MDEV-19139: pushdown condition with Item_func_set_user_var

The bug occurs because Item_func_set_user var is allowed to be pushed
into materialized derived table/view.
To fix it excl_dep_on_table() as added to Item_func_set_user_var class
to prevent pushdown.

mysql-test/r/derived_cond_pushdown.result

@@ -10501,4 +10501,53 @@ a	b	max_d	c
 1	2	3	1
 5	6	1	5
 DROP TABLE t1,t2;
+#
+# MDEV-19139: pushdown condition with Item_func_set_user_var
+#
+CREATE TABLE t1 (a INT, b INT);
+CREATE VIEW v1 AS SELECT a, MAX(b) FROM t1 GROUP BY a;
+SELECT * FROM (SELECT 1 FROM v1 UNION (SELECT 1 FROM v1 WHERE @a := uuid())) dt;
+1
+EXPLAIN FORMAT=JSON
+SELECT * FROM (SELECT 1 FROM v1 UNION (SELECT 1 FROM v1 WHERE @a := uuid())) dt;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<derived2>",
+      "access_type": "ALL",
+      "rows": 2,
+      "filtered": 100,
+      "materialized": {
+        "query_block": {
+          "union_result": {
+            "table_name": "<union2,3>",
+            "access_type": "ALL",
+            "query_specifications": [
+              {
+                "query_block": {
+                  "select_id": 2,
+                  "table": {
+                    "message": "no matching row in const table"
+                  }
+                }
+              },
+              {
+                "query_block": {
+                  "select_id": 3,
+                  "table": {
+                    "message": "no matching row in const table"
+                  }
+                }
+              }
+            ]
+          }
+        }
+      }
+    }
+  }
+}
+DROP TABLE t1;
+DROP VIEW v1;
 # End of 10.2 tests

mysql-test/t/derived_cond_pushdown.test

@@ -2124,4 +2124,18 @@ WHERE t1.a=tab.c AND
 
 DROP TABLE t1,t2;
 
+--echo #
+--echo # MDEV-19139: pushdown condition with Item_func_set_user_var
+--echo #
+
+CREATE TABLE t1 (a INT, b INT);
+CREATE VIEW v1 AS SELECT a, MAX(b) FROM t1 GROUP BY a;
+
+SELECT * FROM (SELECT 1 FROM v1 UNION (SELECT 1 FROM v1 WHERE @a := uuid())) dt;
+EXPLAIN FORMAT=JSON
+SELECT * FROM (SELECT 1 FROM v1 UNION (SELECT 1 FROM v1 WHERE @a := uuid())) dt;
+
+DROP TABLE t1;
+DROP VIEW v1;
+
 --echo # End of 10.2 tests

sql/item_func.h

@@ -1992,6 +1992,7 @@ class Item_func_set_user_var :public Item_func_user_var
   void cleanup();
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_func_set_user_var>(thd, mem_root, this); }
+  bool excl_dep_on_table(table_map tab_map) { return false; }
 };