Take into account the number of null values in any used column when

calculating selectivity of conditions.

mysql-test/r/selectivity.result

@@ -6,12 +6,37 @@ select @@session.use_stat_tables;
 COMPLEMENTARY
 set @save_use_stat_tables=@@use_stat_tables;
 set use_stat_tables='preferably';
-DROP DATABASE IF EXISTS dbt3_s001;
-CREATE DATABASE dbt3_s001;
-use dbt3_s001;
 set @save_optimizer_use_condition_selectivity=@@optimizer_use_condition_selectivity;
 set @save_histogram_size=@@histogram_size;
 set @save_histogram_type=@@histogram_type;
+set optimizer_use_condition_selectivity=3;
+create table t1 (a int);
+insert into t1 values
+(9), (3), (2), (NULL), (NULL), (2), (NULL), (1), (5), (NULL);
+analyze table t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+select * from mysql.column_stats;
+db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
+test	t1	a	1	9	0.4000	4.0000	1.2000	0	NULL	NULL
+flush table t1;
+explain extended 
+select * from t1 where a is null;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10	40.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where isnull(`test`.`t1`.`a`)
+explain extended 
+select * from t1 where a is not null;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10	60.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` is not null)
+drop table t1;
+set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
+DROP DATABASE IF EXISTS dbt3_s001;
+CREATE DATABASE dbt3_s001;
+use dbt3_s001;
 === Q15 ===
 create view revenue0 (supplier_no, total_revenue) as
 select l_suppkey, sum(l_extendedprice * (1 - l_discount))

mysql-test/r/selectivity_innodb.result

@@ -9,12 +9,37 @@ select @@session.use_stat_tables;
 COMPLEMENTARY
 set @save_use_stat_tables=@@use_stat_tables;
 set use_stat_tables='preferably';
-DROP DATABASE IF EXISTS dbt3_s001;
-CREATE DATABASE dbt3_s001;
-use dbt3_s001;
 set @save_optimizer_use_condition_selectivity=@@optimizer_use_condition_selectivity;
 set @save_histogram_size=@@histogram_size;
 set @save_histogram_type=@@histogram_type;
+set optimizer_use_condition_selectivity=3;
+create table t1 (a int);
+insert into t1 values
+(9), (3), (2), (NULL), (NULL), (2), (NULL), (1), (5), (NULL);
+analyze table t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+select * from mysql.column_stats;
+db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_length	avg_frequency	hist_size	hist_type	histogram
+test	t1	a	1	9	0.4000	4.0000	1.2000	0	NULL	NULL
+flush table t1;
+explain extended 
+select * from t1 where a is null;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10	40.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where isnull(`test`.`t1`.`a`)
+explain extended 
+select * from t1 where a is not null;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10	60.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` is not null)
+drop table t1;
+set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
+DROP DATABASE IF EXISTS dbt3_s001;
+CREATE DATABASE dbt3_s001;
+use dbt3_s001;
 === Q15 ===
 create view revenue0 (supplier_no, total_revenue) as
 select l_suppkey, sum(l_extendedprice * (1 - l_discount))

mysql-test/t/selectivity.test

@@ -7,6 +7,34 @@ set @save_use_stat_tables=@@use_stat_tables;
 
 set use_stat_tables='preferably';
 
+set @save_optimizer_use_condition_selectivity=@@optimizer_use_condition_selectivity;
+set @save_histogram_size=@@histogram_size;
+set @save_histogram_type=@@histogram_type;
+
+# check that statistics on nulls is used
+
+set optimizer_use_condition_selectivity=3;
+
+create table t1 (a int);
+insert into t1 values
+  (9), (3), (2), (NULL), (NULL), (2), (NULL), (1), (5), (NULL);
+
+analyze table t1;
+
+select * from mysql.column_stats;
+
+flush table t1;
+
+explain extended 
+select * from t1 where a is null;
+
+explain extended 
+select * from t1 where a is not null;
+
+drop table t1;
+
+set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
+
 --disable_warnings
 DROP DATABASE IF EXISTS dbt3_s001;
 --enable_warnings
@@ -15,10 +43,6 @@ CREATE DATABASE dbt3_s001;
 
 use dbt3_s001;
 
-set @save_optimizer_use_condition_selectivity=@@optimizer_use_condition_selectivity;
-set @save_histogram_size=@@histogram_size;
-set @save_histogram_type=@@histogram_type;
-
 --disable_query_log
 --disable_result_log
 --disable_warnings

sql/sql_statistics.cc

@@ -3342,11 +3342,26 @@ double get_column_range_cardinality(Field *field,
   double res;
   TABLE *table= field->table;
   Column_statistics *col_stats= table->field[field->field_index]->read_stats;
+  double tab_records= table->stat_records();
 
   if (!col_stats)
-    res= table->stat_records();
+    return tab_records;
+
+  double col_nulls= tab_records * col_stats->get_nulls_ratio();
+
+  double col_non_nulls= tab_records - col_nulls;
+
+  if (col_non_nulls < 1)
+    res= 0;
   else if (min_endp && max_endp && min_endp->length == max_endp->length &&
            !memcmp(min_endp->key, max_endp->key, min_endp->length))
+  { 
+    if (field->null_ptr && min_endp->key[0])
+    {
+      /* This is null single point range */
+      res= col_nulls;
+    }
+    else
     {
       double avg_frequency= col_stats->get_avg_frequency();
       res= avg_frequency;   
@@ -3358,9 +3373,10 @@ double get_column_range_cardinality(Field *field,
         {
           double pos= field->middle_point_pos(col_stats->min_value,
                                               col_stats->max_value);
-        res= table->stat_records() * 
+          res= col_non_nulls * 
 	       hist->point_selectivity(pos,
-                                     avg_frequency / table->stat_records());
+                                       avg_frequency / col_non_nulls);
+        }
       }
     }
   }  
@@ -3394,10 +3410,10 @@ double get_column_range_cardinality(Field *field,
         sel= (max_mp_pos - min_mp_pos);
       else
         sel= hist->range_selectivity(min_mp_pos, max_mp_pos);
-      res= table->stat_records() * sel;
+      res= col_non_nulls * sel;
     }
     else
-      res= table->stat_records();
+      res= col_non_nulls;
   }
   return res;
 }