db: reduce allocations of Iterator.prefixOrFullSeekKey

db.go

@@ -670,11 +670,12 @@ func (d *DB) commitWrite(b *Batch, syncWG *sync.WaitGroup, syncErr *error) (*mem
 }
 
 type iterAlloc struct {
-	dbi     Iterator
-	keyBuf  []byte
-	merging mergingIter
-	mlevels [3 + numLevels]mergingIterLevel
-	levels  [3 + numLevels]levelIter
+	dbi                  Iterator
+	keyBuf               []byte
+	prefixOrFullSeekKey  []byte
+	merging              mergingIter
+	mlevels              [3 + numLevels]mergingIterLevel
+	levels               [3 + numLevels]levelIter
 }
 
 var iterAllocPool = sync.Pool{
@@ -709,17 +710,18 @@ func (d *DB) newIterInternal(batch *Batch, s *Snapshot, o *IterOptions) *Iterato
 	buf := iterAllocPool.Get().(*iterAlloc)
 	dbi := &buf.dbi
 	*dbi = Iterator{
-		alloc:     buf,
-		cmp:       d.cmp,
-		equal:     d.equal,
-		iter:      &buf.merging,
-		merge:     d.merge,
-		split:     d.split,
-		readState: readState,
-		keyBuf:    buf.keyBuf,
-		batch:     batch,
-		newIters:  d.newIters,
-		seqNum:    seqNum,
+		alloc:               buf,
+		cmp:                 d.cmp,
+		equal:               d.equal,
+		iter:                &buf.merging,
+		merge:               d.merge,
+		split:               d.split,
+		readState:           readState,
+		keyBuf:              buf.keyBuf,
+		prefixOrFullSeekKey: buf.prefixOrFullSeekKey,
+		batch:               batch,
+		newIters:            d.newIters,
+		seqNum:              seqNum,
 	}
 	if o != nil {
 		dbi.opts = *o

iterator.go

@@ -863,6 +863,11 @@ func (i *Iterator) Close() error {
 		} else {
 			alloc.keyBuf = i.keyBuf
 		}
+		if cap(i.prefixOrFullSeekKey) >= maxKeyBufCacheSize {
+			alloc.prefixOrFullSeekKey = nil
+		} else {
+			alloc.prefixOrFullSeekKey = i.prefixOrFullSeekKey
+		}
 		*i = Iterator{}
 		iterAllocPool.Put(alloc)
 	}