ldb sometimes specify a string-append merge operator (#5607)

Summary:
Right now, ldb cannot scan a DB with merge operands with default ldb. There is no hard to give a general merge operator so that it can at least print out something
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5607

Test Plan: Run ldb against a DB with merge operands and see the outputs.

Differential Revision: D16442634

fbshipit-source-id: c66c414ec07f219cfc6e6ec2cc14c783ee95df54

HISTORY.md

@@ -18,6 +18,7 @@
 * Rate limited deletion of WALs is only enabled if DBOptions::wal_dir is not set, or explicitly set to db_name passed to DB::Open and DBOptions::db_paths is empty, or same as db_paths[0].path
 * Overload GetAllKeyVersions() to support non-default column family.
 * Added new APIs ExportColumnFamily() and CreateColumnFamilyWithImport() to support export and import of a Column Family. https://github.com/facebook/rocksdb/issues/3469
+* ldb sometimes uses a string-append merge operator if no merge operator is passed in. This is to allow users to print keys from a DB with a merge operator.
 
 ### New Features
 * Add an option `snap_refresh_nanos` (default to 0) to periodically refresh the snapshot list in compaction jobs. Assign to 0 to disable the feature.

tools/ldb_cmd.cc

@@ -31,6 +31,7 @@
 #include "util/coding.h"
 #include "util/stderr_logger.h"
 #include "util/string_util.h"
+#include "utilities/merge_operators.h"
 #include "utilities/ttl/db_ttl_impl.h"
 
 #include <cstdlib>
@@ -353,11 +354,24 @@ void LDBCommand::OpenDB() {
           stderr,
           "wal_dir loaded from the option file doesn't exist. Ignore it.\n");
     }
+
+    // If merge operator is not set, set a string append operator. There is
+    // no harm doing it.
+    for (auto& cf_entry : column_families_) {
+      if (!cf_entry.options.merge_operator) {
+        cf_entry.options.merge_operator =
+            MergeOperators::CreateStringAppendOperator(':');
+      }
+    }
   }
   options_ = PrepareOptionsForOpenDB();
   if (!exec_state_.IsNotStarted()) {
     return;
   }
+  if (column_families_.empty() && !options_.merge_operator) {
+    // No harm to add a general merge operator if it is not specified.
+    options_.merge_operator = MergeOperators::CreateStringAppendOperator(':');
+  }
   // Open the DB.
   Status st;
   std::vector<ColumnFamilyHandle*> handles_opened;