Only store primary keys in secondary index sets

1 files changed, 23 insertions, 5 deletions

diff --git a/log_db/src/primary_memtable.rs b/log_db/src/primary_memtable.rs
index 0c94ceb..141238f 100644
--- a/log_db/src/primary_memtable.rs
+++ b/log_db/src/primary_memtable.rs
@@ -3,15 +3,29 @@ use priority_queue::PriorityQueue;
 use std::collections::BTreeMap;
 
 pub struct PrimaryMemtable {
+    /// Maximum number of records that can be stored in the memtable
+    /// before evicting the oldest records. The oldest record is
+    /// determined by the `evict_policy`.
     capacity: usize,
-    /// Running counter of memtable operations, used as priority
-    /// in evict_queue.
-    n_operations: u64,
+    /// Map of records indexed by key. Used as a shared heap of records
+    /// for all secondary memtables also. Secondary memtables store an
+    /// IndexableValue as their record value, which is used to get
+    /// the actual record from the primary memtable `records` map.
+    ///
+    /// Note: it must be invariant that all memtables (primary and secondary)
+    /// contain the same keys.
     records: BTreeMap<IndexableValue, Record>,
-    /// A max heap priority queue of keys. Note: n_operations must
-    /// be negated upon append to evict oldest values first.
+    /// A max heap priority queue of keys. The record with least priority is evicted
+    /// from the primary memtable and any secondary memtables that reference it, when
+    /// the memtable reaches capacity.
+    ///
+    /// Note: n_operations must be negated upon append to evict oldest values first.
     evict_queue: PriorityQueue<IndexableValue, i64>,
+    /// Policy for prioritizing records for eviction.
     evict_policy: MemtableEvictPolicy,
+    /// Running counter of memtable operations, used as priority
+    /// in evict_queue.
+    n_operations: u64,
 }
 
 impl PrimaryMemtable {
@@ -59,6 +73,10 @@ impl PrimaryMemtable {
         self.records.get(key)
     }
 
+    pub fn get_without_update(&self, key: &IndexableValue) -> Option<&Record> {
+        self.records.get(key)
+    }
+
     fn set_priority(&mut self, key: &IndexableValue) {
         let priority = self.get_and_increment_current_priority();
         match self.evict_queue.get(key) {