Prefix scan: db_bench and bug fixes

Summary: If use_prefix_filters is set and read_range>1, then the random seeks will set a the prefix filter to be the prefix of the key which was randomly selected as the target.  Still need to add statistics (perhaps in a separate diff).

Test Plan: ./db_bench --benchmarks=fillseq,prefixscanrandom --num=10000000 --statistics=1 --use_prefix_blooms=1 --use_prefix_api=1 --bloom_bits=10

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb, haobo

Differential Revision: https://reviews.facebook.net/D12273

include/leveldb/statistics.h

@@ -62,13 +62,21 @@ enum Tickers {
   NUMBER_MERGE_FAILURES = 22,
   SEQUENCE_NUMBER = 23,
 
-  TICKER_ENUM_MAX = 24
+  // number of times bloom was checked before creating iterator on a
+  // file, and the number of times the check was useful in avoiding
+  // iterator creation (and thus likely IOPs).
+  BLOOM_FILTER_PREFIX_CHECKED = 24,
+  BLOOM_FILTER_PREFIX_USEFUL = 25,
+
+  TICKER_ENUM_MAX = 26
 };
 
 const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
   { BLOCK_CACHE_MISS, "rocksdb.block.cache.miss" },
   { BLOCK_CACHE_HIT, "rocksdb.block.cache.hit" },
   { BLOOM_FILTER_USEFUL, "rocksdb.bloom.filter.useful" },
+  { BLOOM_FILTER_PREFIX_CHECKED, "rocksdb.bloom.filter.prefix.checked" },
+  { BLOOM_FILTER_PREFIX_USEFUL, "rocksdb.bloom.filter.prefix.useful" },
   { COMPACTION_KEY_DROP_NEWER_ENTRY, "rocksdb.compaction.key.drop.new" },
   { COMPACTION_KEY_DROP_OBSOLETE, "rocksdb.compaction.key.drop.obsolete" },
   { COMPACTION_KEY_DROP_USER, "rocksdb.compaction.key.drop.user" },

include/leveldb/status.h

@@ -91,7 +91,7 @@ class Status {
     kNotSupported = 3,
     kInvalidArgument = 4,
     kIOError = 5,
-    kMergeInProgress = 6
+    kMergeInProgress = 6,
   };
 
   Code code() const {