MDEV-19574 innodb_stats_method is not honored when innodb_stats_persistent=ON

Problem:
=======
 InnoDB persistent statistics doesn't take innodb_stats_method
variable while calculating n_diff_pfx for the n-prefix index
columns.

Solution:
=========
While calculating the persistent statistics, InnoDB consider all
NULLS as different value when innodb_stats_method is
set to NULLS_UNEQUAL or NULLS_IGNORED. InnoDB statistics depends
on the value of "innodb_stats_method" variable when the
statistics were last recalculated.

dict_stats_analyze_index_level(),
dict_stats_scan_page(),
dict_stats_analyze_index_below_cur(),
dict_stats_analyze_index_for_n_prefix(): All function accepts
the parameter to indicate the innodb_stats_method. It also
calculates the number of non-null key value for each level or leaf page

dict_stats_index_set_n_diff(): calculates the number of non-null
values similar to number of different key values.

Append the non-default innodb_stats_method variable name
for n_diff_pfx statistics description in mysql.innodb_index_stats.

IndexScanStatistics: Structure to store the various statistical
metric gathered during index scan fetch operation. This structure
is being used in dict_stats_analyze_index_level() to fetch the
statistics

Author: Thirunarayanan

mysql-test/suite/innodb/r/innodb_stats_method,NULLS_EQUAL.rdiff

@@ -0,0 +1,31 @@
+--- innodb_stats_method.result
++++ innodb_stats_method,NULLS_EQUAL.result
+@@ -7,21 +7,21 @@
+ SELECT stat_name, stat_value, stat_description
+ FROM mysql.innodb_index_stats WHERE database_name="test" and table_name="t1";
+ stat_name	stat_value	stat_description
+-n_diff_pfx01	16341	DB_ROW_ID NULLS_UNEQUAL
++n_diff_pfx01	16341	DB_ROW_ID
+ n_leaf_pages	37	Number of leaf pages in the index
+ size	97	Number of pages in the index
+-n_diff_pfx01	16384	f1 NULLS_UNEQUAL
+-n_diff_pfx02	16384	f1,f3 NULLS_UNEQUAL
+-n_diff_pfx03	16384	f1,f3,DB_ROW_ID NULLS_UNEQUAL
++n_diff_pfx01	16384	f1
++n_diff_pfx02	16384	f1,f3
++n_diff_pfx03	16384	f1,f3,DB_ROW_ID
+ n_leaf_pages	1	Number of leaf pages in the index
+ size	19	Number of pages in the index
+-n_diff_pfx01	16384	f3 NULLS_UNEQUAL
+-n_diff_pfx02	16384	f3,DB_ROW_ID NULLS_UNEQUAL
++n_diff_pfx01	1	f3
++n_diff_pfx02	16384	f3,DB_ROW_ID
+ n_leaf_pages	1	Number of leaf pages in the index
+ size	15	Number of pages in the index
+ SHOW KEYS FROM t1;
+ Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment	Ignored
+ t1	1	f1	1	f1	A	16341	NULL	NULL		BTREE			NO
+ t1	1	f1	2	f3	A	16341	NULL	NULL	YES	BTREE			NO
+-t1	1	f3	1	f3	A	16341	NULL	NULL	YES	BTREE			NO
++t1	1	f3	1	f3	A	2	NULL	NULL	YES	BTREE			NO
+ DROP TABLE t1;

