optimize batched iter

Signed-off-by: ekexium <[email protected]>

internal/unionstore/memdb_art.go

@@ -203,7 +203,8 @@ type snapshotBatchedIter struct {
 	reverse               bool
 
 	// current batch
-	kvs       []KvPair
+	keys      [][]byte
+	values    [][]byte
 	pos       int
 	batchSize int
 	nextKey   []byte
@@ -216,10 +217,9 @@ func (db *artDBWithContext) BatchedSnapshotIter(lower, upper []byte, reverse boo
 		lower:                 lower,
 		upper:                 upper,
 		reverse:               reverse,
-		batchSize:             4,
+		batchSize:             32,
 	}
 
-	// Position at first key immediately
 	iter.fillBatch()
 	return iter
 }
@@ -233,10 +233,12 @@ func (it *snapshotBatchedIter) fillBatch() error {
 	it.db.RLock()
 	defer it.db.RUnlock()
 
-	if it.kvs == nil {
-		it.kvs = make([]KvPair, 0, it.batchSize)
+	if it.keys == nil || it.values == nil || cap(it.keys) < it.batchSize || cap(it.values) < it.batchSize {
+		it.keys = make([][]byte, 0, it.batchSize)
+		it.values = make([][]byte, 0, it.batchSize)
 	} else {
-		it.kvs = it.kvs[:0]
+		it.keys = it.keys[:0]
+		it.values = it.values[:0]
 	}
 
 	var snapshotIter Iterator
@@ -256,24 +258,38 @@ func (it *snapshotBatchedIter) fillBatch() error {
 	defer snapshotIter.Close()
 
 	// fill current batch
+	// Further optimization: let the underlying memdb support batch iter.
 	for i := 0; i < it.batchSize && snapshotIter.Valid(); i++ {
-		it.kvs = append(it.kvs, KvPair{
-			Key:   snapshotIter.Key(),
-			Value: snapshotIter.Value(),
-		})
+		it.keys = it.keys[:i+1]
+		it.values = it.values[:i+1]
+		it.keys[i] = snapshotIter.Key()
+		it.values[i] = snapshotIter.Value()
 		if err := snapshotIter.Next(); err != nil {
 			return err
 		}
 	}
 
 	// update state
 	it.pos = 0
-	if len(it.kvs) > 0 {
-		lastKV := it.kvs[len(it.kvs)-1]
+	if len(it.keys) > 0 {
+		lastKey := it.keys[len(it.keys)-1]
+		keyLen := len(lastKey)
+
 		if it.reverse {
-			it.nextKey = append([]byte(nil), lastKV.Key...)
+			if cap(it.nextKey) >= keyLen {
+				it.nextKey = it.nextKey[:keyLen]
+			} else {
+				it.nextKey = make([]byte, keyLen)
+			}
+			copy(it.nextKey, lastKey)
 		} else {
-			it.nextKey = append(append([]byte(nil), lastKV.Key...), 0)
+			if cap(it.nextKey) >= keyLen+1 {
+				it.nextKey = it.nextKey[:keyLen+1]
+			} else {
+				it.nextKey = make([]byte, keyLen+1)
+			}
+			copy(it.nextKey, lastKey)
+			it.nextKey[keyLen] = 0
 		}
 	} else {
 		it.nextKey = nil
@@ -285,7 +301,7 @@ func (it *snapshotBatchedIter) fillBatch() error {
 
 func (it *snapshotBatchedIter) Valid() bool {
 	return it.snapshotTruncateSeqNo == it.db.SnapshotSeqNo &&
-		it.pos < len(it.kvs)
+		it.pos < len(it.keys)
 }
 
 func (it *snapshotBatchedIter) Next() error {
@@ -300,7 +316,7 @@ func (it *snapshotBatchedIter) Next() error {
 	}
 
 	it.pos++
-	if it.pos >= len(it.kvs) {
+	if it.pos >= len(it.keys) {
 		return it.fillBatch()
 	}
 	return nil
@@ -310,17 +326,18 @@ func (it *snapshotBatchedIter) Key() []byte {
 	if !it.Valid() {
 		return nil
 	}
-	return it.kvs[it.pos].Key
+	return it.keys[it.pos]
 }
 
 func (it *snapshotBatchedIter) Value() []byte {
 	if !it.Valid() {
 		return nil
 	}
-	return it.kvs[it.pos].Value
+	return it.values[it.pos]
 }
 
 func (it *snapshotBatchedIter) Close() {
-	it.kvs = nil
+	it.keys = nil
+	it.values = nil
 	it.nextKey = nil
 }