optimize batched iter

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

internal/unionstore/memdb_art.go

@@ -219,7 +219,6 @@ func (db *artDBWithContext) BatchedSnapshotIter(lower, upper []byte, reverse boo
 		batchSize:             4,
 	}
 
-	// Position at first key immediately
 	iter.fillBatch()
 	return iter
 }
@@ -269,11 +268,24 @@ func (it *snapshotBatchedIter) fillBatch() error {
 	// update state
 	it.pos = 0
 	if len(it.kvs) > 0 {
-		lastKV := it.kvs[len(it.kvs)-1]
+		lastKV := &it.kvs[len(it.kvs)-1]
+		keyLen := len(lastKV.Key)
+
 		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, lastKV.Key)
 		} 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, lastKV.Key)
+			it.nextKey[keyLen] = 0
 		}
 	} else {
 		it.nextKey = nil