mysql-test/suite/innodb/r/innodb_stats_method,NULLS_IGNORED.rdiff

@@ -0,0 +1,25 @@
+--- innodb_stats_method.result
++++ innodb_stats_method,NULLS_IGNORED.result
+@@ -7,16 +7,16 @@
+ SELECT stat_name, stat_value, stat_description
+ FROM mysql.innodb_index_stats WHERE database_name="test" and table_name="t1";
+ stat_name	stat_value	stat_description
+-n_diff_pfx01	16341	DB_ROW_ID NULLS_UNEQUAL
++n_diff_pfx01	16341	DB_ROW_ID NULLS_IGNORED
+ n_leaf_pages	37	Number of leaf pages in the index
+ size	97	Number of pages in the index
+-n_diff_pfx01	16384	f1 NULLS_UNEQUAL
+-n_diff_pfx02	16384	f1,f3 NULLS_UNEQUAL
+-n_diff_pfx03	16384	f1,f3,DB_ROW_ID NULLS_UNEQUAL
++n_diff_pfx01	16384	f1 NULLS_IGNORED
++n_diff_pfx02	16384	f1,f3 NULLS_IGNORED
++n_diff_pfx03	16384	f1,f3,DB_ROW_ID NULLS_IGNORED
+ n_leaf_pages	1	Number of leaf pages in the index
+ size	19	Number of pages in the index
+-n_diff_pfx01	16384	f3 NULLS_UNEQUAL
+-n_diff_pfx02	16384	f3,DB_ROW_ID NULLS_UNEQUAL
++n_diff_pfx01	16384	f3 NULLS_IGNORED
++n_diff_pfx02	16384	f3,DB_ROW_ID NULLS_IGNORED
+ n_leaf_pages	1	Number of leaf pages in the index
+ size	15	Number of pages in the index
+ SHOW KEYS FROM t1;

mysql-test/suite/innodb/r/innodb_stats_method.result

@@ -0,0 +1,27 @@
+CREATE TABLE t1(f1 INT NOT NULL, f2 INT NOT NULL,
+f3 INT, KEY(f1, f3), key(f3)) STATS_PERSISTENT=1,ENGINE=INNODB;
+INSERT INTO t1 SELECT seq, seq, NULL from seq_1_to_16384;
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+SELECT stat_name, stat_value, stat_description
+FROM mysql.innodb_index_stats WHERE database_name="test" and table_name="t1";
+stat_name	stat_value	stat_description
+n_diff_pfx01	16341	DB_ROW_ID NULLS_UNEQUAL
+n_leaf_pages	37	Number of leaf pages in the index
+size	97	Number of pages in the index
+n_diff_pfx01	16384	f1 NULLS_UNEQUAL
+n_diff_pfx02	16384	f1,f3 NULLS_UNEQUAL
+n_diff_pfx03	16384	f1,f3,DB_ROW_ID NULLS_UNEQUAL
+n_leaf_pages	1	Number of leaf pages in the index
+size	19	Number of pages in the index
+n_diff_pfx01	16384	f3 NULLS_UNEQUAL
+n_diff_pfx02	16384	f3,DB_ROW_ID NULLS_UNEQUAL
+n_leaf_pages	1	Number of leaf pages in the index
+size	15	Number of pages in the index
+SHOW KEYS FROM t1;
+Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment	Ignored
+t1	1	f1	1	f1	A	16341	NULL	NULL		BTREE			NO
+t1	1	f1	2	f3	A	16341	NULL	NULL	YES	BTREE			NO
+t1	1	f3	1	f3	A	16341	NULL	NULL	YES	BTREE			NO
+DROP TABLE t1;

mysql-test/suite/innodb/t/innodb_stats_method.combinations

@@ -0,0 +1,11 @@
+[NULLS_EQUAL]
+--innodb_stats_method=nulls_equal
+--use_stat_tables=never
+
+[NULLS_UNEQUAL]
+--innodb_stats_method=nulls_unequal
+--use_stat_tables=never
+
+[NULLS_IGNORED]
+--innodb_stats_method=nulls_ignored
+--use_stat_tables=never

mysql-test/suite/innodb/t/innodb_stats_method.test

@@ -0,0 +1,11 @@
+--source include/have_innodb.inc
+--source include/have_sequence.inc
+
+CREATE TABLE t1(f1 INT NOT NULL, f2 INT NOT NULL,
+                f3 INT, KEY(f1, f3), key(f3)) STATS_PERSISTENT=1,ENGINE=INNODB;
+INSERT INTO t1 SELECT seq, seq, NULL from seq_1_to_16384;
+ANALYZE TABLE t1;
+SELECT stat_name, stat_value, stat_description
+FROM mysql.innodb_index_stats WHERE database_name="test" and table_name="t1";
+SHOW KEYS FROM t1;
+DROP TABLE t1;