Bug#32855925 SELECT #2 WILL BE DISPLAYED TWICE WHEN WE EXPLAIN THE SQL

If the argument to a window function contains a subquery, the access path
of that subquery would be printed twice when doing 'EXPLAIN FORMAT=TREE'.
When using the Hypergraph optimizer, the subquery path was not printed at
all, whether using FORMAT=TREE or FORMAT=JSON.

This commit fixes this by ensuring that we ignore duplicate paths,
and (for Hypergraph) by traversing the structures needed to find the
relevant Item_subselect objects.

Change-Id: I2abedcf690294f98ce169b74e53f042f46c47a45

Author: Jan Wedvik

mysql-test/r/explain_json_hypergraph.result

@@ -963,3 +963,73 @@ EXPLAIN
 Warnings:
 Note	1276	Field or reference 'test.x1.a' of SELECT #2 was resolved in SELECT #1
 DROP TABLE t1;
+#
+# Bug#34569685 No explain output for subquery
+#
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+ANALYZE TABLE  t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+EXPLAIN FORMAT=TREE SELECT LAST_VALUE((SELECT x1.a FROM t1))
+OVER (PARTITION BY b) FROM t1 x1;
+EXPLAIN
+-> Window aggregate with buffering: last_value(`(select #2)`) OVER (PARTITION BY x1.b )   (...)
+    -> Sort: x1.b  (...)
+        -> Table scan on x1  (...)
+    -> Select #2 (subquery in projection; dependent)
+        -> Table scan on t1  (...)
+
+Warnings:
+Note	1276	Field or reference 'test.x1.a' of SELECT #2 was resolved in SELECT #1
+EXPLAIN FORMAT=JSON SELECT LAST_VALUE((SELECT x1.a FROM t1))
+OVER (PARTITION BY b) FROM t1 x1;
+EXPLAIN
+{
+  "query": "/* select#1 */ select last_value(`(select #2)`) OVER (PARTITION BY `test`.`x1`.`b` )  AS `LAST_VALUE((SELECT x1.a FROM t1))\nOVER (PARTITION BY b)` from `test`.`t1` `x1`",
+  "inputs": [
+    {
+      "inputs": [
+        {
+          "operation": "Table scan on x1",
+          "table_name": "x1",
+          "access_type": "table",
+          "estimated_rows": 1.0,
+          "estimated_total_cost": 0.25,
+          "estimated_first_row_cost": 0.25
+        }
+      ],
+      "operation": "Sort: x1.b",
+      "access_type": "sort",
+      "sort_fields": [
+        "x1.b"
+      ],
+      "estimated_rows": 1.0,
+      "estimated_total_cost": 0.35,
+      "estimated_first_row_cost": 0.35
+    },
+    {
+      "heading": "Select #2 (subquery in projection; dependent)",
+      "subquery": true,
+      "dependent": true,
+      "operation": "Table scan on t1",
+      "table_name": "t1",
+      "access_type": "table",
+      "estimated_rows": 1.0,
+      "subquery_location": "projection",
+      "estimated_total_cost": 0.25,
+      "estimated_first_row_cost": 0.25
+    }
+  ],
+  "buffering": true,
+  "functions": [
+    "last_value(`(select #2)`) OVER (PARTITION BY x1.b ) "
+  ],
+  "operation": "Window aggregate with buffering: last_value(`(select #2)`) OVER (PARTITION BY x1.b ) ",
+  "access_type": "window",
+  "estimated_rows": 1.0,
+  "estimated_total_cost": 0.44999999999999996,
+  "estimated_first_row_cost": 0.44999999999999996
+}
+Warnings:
+Note	1276	Field or reference 'test.x1.a' of SELECT #2 was resolved in SELECT #1
+DROP TABLE t1;

mysql-test/r/explain_tree.result

@@ -1299,3 +1299,22 @@ EXPLAIN
 Warnings:
 Note	1276	Field or reference 'test.x1.a' of SELECT #2 was resolved in SELECT #1
 DROP TABLE t1;
+#
+# Bug#34569685 No explain output for subquery
+#
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+ANALYZE TABLE  t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+EXPLAIN FORMAT=TREE SELECT LAST_VALUE((SELECT x1.a FROM t1))
+OVER (PARTITION BY b) FROM t1 x1;
+EXPLAIN
+-> Window aggregate with buffering: last_value((select #2)) OVER (PARTITION BY x1.b ) 
+    -> Sort: x1.b  (...)
+        -> Table scan on x1  (...)
+    -> Select #2 (subquery in projection; dependent)
+        -> Covering index scan on t1 using PRIMARY  (...)
+
+Warnings:
+Note	1276	Field or reference 'test.x1.a' of SELECT #2 was resolved in SELECT #1
+DROP TABLE t1;

mysql-test/r/explain_tree_hypergraph.result

@@ -97,9 +97,6 @@ EXPLAIN
     -> Select #2 (subquery in condition; uncacheable)
         -> Aggregate: max((x2.a + rand(0)))  (cost=1.25..1.25 rows=1)
             -> Table scan on x2  (cost=0.025..0.25 rows=10)
--> Select #2 (subquery in projection; uncacheable)
-    -> Aggregate: max((x2.a + rand(0)))  (cost=1.25..1.25 rows=1)
-        -> Table scan on x2  (cost=0.025..0.25 rows=10)
 
 DROP TABLE num,t1;
 #

mysql-test/t/explain_json_hypergraph.test

@@ -193,3 +193,23 @@ SELECT * FROM t1 x1 JOIN t1 x2 ON x2.a<
 --eval EXPLAIN FORMAT=JSON $query
 
