Removing alloc on all .next() in MultiValueColumn

src/indexer/flat_map_with_buffer.rs

@@ -0,0 +1,69 @@
+pub struct FlatMapWithBuffer<T, F, Iter> {
+    buffer: Vec<T>,
+    fill_buffer: F,
+    underlying_it: Iter,
+}
+
+impl<T, F, Iter, I> Iterator for FlatMapWithBuffer<T, F, Iter>
+where
+    Iter: Iterator<Item = I>,
+    F: Fn(I, &mut Vec<T>),
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while self.buffer.is_empty() {
+            let next_el = self.underlying_it.next()?;
+            (self.fill_buffer)(next_el, &mut self.buffer);
+            // We will pop elements, so we reverse the buffer first.
+            self.buffer.reverse();
+        }
+        self.buffer.pop()
+    }
+}
+
+pub trait FlatMapWithBufferIter: Iterator {
+    /// Function similar to `flat_map`, but allows reusing a shared `Vec`.
+    fn flat_map_with_buffer<F, T>(self, fill_buffer: F) -> FlatMapWithBuffer<T, F, Self>
+    where
+        F: Fn(Self::Item, &mut Vec<T>),
+        Self: Sized,
+    {
+        FlatMapWithBuffer {
+            buffer: Vec::with_capacity(10),
+            fill_buffer,
+            underlying_it: self,
+        }
+    }
+}
+
+impl<T: ?Sized> FlatMapWithBufferIter for T where T: Iterator {}
+
+#[cfg(test)]
+mod tests {
+    use crate::indexer::flat_map_with_buffer::FlatMapWithBufferIter;
+
+    #[test]
+    fn test_flat_map_with_buffer_empty() {
+        let mut empty_iter = std::iter::empty::<usize>()
+            .flat_map_with_buffer(|_val: usize, _buffer: &mut Vec<usize>| {});
+        assert!(empty_iter.next().is_none());
+    }
+
+    #[test]
+    fn test_flat_map_with_buffer_simple() {
+        let vals: Vec<usize> = (1..5)
+            .flat_map_with_buffer(|val: usize, buffer: &mut Vec<usize>| buffer.extend(0..val))
+            .collect();
+        assert_eq!(&[0, 0, 1, 0, 1, 2, 0, 1, 2, 3], &vals[..]);
+    }
+
+    #[test]
+    fn test_flat_map_filling_no_elements_does_not_stop_iterator() {
+        let vals: Vec<usize> = [2, 0, 0, 3]
+            .into_iter()
+            .flat_map_with_buffer(|val: usize, buffer: &mut Vec<usize>| buffer.extend(0..val))
+            .collect();
+        assert_eq!(&[0, 1, 0, 1, 2], &vals[..]);
+    }
+}

src/indexer/mod.rs

@@ -3,6 +3,7 @@ pub mod delete_queue;
 pub mod demuxer;
 pub mod doc_id_mapping;
 mod doc_opstamp_mapping;
+mod flat_map_with_buffer;
 pub mod index_writer;
 mod index_writer_status;
 mod json_term_writer;

src/indexer/sorted_doc_id_multivalue_column.rs

@@ -2,10 +2,11 @@ use std::cmp;
 
 use fastfield_codecs::Column;
 
+use super::flat_map_with_buffer::FlatMapWithBufferIter;
 use crate::fastfield::MultiValuedFastFieldReader;
 use crate::indexer::doc_id_mapping::SegmentDocIdMapping;
 use crate::schema::Field;
-use crate::SegmentReader;
+use crate::{DocAddress, SegmentReader};
 
 pub(crate) struct RemappedDocIdMultiValueColumn<'a> {
     doc_id_mapping: &'a SegmentDocIdMapping,
@@ -74,11 +75,9 @@ impl<'a> Column for RemappedDocIdMultiValueColumn<'a> {
         Box::new(
             self.doc_id_mapping
                 .iter_old_doc_addrs()
-                .flat_map(|old_doc_addr| {
+                .flat_map_with_buffer(|old_doc_addr: DocAddress, buffer| {
                     let ff_reader = &self.fast_field_readers[old_doc_addr.segment_ord as usize];
-                    let mut vals = Vec::new();
-                    ff_reader.get_vals(old_doc_addr.doc_id, &mut vals);
-                    vals.into_iter()
+                    ff_reader.get_vals(old_doc_addr.doc_id, buffer);
                 }),
         )
     }