunk {
     }
 
     /// `compact` will convert the chunk to compact format.
-    /// Compact format means that `visibility == None`.
+    /// Compacting removes the hidden rows, and returns a new visibility
+    /// mask which indicates this.
+    ///
+    /// `compact` has trade-offs:
+    ///
+    /// Cost:
+    /// It has to rebuild the each column, meaning it will incur cost
+    /// of copying over bytes from the original column array to the new one.
+    ///
+    /// Benefit:
+    /// The main benefit is that the data chunk is smaller, taking up less memory.
+    /// We can also save the cost of iterating over many hidden rows.
     pub fn compact(self) -> Self {
         match &self.vis2 {
             Vis::Compact(_) => self,

src/common/src/array/mod.rs

@@ -496,7 +496,8 @@ macro_rules! impl_array_builder {
                 }
             }
 
-            /// Append a [`Datum`] or [`DatumRef`] multiple times, return error while type not match.
+            /// Append a [`Datum`] or [`DatumRef`] multiple times,
+            /// panicking if the datum's type does not match the array builder's type.
             pub fn append_datum_n(&mut self, n: usize, datum: impl ToDatumRef) {
                 match datum.to_datum_ref() {
                     None => match self {

src/common/src/array/vis.rs

@@ -17,11 +17,15 @@ use itertools::repeat_n;
 
 use crate::buffer::{Bitmap, BitmapBuilder};
 
-/// `Vis` is a visibility bitmap of rows. When all rows are visible, it is considered compact and
-/// is represented by a single cardinality number rather than that many of ones.
+/// `Vis` is a visibility bitmap of rows.
 #[derive(Clone, PartialEq, Debug)]
 pub enum Vis {
+    /// Non-compact variant.
+    /// Certain rows are hidden using this bitmap.
     Bitmap(Bitmap),
+
+    /// Compact variant which just stores cardinality of rows.
+    /// This can be used when all rows are visible.
     Compact(usize), // equivalent to all ones of this size
 }