MDEV-28246 Optimizer uses all partitions after upgrade to 10.3

Cause: a copy of the joined TABLE_LIST is created during multi_update::prepare
and TABLE::pos_in_table_list of the tables are set to point to the new
TABLE_LIST object. This prevents some optimization steps to perform correctly.
Solution: do not update pos_in_table_list during multi_update::prepare

mysql-test/main/multi_update.result

@@ -1161,3 +1161,93 @@ ERROR 21000: Subquery returns more than 1 row
 update t1 set a= (select 2 from t1 having (a = 3));
 ERROR 21000: Subquery returns more than 1 row
 drop tables t1;
+#
+# MDEV-28246 Optimizer uses all partitions during an update in MariaDB 10.6.x but not in 10.2.x
+#
+CREATE TABLE t1 (
+part INT(1),
+a INT(1),
+b INT(1),
+PRIMARY KEY (a,part),
+INDEX b (b,part)
+) PARTITION BY LIST (part) (
+PARTITION Current VALUES IN (0),
+PARTITION Relevant VALUES IN (1),
+PARTITION Archive VALUES IN (2)
+);
+CREATE TABLE t2 LIKE t1;
+INSERT INTO t1 (part,a,b) VALUES (0,0,0),(1,1,1),(2,2,2);
+INSERT INTO t2 (part,a,b) VALUES (0,0,0),(1,1,1),(2,2,2);
+# Expecting partition "Current"
+EXPLAIN FORMAT=JSON UPDATE t2 JOIN t1 USING(a) SET t2.part=3 WHERE t2.part=0 AND t1.part=0;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t2",
+      "partitions": ["Current"],
+      "access_type": "system",
+      "possible_keys": ["PRIMARY"],
+      "rows": 1,
+      "filtered": 100
+    },
+    "table": {
+      "table_name": "t1",
+      "partitions": ["Current"],
+      "access_type": "system",
+      "possible_keys": ["PRIMARY"],
+      "rows": 1,
+      "filtered": 100
+    }
+  }
+}
+# Expecting partition "Relevant"
+EXPLAIN FORMAT=JSON UPDATE t2 JOIN t1 USING(a) SET t2.part=2 WHERE t2.part=1 AND t1.part=1;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t2",
+      "partitions": ["Relevant"],
+      "access_type": "system",
+      "possible_keys": ["PRIMARY"],
+      "rows": 1,
+      "filtered": 100
+    },
+    "table": {
+      "table_name": "t1",
+      "partitions": ["Relevant"],
+      "access_type": "system",
+      "possible_keys": ["PRIMARY"],
+      "rows": 1,
+      "filtered": 100
+    }
+  }
+}
+# Expecting partition "Archive"
+EXPLAIN FORMAT=JSON UPDATE t2 JOIN t1 USING(a) SET t2.part=3 WHERE t2.part=2 AND t1.part=2;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t2",
+      "partitions": ["Archive"],
+      "access_type": "system",
+      "possible_keys": ["PRIMARY"],
+      "rows": 1,
+      "filtered": 100
+    },
+    "table": {
+      "table_name": "t1",
+      "partitions": ["Archive"],
+      "access_type": "system",
+      "possible_keys": ["PRIMARY"],
+      "rows": 1,
+      "filtered": 100
+    }
+  }
+}
+DROP TABLES t1, t2;

mysql-test/main/multi_update.test

@@ -1098,3 +1098,34 @@ select a from t1 where a= (select 2 from t1 having (a = 3));
 --error ER_SUBQUERY_NO_1_ROW
 update t1 set a= (select 2 from t1 having (a = 3));
 drop tables t1;
+
+--echo #
+--echo # MDEV-28246 Optimizer uses all partitions during an update in MariaDB 10.6.x but not in 10.2.x
+--echo #
+--source include/have_partition.inc
+CREATE TABLE t1 (
+  part INT(1),
+  a INT(1),
+  b INT(1),
+  PRIMARY KEY (a,part),
+  INDEX b (b,part)
+) PARTITION BY LIST (part) (
+  PARTITION Current VALUES IN (0),
+  PARTITION Relevant VALUES IN (1),
+  PARTITION Archive VALUES IN (2)
+);
+
+CREATE TABLE t2 LIKE t1;
+INSERT INTO t1 (part,a,b) VALUES (0,0,0),(1,1,1),(2,2,2);
+INSERT INTO t2 (part,a,b) VALUES (0,0,0),(1,1,1),(2,2,2);
+
+--echo # Expecting partition "Current"
+EXPLAIN FORMAT=JSON UPDATE t2 JOIN t1 USING(a) SET t2.part=3 WHERE t2.part=0 AND t1.part=0;
+
+--echo # Expecting partition "Relevant"
+EXPLAIN FORMAT=JSON UPDATE t2 JOIN t1 USING(a) SET t2.part=2 WHERE t2.part=1 AND t1.part=1;
+
+--echo # Expecting partition "Archive"
+EXPLAIN FORMAT=JSON UPDATE t2 JOIN t1 USING(a) SET t2.part=3 WHERE t2.part=2 AND t1.part=2;
+
+DROP TABLES t1, t2;

sql/sql_update.cc

@@ -1977,10 +1977,9 @@ int multi_update::prepare(List<Item> &not_used_values,
       if (!tl)
 	DBUG_RETURN(1);
       update.link_in_list(tl, &tl->next_local);
-      tl->shared= table_count++;
+      table_ref->shared= tl->shared= table_count++;
       table->no_keyread=1;
       table->covering_keys.clear_all();
-      table->pos_in_table_list= tl;
       table->prepare_triggers_for_update_stmt_or_event();
       table->reset_default_fields();
     }