[FEAT MERGE] implement dynamic sampling

src/sql/optimizer/ob_opt_default_stat.h

@@ -23,6 +23,8 @@ const double LN_2 = 0.69314718055994530941723212145818;
 
 const double DEFAULT_CACHE_HIT_RATE = 0.8;
 
+const int64_t DEFAULT_TABLE_ROW_COUNT = 1;
+
 const int64_t DEFAULT_ROW_SIZE = 200;
 
 const int64_t DEFAULT_COLUMN_SIZE = 10;