feat: add transaction histogram

Add transaction histogram for tracking operations per transaction:
- num_op_per_transaction: Distribution of operation count per transaction

Extended test/transaction_statistics.erl with histogram test case
verifying histogram data structure.

Author: benoitc

c_src/atoms.h

@@ -422,6 +422,9 @@ extern ERL_NIF_TERM ATOM_WAL_FILE_SYNC_MICROS;
 extern ERL_NIF_TERM ATOM_BYTES_PER_READ;
 extern ERL_NIF_TERM ATOM_BYTES_PER_WRITE;
 
+// Transaction Histogram
+extern ERL_NIF_TERM ATOM_NUM_OP_PER_TRANSACTION;
+
 // Compaction Statistics Tickers
 extern ERL_NIF_TERM ATOM_COMPACT_READ_BYTES;
 extern ERL_NIF_TERM ATOM_COMPACT_WRITE_BYTES;

c_src/erocksdb.cc

@@ -624,6 +624,9 @@ ERL_NIF_TERM ATOM_WAL_FILE_SYNC_MICROS;
 ERL_NIF_TERM ATOM_BYTES_PER_READ;
 ERL_NIF_TERM ATOM_BYTES_PER_WRITE;
 
+// Transaction Histogram
+ERL_NIF_TERM ATOM_NUM_OP_PER_TRANSACTION;
+
 // Compaction Statistics Tickers
 ERL_NIF_TERM ATOM_COMPACT_READ_BYTES;
 ERL_NIF_TERM ATOM_COMPACT_WRITE_BYTES;
@@ -1144,6 +1147,9 @@ try
   ATOM(erocksdb::ATOM_BYTES_PER_READ, "bytes_per_read");
   ATOM(erocksdb::ATOM_BYTES_PER_WRITE, "bytes_per_write");
 
+  // Transaction Histogram
+  ATOM(erocksdb::ATOM_NUM_OP_PER_TRANSACTION, "num_op_per_transaction");
+
   // Compaction Statistics Tickers
   ATOM(erocksdb::ATOM_COMPACT_READ_BYTES, "compact_read_bytes");
   ATOM(erocksdb::ATOM_COMPACT_WRITE_BYTES, "compact_write_bytes");

c_src/statistics.cc

@@ -777,6 +777,12 @@ bool HistogramAtomToEnum(ERL_NIF_TERM atom, rocksdb::Histograms* histogram)
         *histogram = rocksdb::BYTES_PER_WRITE;
         return true;
     }
+    // Transaction Histogram
+    else if (atom == ATOM_NUM_OP_PER_TRANSACTION)
+    {
+        *histogram = rocksdb::NUM_OP_PER_TRANSACTION;
+        return true;
+    }
     return false;
 }
 

src/rocksdb.erl

@@ -554,6 +554,8 @@
       bytes_per_read |
       bytes_per_write.
 
+-type transaction_histogram() :: num_op_per_transaction.
+
 -type compaction_ticker() :: compact_read_bytes |
       compact_write_bytes |
       flush_write_bytes |
@@ -2231,7 +2233,7 @@ statistics_ticker(_Statistics, _Ticker) ->
 %% Returns histogram information including median, percentiles, average, etc.
 %% For integrated BlobDB, relevant histograms are blob_db_blob_file_write_micros,
 %% blob_db_blob_file_read_micros, blob_db_compression_micros, etc.
--spec statistics_histogram(statistics_handle(), blob_db_histogram() | core_operation_histogram() | io_sync_histogram()) -> {ok, histogram_info()}.
+-spec statistics_histogram(statistics_handle(), blob_db_histogram() | core_operation_histogram() | io_sync_histogram() | transaction_histogram()) -> {ok, histogram_info()}.
 statistics_histogram(_Statistics, _Histogram) ->
   ?nif_stub.
 

test/transaction_statistics.erl

@@ -184,3 +184,40 @@ all_transaction_tickers_readable_test() ->
   end,
   ok.
 
+num_op_per_transaction_histogram_test() ->
+  ?rm_rf("test_txn_ops_hist"),
+  {ok, Stats} = rocksdb:new_statistics(),
+  {ok, Db, _} = rocksdb:open_pessimistic_transaction_db(
+    "test_txn_ops_hist",
+    [{create_if_missing, true},
+     {statistics, Stats}],
+    [{"default", []}]),
+  try
+    %% Create a transaction with multiple operations
+    {ok, Txn} = rocksdb:pessimistic_transaction(Db, []),
+    lists:foreach(
+      fun(I) ->
+        Key = <<$k, (integer_to_binary(I))/binary>>,
+        Value = list_to_binary(lists:duplicate(50, $v)),
+        ok = rocksdb:pessimistic_transaction_put(Txn, Key, Value)
+      end,
+      lists:seq(1, 10)),
+    ok = rocksdb:pessimistic_transaction_commit(Txn),
+    ok = rocksdb:release_pessimistic_transaction(Txn),
+
+    %% Check num_op_per_transaction histogram is readable
+    {ok, OpHist} = rocksdb:statistics_histogram(Stats, num_op_per_transaction),
+    ?assert(is_map(OpHist)),
+    ?assert(is_float(maps:get(median, OpHist))),
+    ?assert(is_float(maps:get(percentile95, OpHist))),
+    ?assert(is_float(maps:get(percentile99, OpHist))),
+    ?assert(is_float(maps:get(average, OpHist))),
+    ?assert(is_integer(maps:get(count, OpHist))),
+    ?assert(is_integer(maps:get(sum, OpHist)))
+  after
+    ok = rocksdb:close(Db),
+    ok = rocksdb:release_statistics(Stats),
+    ?rm_rf("test_txn_ops_hist")
+  end,
+  ok.
+