Reduce extra key comparision in DBIter::Next()

Summary: Now DBIter::Next() always compares with current key with itself first, which is unnecessary if the last key is not a merge key. I made the change and didn't see db_iter_test fails. Want to hear whether people have any idea what I miss.

Test Plan: Run all unit tests

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D48279

db/db_iter.cc

@@ -186,9 +186,17 @@ void DBIter::Next() {
     if (!iter_->Valid()) {
       iter_->SeekToFirst();
     }
+  } else if (iter_->Valid() && !current_entry_is_merged_) {
+    // If the current value is not a merge, the iter position is the
+    // current key, which is already returned. We can safely issue a
+    // Next() without checking the current key.
+    // If the current key is a merge, very likely iter already points
+    // to the next internal position.
+    iter_->Next();
   }
 
-  // If the current value is merged, we might already hit end of iter_
+  // Now we point to the next internal position, for both of merge and
+  // not merge cases.
   if (!iter_->Valid()) {
     valid_ = false;
     return;

db/db_test.cc

@@ -1910,9 +1910,10 @@ TEST_F(DBTest, IterReseek) {
   DestroyAndReopen(options);
   CreateAndReopenWithCF({"pikachu"}, options);
 
-  // insert two keys with same userkey and verify that
+  // insert three keys with same userkey and verify that
   // reseek is not invoked. For each of these test cases,
   // verify that we can find the next key "b".
+  ASSERT_OK(Put(1, "a", "zero"));
   ASSERT_OK(Put(1, "a", "one"));
   ASSERT_OK(Put(1, "a", "two"));
   ASSERT_OK(Put(1, "b", "bone"));