ldb dump can print histogram of value size

Summary:
Make "ldb dump --count_only" print histogram of value size. Also, fix a bug that "ldb dump --path=<db_path>" doesn't work.
Closes https://github.com/facebook/rocksdb/pull/2944

Differential Revision: D5954527

Pulled By: siying

fbshipit-source-id: c620a444ec544258b8d113f5f663c375dd53d6be

tools/ldb_cmd.cc

@@ -1353,6 +1353,9 @@ DBDumperCommand::DBDumperCommand(
   itr = options.find(ARG_PATH);
   if (itr != options.end()) {
     path_ = itr->second;
+    if (db_path_.empty()) {
+      db_path_ = path_;
+    }
   }
 }
 
@@ -1485,6 +1488,8 @@ void DBDumperCommand::DoDumpCommand() {
             ReadableTime(ttl_start).c_str(), ReadableTime(ttl_end).c_str());
   }
 
+  HistogramImpl vsize_hist;
+
   for (; iter->Valid(); iter->Next()) {
     int rawtime = 0;
     // If end marker was specified, we stop before it
@@ -1529,7 +1534,9 @@ void DBDumperCommand::DoDumpCommand() {
 
     }
 
-
+    if (count_only_) {
+      vsize_hist.Add(iter->value().size());
+    }
 
     if (!count_only_ && !count_delim_) {
       if (is_db_ttl_ && timestamp_) {
@@ -1551,6 +1558,11 @@ void DBDumperCommand::DoDumpCommand() {
   } else {
     fprintf(stdout, "Keys in range: %lld\n", (long long) count);
   }
+
+  if (count_only_) {
+    fprintf(stdout, "Value size distribution: \n");
+    fprintf(stdout, "%s\n", vsize_hist.ToString().c_str());
+  }
   // Clean up
   delete iter;
 }