MDEV-3818: Query against view over IS tables worse than equivalent query without view

Analysis:
The reason for the suboptimal plan when querying IS tables through a view
was that the view columns that participate in an equality are wrapped by
an Item_direct_view_ref and were not recognized as being direct column
references.

Solution:
Use the original Item_field objects via the real_item() method.

mysql-test/r/information_schema.result

@@ -1934,6 +1934,26 @@ event_object_table	trigger_name
 # Switching to connection 'default'.
 #
 #
+# MDEV-3818: Query against view over IS tables worse than equivalent query without view
+#
+CREATE VIEW v1 AS SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS;
+explain extended
+SELECT column_name FROM v1
+WHERE (TABLE_SCHEMA = "osm") AND (TABLE_NAME = "test");
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	COLUMNS	ALL	NULL	TABLE_SCHEMA,TABLE_NAME	NULL	NULL	NULL	NULL	Using where; Open_frm_only; Scanned 0 databases
+Warnings:
+Note	1003	select `information_schema`.`COLUMNS`.`COLUMN_NAME` AS `COLUMN_NAME` from `INFORMATION_SCHEMA`.`COLUMNS` where ((`information_schema`.`COLUMNS`.`TABLE_SCHEMA` = 'osm') and (`information_schema`.`COLUMNS`.`TABLE_NAME` = 'test'))
+explain extended
+SELECT INFORMATION_SCHEMA.COLUMNS.COLUMN_NAME AS COLUMN_NAME
+FROM INFORMATION_SCHEMA.COLUMNS
+WHERE (information_schema.COLUMNS.TABLE_SCHEMA = 'osm') and (information_schema.COLUMNS.TABLE_NAME = 'test');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	COLUMNS	ALL	NULL	TABLE_SCHEMA,TABLE_NAME	NULL	NULL	NULL	NULL	Using where; Open_frm_only; Scanned 0 databases
+Warnings:
+Note	1003	select `information_schema`.`COLUMNS`.`COLUMN_NAME` AS `COLUMN_NAME` from `INFORMATION_SCHEMA`.`COLUMNS` where ((`information_schema`.`COLUMNS`.`TABLE_SCHEMA` = 'osm') and (`information_schema`.`COLUMNS`.`TABLE_NAME` = 'test'))
+drop view v1;
+#
 # Clean-up.
 drop database mysqltest;
 #

mysql-test/t/information_schema.test

@@ -1789,6 +1789,24 @@ disconnect con12828477_1;
 disconnect con12828477_2;
 disconnect con12828477_3;
 
+
+--echo #
+--echo # MDEV-3818: Query against view over IS tables worse than equivalent query without view
+--echo #
+
+CREATE VIEW v1 AS SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS;
+
+explain extended
+SELECT column_name FROM v1
+WHERE (TABLE_SCHEMA = "osm") AND (TABLE_NAME = "test");
+
+explain extended
+SELECT INFORMATION_SCHEMA.COLUMNS.COLUMN_NAME AS COLUMN_NAME
+FROM INFORMATION_SCHEMA.COLUMNS
+WHERE (information_schema.COLUMNS.TABLE_SCHEMA = 'osm') and (information_schema.COLUMNS.TABLE_NAME = 'test');
+
+drop view v1;
+
 --echo #
 --echo # Clean-up.
 drop database mysqltest;

sql/sql_show.cc

@@ -3184,13 +3184,13 @@ bool get_lookup_value(THD *thd, Item_func *item_func,
     Item_field *item_field;
     CHARSET_INFO *cs= system_charset_info;
 
-    if (item_func->arguments()[0]->type() == Item::FIELD_ITEM &&
+    if (item_func->arguments()[0]->real_item()->type() == Item::FIELD_ITEM &&
         item_func->arguments()[1]->const_item())
     {
       idx_field= 0;
       idx_val= 1;
     }
-    else if (item_func->arguments()[1]->type() == Item::FIELD_ITEM &&
+    else if (item_func->arguments()[1]->real_item()->type() == Item::FIELD_ITEM &&
              item_func->arguments()[0]->const_item())
     {
       idx_field= 1;
@@ -3199,7 +3199,7 @@ bool get_lookup_value(THD *thd, Item_func *item_func,
     else
       return 0;
 
-    item_field= (Item_field*) item_func->arguments()[idx_field];
+    item_field= (Item_field*) item_func->arguments()[idx_field]->real_item();
     if (table->table != item_field->field->table)
       return 0;
     tmp_str= item_func->arguments()[idx_val]->val_str(&str_buff);