Bug#27637968 PROVIDE A DEFAULT NUMBER OF BUCKETS FOR HISTOGRAM STATISTICS

Problem: If a user wants to create histogram statistics, he/she MUST
specify the maximum number of buckets to be created. However, the server
should provide a default value since it's difficult for users to know
what a reasonable number of buckets is.

Fix: Make the clause [WITH n BUCKETS] optional. If it isn't specified,
a default value of 100 will be used.

Change-Id: I6e00adbb37ebd412b2908ad731daa27713171116

Author: Erik Froseth

mysql-test/r/histograms.result

@@ -3112,3 +3112,18 @@ should_be_true
 SET cte_max_recursion_depth = DEFAULT;
 SET histogram_generation_max_mem_size = DEFAULT;
 DROP TABLE t1;
+#
+# Bug#27637968 PROVIDE A DEFAULT NUMBER OF BUCKETS FOR HISTOGRAM
+#              STATISTICS
+#
+CREATE TABLE t1 (col1 INT);
+INSERT INTO t1 VALUES (10), (20), (30);
+ANALYZE TABLE t1 UPDATE HISTOGRAM ON col1;
+Table	Op	Msg_type	Msg_text
+test.t1	histogram	status	Histogram statistics created for column 'col1'.
+SELECT JSON_EXTRACT(histogram, '$."number-of-buckets-specified"')
+FROM INFORMATION_SCHEMA.column_statistics
+WHERE table_name = "t1" AND column_name = "col1";
+JSON_EXTRACT(histogram, '$."number-of-buckets-specified"')
+100
+DROP TABLE t1;

mysql-test/t/histograms.test

@@ -1932,3 +1932,16 @@ SET cte_max_recursion_depth = DEFAULT;
 SET histogram_generation_max_mem_size = DEFAULT;
 
 DROP TABLE t1;
+
+--echo #
+--echo # Bug#27637968 PROVIDE A DEFAULT NUMBER OF BUCKETS FOR HISTOGRAM
+--echo #              STATISTICS
+--echo #
+CREATE TABLE t1 (col1 INT);
+INSERT INTO t1 VALUES (10), (20), (30);
+ANALYZE TABLE t1 UPDATE HISTOGRAM ON col1;
+
+SELECT JSON_EXTRACT(histogram, '$."number-of-buckets-specified"')
+FROM INFORMATION_SCHEMA.column_statistics
+WHERE table_name = "t1" AND column_name = "col1";
+DROP TABLE t1;

sql/sql_yacc.yy

@@ -120,6 +120,11 @@ using std::max;
 /// The maximum number of histogram buckets.
 static const int MAX_NUMBER_OF_HISTOGRAM_BUCKETS= 1024;
 
+/// The default number of histogram buckets when the user does not specify it
+/// explicitly. A value of 100 is chosen because the gain in accuracy above this
+/// point seems to be generally low.
+static const int DEFAULT_NUMBER_OF_HISTOGRAM_BUCKETS= 100;
+
 int yylex(void *yylval, void *yythd);
 
 #define yyoverflow(A,B,C,D,E,F,G,H)           \
@@ -1294,6 +1299,7 @@ void warn_about_deprecated_national(THD *thd)
         trg_action_time trg_event
         view_check_option
         signed_num
+        opt_num_buckets
 
 
 %type <order_direction> order_dir
@@ -8262,21 +8268,12 @@ analyze_table_stmt:
           }
         ;
 
-opt_histogram:
-          /* empty */
+opt_num_buckets:
+          /* empty */ { $$= DEFAULT_NUMBER_OF_HISTOGRAM_BUCKETS; }
+        | WITH NUM BUCKETS_SYM
           {
-            $$.command= Sql_cmd_analyze_table::Histogram_command::NONE;
-            $$.columns= nullptr;
-            $$.num_buckets= 0;
-          }
-        | UPDATE_SYM HISTOGRAM_SYM ON_SYM ident_string_list WITH NUM BUCKETS_SYM
-          {
-            $$.command=
-              Sql_cmd_analyze_table::Histogram_command::UPDATE_HISTOGRAM;
-            $$.columns= $4;
-
             int error;
-            longlong num= my_strtoll10($6.str, nullptr, &error);
+            longlong num= my_strtoll10($2.str, nullptr, &error);
             MYSQL_YYABORT_UNLESS(error <= 0);
 
             if (num < 1 || num > MAX_NUMBER_OF_HISTOGRAM_BUCKETS)
@@ -8286,7 +8283,23 @@ opt_histogram:
               MYSQL_YYABORT;
             }
 
-            $$.num_buckets= static_cast<int>(num);
+            $$= num;
+          }
+        ;
+
+opt_histogram:
+          /* empty */
+          {
+            $$.command= Sql_cmd_analyze_table::Histogram_command::NONE;
+            $$.columns= nullptr;
+            $$.num_buckets= 0;
+          }
+        | UPDATE_SYM HISTOGRAM_SYM ON_SYM ident_string_list opt_num_buckets
+          {
+            $$.command=
+              Sql_cmd_analyze_table::Histogram_command::UPDATE_HISTOGRAM;
+            $$.columns= $4;
+            $$.num_buckets= $5;
           }
         | DROP HISTOGRAM_SYM ON_SYM ident_string_list
           {