Fix for BUG#17523: natural join and information schema.

The cause of the bug was an ASSERT that checked the consistency
of TABLE_SHARE::db and TABLE_LIST::db and failed for I_S tables.
The fix relaxes the requirement for consistency for I_S.

mysql-test/r/join.result

@@ -677,6 +677,11 @@ select t1.b from v1a;
 ERROR 42S22: Unknown column 't1.b' in 'field list'
 select * from v1a join v1b on t1.b = t2.b;
 ERROR 42S22: Unknown column 't1.b' in 'on clause'
+select * from information_schema.statistics join information_schema.columns
+using(table_name,column_name) where table_name='user';
+TABLE_NAME	COLUMN_NAME	TABLE_CATALOG	TABLE_SCHEMA	NON_UNIQUE	INDEX_SCHEMA	INDEX_NAME	SEQ_IN_INDEX	COLLATION	CARDINALITY	SUB_PART	PACKED	NULLABLE	INDEX_TYPE	COMMENT	TABLE_CATALOG	TABLE_SCHEMA	ORDINAL_POSITION	COLUMN_DEFAULT	IS_NULLABLE	DATA_TYPE	CHARACTER_MAXIMUM_LENGTH	CHARACTER_OCTET_LENGTH	NUMERIC_PRECISION	NUMERIC_SCALE	CHARACTER_SET_NAME	COLLATION_NAME	COLUMN_TYPE	COLUMN_KEY	EXTRA	PRIVILEGES	COLUMN_COMMENT
+user	Host	NULL	mysql	0	mysql	PRIMARY	1	A	NULL	NULL	NULL		BTREE		NULL	mysql	1		NO	char	20	60	NULL	NULL	utf8	utf8_bin	char(60)	PRI		select,insert,update,references	
+user	User	NULL	mysql	0	mysql	PRIMARY	2	A	5	NULL	NULL		BTREE		NULL	mysql	2		NO	char	5	16	NULL	NULL	utf8	utf8_bin	char(16)	PRI		select,insert,update,references	
 drop table t1;
 drop table t2;
 drop table t3;

mysql-test/t/join.test

@@ -523,6 +523,12 @@ select t1.b from v1a;
 -- error 1054
 select * from v1a join v1b on t1.b = t2.b;
 
+#
+# Bug #17523 natural join and information_schema
+#
+select * from information_schema.statistics join information_schema.columns
+              using(table_name,column_name) where table_name='user';
+
 drop table t1;
 drop table t2;
 drop table t3;

sql/table.cc

@@ -2595,8 +2595,15 @@ const char *Natural_join_column::db_name()
   if (view_field)
     return table_ref->view_db.str;
 
+  /*
+    Test that TABLE_LIST::db is the same as st_table_share::db to
+    ensure consistency. An exception are I_S schema tables, which
+    are inconsistent in this respect.
+  */
   DBUG_ASSERT(!strcmp(table_ref->db,
-                      table_ref->table->s->db));
+                      table_ref->table->s->db) ||
+              (table_ref->schema_table &&
+               table_ref->table->s->db[0] == 0));
   return table_ref->db;
 }
 
@@ -2798,7 +2805,15 @@ const char *Field_iterator_table_ref::db_name()
   else if (table_ref->is_natural_join)
     return natural_join_it.column_ref()->db_name();
 
-  DBUG_ASSERT(!strcmp(table_ref->db, table_ref->table->s->db));
+  /*
+    Test that TABLE_LIST::db is the same as st_table_share::db to
+    ensure consistency. An exception are I_S schema tables, which
+    are inconsistent in this respect.
+  */
+  DBUG_ASSERT(!strcmp(table_ref->db, table_ref->table->s->db) ||
+              (table_ref->schema_table &&
+               table_ref->table->s->db[0] == 0));
+
   return table_ref->db;
 }