add json statistics test and change histogram column type to blob

mysql-test/main/statistics_json.test

+--source include/have_stat_tables.inc
+--echo #
+--echo # Test that JSON is a valid histogram type and we can store JSON strings in mysql.column_stats
+--echo #
+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+
+set @save_histogram_type=@@histogram_type;
+
+CREATE TABLE t1 (
+  a int NOT NULL PRIMARY KEY,
+  b varchar(32)
+) ENGINE=MYISAM;
+
+SET histogram_type='JSON';
+SELECT @@histogram_type;
+
+INSERT INTO t1 VALUES
+  (7, 'xxxxxxxxxxxxxxxxxxxxxxxxxx'),
+  (17, 'vvvvvvvvvvvvv');
+
+ANALYZE TABLE t1 PERSISTENT FOR COLUMNS(b) INDEXES();
+DESCRIBE mysql.column_stats;
+SELECT * FROM mysql.column_stats;
+
+set histogram_type=@save_histogram_type;
+DROP TABLE t1;
\ No newline at end of file

scripts/mysql_system_tables.sql

@@ -314,7 +314,7 @@ DROP TABLE tmp_proxies_priv;
 
 CREATE TABLE IF NOT EXISTS table_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, cardinality bigint(21) unsigned DEFAULT NULL, PRIMARY KEY (db_name,table_name) ) engine=Aria transactional=0 CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Tables';
 
-CREATE TABLE IF NOT EXISTS column_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, column_name varchar(64) NOT NULL, min_value varbinary(255) DEFAULT NULL, max_value varbinary(255) DEFAULT NULL, nulls_ratio decimal(12,4) DEFAULT NULL, avg_length decimal(12,4) DEFAULT NULL, avg_frequency decimal(12,4) DEFAULT NULL, hist_size tinyint unsigned, hist_type enum('SINGLE_PREC_HB','DOUBLE_PREC_HB','JSON'), histogram varbinary(255), PRIMARY KEY (db_name,table_name,column_name) ) engine=Aria transactional=0 CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Columns';
+CREATE TABLE IF NOT EXISTS column_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, column_name varchar(64) NOT NULL, min_value varbinary(255) DEFAULT NULL, max_value varbinary(255) DEFAULT NULL, nulls_ratio decimal(12,4) DEFAULT NULL, avg_length decimal(12,4) DEFAULT NULL, avg_frequency decimal(12,4) DEFAULT NULL, hist_size tinyint unsigned, hist_type enum('SINGLE_PREC_HB','DOUBLE_PREC_HB','JSON'), histogram blob, PRIMARY KEY (db_name,table_name,column_name) ) engine=Aria transactional=0 CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Columns';
 
 CREATE TABLE IF NOT EXISTS index_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, index_name varchar(64) NOT NULL, prefix_arity int(11) unsigned NOT NULL, avg_frequency decimal(12,4) DEFAULT NULL, PRIMARY KEY (db_name,table_name,index_name,prefix_arity) ) engine=Aria transactional=0 CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Indexes';
 

sql/sql_statistics.cc

@@ -183,7 +183,7 @@ TABLE_FIELD_TYPE column_stat_fields[COLUMN_STAT_N_FIELDS] =
   },
   {
     { STRING_WITH_LEN("histogram") },
-    { STRING_WITH_LEN("varbinary(255)") },
+    { STRING_WITH_LEN("blob") },
     { NULL, 0 }
   }
 };
@@ -1071,7 +1071,8 @@ class Column_stat: public Stat_table
           break;
         case COLUMN_STAT_HISTOGRAM:
           if (stats->histogram.get_type() == JSON) {
-            stat_field->store((char *) "hello_world", 11, &my_charset_bin);
+            const char* val = "{'hello': 'world'}";
+            stat_field->store(val, strlen(val), &my_charset_bin);
           } else {
             stat_field->store((char *) stats->histogram.get_values(),
                               stats->histogram.get_size(), &my_charset_bin);