Fix memory bloat in my_core::handler

Summary:
In 8.0 sizeof(my_core::handler) == 5680. This is a regression from 5.6
where the size was 912 bytes.
This is because 8.0 introduced m_random_number_engine
field of type std::mt19937 inline. The size of this type is 5000 bytes.
This regressed workloads which started taking > 50 MB more as many
handlers are initialized when running workloads.

m_random_number_engine is only used when generating histogram
samples as part of running "ANALYZE TABLE UPDATE HISTOGRAM".
Since histograms are not yet widely used and the generator is needed
only when updating histogram, convert it into a pointer field. The
object of std::mt19937 is now dynamically created when running
UPDATE HISTOGRAM, This should be ok as histograms will not be
refreshed very often.

Reviewed By: yizhang82

Differential Revision: D27251616

fbshipit-source-id: 1c122177a15

sql/handler.cc

@@ -3231,21 +3231,28 @@ int handler::ha_sample_init(void *&scan_ctx, double sampling_percentage,
   DBUG_ASSERT(sampling_percentage >= 0.0);
   DBUG_ASSERT(sampling_percentage <= 100.0);
   DBUG_ASSERT(inited == NONE);
+  DBUG_ASSERT(m_random_number_engine == nullptr);
 
-  // Initialise the random number generator.
-  m_random_number_engine.seed(sampling_seed);
   m_sampling_percentage = sampling_percentage;
 
   int result = sample_init(scan_ctx, sampling_percentage, sampling_seed,
                            sampling_method);
   inited = (result != 0) ? NONE : SAMPLING;
+
+  // Initialise the random number generator on successful initialization.
+  if (result == 0) {
+    m_random_number_engine = new std::mt19937();
+    m_random_number_engine->seed(sampling_seed);
+  }
   return result;
 }
 
 int handler::ha_sample_end(void *scan_ctx) {
   DBUG_TRACE;
   DBUG_ASSERT(inited == SAMPLING);
   inited = NONE;
+  delete m_random_number_engine;
+  m_random_number_engine = nullptr;
   int result = sample_end(scan_ctx);
   return result;
 }
@@ -3285,7 +3292,8 @@ int handler::sample_next(void *scan_ctx MY_ATTRIBUTE((unused)), uchar *buf) {
   int res = rnd_next(buf);
 
   std::uniform_real_distribution<double> rnd(0.0, 1.0);
-  while (!res && rnd(m_random_number_engine) > (m_sampling_percentage / 100.0))
+  DBUG_ASSERT(m_random_number_engine);
+  while (!res && rnd(*m_random_number_engine) > (m_sampling_percentage / 100.0))
     res = rnd_next(buf);
 
   return res;

sql/handler.h

@@ -4286,7 +4286,7 @@ class handler {
   */
   PSI_table *m_psi;
 
-  std::mt19937 m_random_number_engine;
+  std::mt19937 *m_random_number_engine;
   double m_sampling_percentage;
 
   /* TODO(yzha) - we needed these to be public for MYSQL_TABLE_IO_WAIT */
@@ -4408,6 +4408,7 @@ class handler {
         insert_id_for_cur_row(0),
         auto_inc_intervals_count(0),
         m_psi(nullptr),
+        m_random_number_engine(nullptr),
         m_psi_batch_mode(PSI_BATCH_MODE_NONE),
         m_psi_numrows(0),
         m_psi_locker(nullptr),
@@ -4421,6 +4422,7 @@ class handler {
 
   virtual ~handler(void) {
     DBUG_ASSERT(m_psi == nullptr);
+    DBUG_ASSERT(m_random_number_engine == nullptr);
     DBUG_ASSERT(m_psi_batch_mode == PSI_BATCH_MODE_NONE);
     DBUG_ASSERT(m_psi_locker == nullptr);
     DBUG_ASSERT(m_lock_type == F_UNLCK);