DBIter::FindNextUserEntryInternal: do not PrepareValue for `Delete` (#11211)

Summary:
`kTypeDeletion/kTypeDeletionWithTimestamp/kTypeSingleDeletion` does not need access iter value, so omit `PrepareValue`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11211

Reviewed By: ajkr

Differential Revision: D43253068

Pulled By: cbi42

fbshipit-source-id: 1945c7f8a90b6909128a0553b62d9fd1078b0a08

db/db_iter.cc

@@ -341,11 +341,6 @@ bool DBIter::FindNextUserEntryInternal(bool skipping_saved_key,
       } else {
         assert(!skipping_saved_key ||
                CompareKeyForSkip(ikey_.user_key, saved_key_.GetUserKey()) > 0);
-        if (!iter_.PrepareValue()) {
-          assert(!iter_.status().ok());
-          valid_ = false;
-          return false;
-        }
         num_skipped = 0;
         reseek_done = false;
         switch (ikey_.type) {
@@ -369,6 +364,11 @@ bool DBIter::FindNextUserEntryInternal(bool skipping_saved_key,
           case kTypeValue:
           case kTypeBlobIndex:
           case kTypeWideColumnEntity:
+            if (!iter_.PrepareValue()) {
+              assert(!iter_.status().ok());
+              valid_ = false;
+              return false;
+            }
             if (timestamp_lb_) {
               saved_key_.SetInternalKey(ikey_);
             } else {
@@ -397,6 +397,11 @@ bool DBIter::FindNextUserEntryInternal(bool skipping_saved_key,
             return true;
             break;
           case kTypeMerge:
+            if (!iter_.PrepareValue()) {
+              assert(!iter_.status().ok());
+              valid_ = false;
+              return false;
+            }
             saved_key_.SetUserKey(
                 ikey_.user_key,
                 !pin_thru_lifetime_ || !iter_.iter()->IsKeyPinned() /* copy */);