 DROP TABLE t1;
+
+
+--echo #
+--echo # Bug#34569685 No explain output for subquery
+--echo #
+
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+
+ANALYZE TABLE  t1;
+
+let $query =
+SELECT LAST_VALUE((SELECT x1.a FROM t1))
+OVER (PARTITION BY b) FROM t1 x1;
+
+--replace_regex $elide_costs_and_rows
+--eval EXPLAIN FORMAT=TREE $query
+
+--eval EXPLAIN FORMAT=JSON $query
+
+DROP TABLE t1;

mysql-test/t/explain_tree.test

@@ -763,3 +763,17 @@ SELECT * FROM t1 x1 LEFT JOIN t1 x2 ON x2.a<
 (SELECT MIN(x3.a) FROM t1 x3 WHERE x1.a=x3.a);
 
 DROP TABLE t1;
+
+--echo #
+--echo # Bug#34569685 No explain output for subquery
+--echo #
+
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+
+ANALYZE TABLE  t1;
+
+--replace_regex $elide_costs_and_rows
+EXPLAIN FORMAT=TREE SELECT LAST_VALUE((SELECT x1.a FROM t1))
+OVER (PARTITION BY b) FROM t1 x1;
+
+DROP TABLE t1;

sql/join_optimizer/explain_access_path.cc

@@ -255,26 +255,49 @@ static bool AddSubqueryPaths(const Item *item_arg, const char *source_text,
 
     qe->finalize(current_thd);
     AccessPath *path;
+
     if (qe->root_access_path() != nullptr) {
       path = qe->root_access_path();
     } else {
       path = qe->item->root_access_path();
     }
-    Json_object *child_obj = new (std::nothrow) Json_object();
-    if (child_obj == nullptr) return true;
-    // Populate the subquery-specific json fields.
+
     bool error = false;
-    error |= AddMemberToObject<Json_boolean>(child_obj, "subquery", true);
-    error |= AddMemberToObject<Json_string>(child_obj, "subquery_location",
-                                            source_text);
-    if (query_block->is_dependent())
-      error |= AddMemberToObject<Json_boolean>(child_obj, "dependent", true);
-    if (query_block->is_cacheable())
-      error |= AddMemberToObject<Json_boolean>(child_obj, "cacheable", true);
 
-    children->push_back({path, description, query_block->join, child_obj});
+    // Add 'path' if not present in 'children' already.
+    if (std::none_of(children->cbegin(), children->cend(),
+                     [path](const ExplainChild &existing) {
+                       return existing.path == path;
+                     })) {
+      char description[256];
+      if (query_block->is_dependent()) {
+        snprintf(description, sizeof(description),
+                 "Select #%d (subquery in %s; dependent)",
+                 query_block->select_number, source_text);
+      } else if (!query_block->is_cacheable()) {
+        snprintf(description, sizeof(description),
+                 "Select #%d (subquery in %s; uncacheable)",
+                 query_block->select_number, source_text);
+      } else {
+        snprintf(description, sizeof(description),
+                 "Select #%d (subquery in %s; run only once)",
+                 query_block->select_number, source_text);
+      }
+      Json_object *child_obj = new (std::nothrow) Json_object();
+      if (child_obj == nullptr) return true;
+      // Populate the subquery-specific json fields.
+      error |= AddMemberToObject<Json_boolean>(child_obj, "subquery", true);
+      error |= AddMemberToObject<Json_string>(child_obj, "subquery_location",
+                                              source_text);
+      if (query_block->is_dependent())
+        error |= AddMemberToObject<Json_boolean>(child_obj, "dependent", true);
+      if (query_block->is_cacheable())
+        error |= AddMemberToObject<Json_boolean>(child_obj, "cacheable", true);
+
+      children->push_back({path, description, query_block->join, child_obj});
+    }
 
-    return error != 0;
+    return error;
   };
 
   return WalkItem(item_arg, enum_walk::POSTFIX, add_subqueries);
@@ -1517,6 +1540,15 @@ static std::unique_ptr<Json_object> SetObjectMembers(
       error |= obj->add_alias("functions", std::move(funcs));
       error |= AddMemberToObject<Json_string>(obj, "access_type", "window");
       children->push_back({path->window().child});
+      // temp_table_param may be nullptr for secondary engine,
+      // see ExplainWindowForExternalExecutor in hypergraph_optimizer-t.cc.
+      if (path->window().temp_table_param != nullptr) {
+        for (const Func_ptr &func :
+             *path->window().temp_table_param->items_to_copy) {
+          AddSubqueryPaths(func.func(), "projection", children);
+        }
+      }
+
       break;
     }
     case AccessPath::WEEDOUT: {
@@ -1746,6 +1778,24 @@ static std::unique_ptr<Json_object> ExplainAccessPath(
     vector<ExplainChild> children_from_select;
     if (GetAccessPathsFromSelectList(join, &children_from_select))
       return nullptr;
+
+    // Return 'true' if 'children' contains an object with the same 'path'
+    // as 'sel_child'.
+    const auto in_children = [&children](const ExplainChild &sel_child) {
+      return std::any_of(children.cbegin(), children.cend(),
+                         [sel_child](const ExplainChild &child) {
+                           return sel_child.path == child.path;
+                         });
+    };
+
+    // Remove objects from children_from_select where 'children' has
+    // an object with the same 'path', so that we do not print the same path
+    // twice.
+    children_from_select.erase(
+        std::remove_if(children_from_select.begin(), children_from_select.end(),
+                       in_children),
+        children_from_select.end());
+
     if (AddChildrenToObject(obj, children_from_select, join,
                             /*is_root_of_join*/ true,
                             "inputs_from_select_list"))