MDEV-4621 select returns null for information_schema.statistics.collation field

information_schema.statistics.collation column needs OPEN_FULL_TABLE,
because it checks index_flags() for HA_READ_ORDER capability.

mysql-test/r/show.result

@@ -40,5 +40,16 @@ nm	varchar(32)	YES		NULL
 a	int(11)	YES		NULL	
 drop table t1;
 #
+# MDEV-4621 select returns null for information_schema.statistics.collation field
+#
+create table t1 (f varchar(64), key(f));
+select index_name, column_name, collation, cardinality from information_schema.STATISTICS where table_schema='test' and table_name='t1';
+index_name	column_name	collation	cardinality
+f	f	A	NULL
+select index_name, column_name, collation from information_schema.STATISTICS where table_schema='test' and table_name='t1';
+index_name	column_name	collation
+f	f	A
+drop table t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/show.test

@@ -34,6 +34,14 @@ show fields from test.t1 where field in
                           where table_name='t1' group by column_name) dt);
 drop table t1;
 
+--echo #
+--echo # MDEV-4621 select returns null for information_schema.statistics.collation field
+--echo #
+create table t1 (f varchar(64), key(f));
+select index_name, column_name, collation, cardinality from information_schema.STATISTICS where table_schema='test' and table_name='t1';
+select index_name, column_name, collation from information_schema.STATISTICS where table_schema='test' and table_name='t1';
+drop table t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/sql_show.cc

@@ -8967,7 +8967,7 @@ ST_FIELD_INFO stat_fields_info[]=
   {"SEQ_IN_INDEX", 2, MYSQL_TYPE_LONGLONG, 0, 0, "Seq_in_index", OPEN_FRM_ONLY},
   {"COLUMN_NAME", NAME_CHAR_LEN, MYSQL_TYPE_STRING, 0, 0, "Column_name",
    OPEN_FRM_ONLY},
-  {"COLLATION", 1, MYSQL_TYPE_STRING, 0, 1, "Collation", OPEN_FRM_ONLY},
+  {"COLLATION", 1, MYSQL_TYPE_STRING, 0, 1, "Collation", OPEN_FULL_TABLE},
   {"CARDINALITY", MY_INT64_NUM_DECIMAL_DIGITS, MYSQL_TYPE_LONGLONG, 0, 1,
    "Cardinality", OPEN_FULL_TABLE},
   {"SUB_PART", 3, MYSQL_TYPE_LONGLONG, 0, 1, "Sub_part", OPEN_FRM_ONLY},