[RocksDB] Generalize prefix-aware iterator to be used for more than one Seek

Summary: Added a prefix_seek flag in ReadOptions to indicate that Seek is prefix aware(might not return data with different prefix), and also not bound to a specific prefix. Multiple Seeks and range scans can be invoked on the same iterator. If a specific prefix is specified, this flag will be ignored. Just a quick prototype that works for PrefixHashRep, the new lockless memtable could be easily extended with this support too.

Test Plan: test it on Leaf

Reviewers: dhruba, kailiu, sdong, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13929

include/rocksdb/memtablerep.h

@@ -130,6 +130,12 @@ class MemTableRep {
     return GetIterator();
   }
 
+  // Return an iterator that has a special Seek semantics. The result of
+  // a Seek might only include keys with the same prefix as the target key.
+  virtual std::shared_ptr<Iterator> GetDynamicPrefixIterator() {
+    return GetIterator();
+  }
+
  protected:
   // When *key is an internal key concatenated with the value, returns the
   // user key.

include/rocksdb/options.h

@@ -649,6 +649,10 @@ struct ReadOptions {
   // Default: true
   bool fill_cache;
 
+  // If this option is set and memtable implementation allows, Seek
+  // might only return keys with the same prefix as the seek-key
+  bool prefix_seek;
+
   // If "snapshot" is non-nullptr, read as of the supplied snapshot
   // (which must belong to the DB that is being read and which must
   // not have been released).  If "snapshot" is nullptr, use an impliicit
@@ -678,13 +682,14 @@ struct ReadOptions {
   ReadOptions()
       : verify_checksums(false),
         fill_cache(true),
+        prefix_seek(false),
         snapshot(nullptr),
         prefix(nullptr),
         read_tier(kReadAllTier) {
   }
   ReadOptions(bool cksum, bool cache) :
               verify_checksums(cksum), fill_cache(cache),
-              snapshot(nullptr), prefix(nullptr),
+              prefix_seek(false), snapshot(nullptr), prefix(nullptr),
               read_tier(kReadAllTier) {
   }
 };