MDEV-22993: Crash on EXPLAIN with PUSHED DOWN SELECT and subquery

- select_describe() should not attempt to produce query plans
  for subqueries if the query is handled by a Select Handler.

- JOIN::save_explain_data_intern should not add links to Explain_select
  for children selects if:
  1. The whole query is handled by the Select Handler, or
  2. this select (and so its children) is handled by Derived Handler.

mysql-test/suite/federated/federatedx_create_handlers.result

@@ -209,7 +209,6 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	7	
 1	PRIMARY	<derived2>	ref	key0	key0	18	federated.t3.name	2	
 2	PUSHED DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL
-3	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	7	Using temporary
 ANALYZE FORMAT=JSON
 SELECT *
 FROM federated.t3, (SELECT t1.name FROM federated.t1
@@ -251,25 +250,7 @@ ANALYZE
           "select_id": 2,
           "table": {
             "message": "Pushed derived"
-          },
-          "subqueries": [
-            {
-              "query_block": {
-                "select_id": 3,
-                "temporary_table": {
-                  "table": {
-                    "table_name": "t2",
-                    "access_type": "ALL",
-                    "r_loops": 0,
-                    "rows": 7,
-                    "r_rows": null,
-                    "filtered": 100,
-                    "r_filtered": null
-                  }
-                }
-              }
-            }
-          ]
+          }
         }
       }
     }
@@ -319,6 +300,78 @@ select @var;
 @var
 xxx
 select name into outfile 'tmp.txt' from federated.t1;
+#
+# MDEV-22993: Crash on EXPLAIN with PUSHED DOWN SELECT and subquery
+#
+explain
+select * from federated.t1
+where name in (select name from federated.t2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PUSHED SELECT	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL
+explain format=json
+select * from federated.t1
+where name in (select name from federated.t2);
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "message": "Pushed select"
+    }
+  }
+}
+#
+# MDEV-22993, testcase #2: EXPLAIN output doesn't make sense when
+#   derived table pushdown is used.
+#
+create table t5 (a int) engine=myisam;
+insert into t5 values (1),(2);
+# Must not show lines with id=3
+explain 
+select * from t5, 
+(select id from federated.t1
+where name in (select name from federated.t2) or name like 'foo%') as TQ;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t5	ALL	NULL	NULL	NULL	NULL	2	
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	5	Using join buffer (flat, BNL join)
+2	PUSHED DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL
+# Must not show elements with select_id=3
+explain format=json
+select * from t5, 
+(select id from federated.t1
+where name in (select name from federated.t2) or name like 'foo%') as TQ;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t5",
+      "access_type": "ALL",
+      "rows": 2,
+      "filtered": 100
+    },
+    "block-nl-join": {
+      "table": {
+        "table_name": "<derived2>",
+        "access_type": "ALL",
+        "rows": 5,
+        "filtered": 100
+      },
+      "buffer_type": "flat",
+      "buffer_size": "65",
+      "join_type": "BNL",
+      "materialized": {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "message": "Pushed derived"
+          }
+        }
+      }
+    }
+  }
+}
+drop table t5;
 DROP TABLE federated.t1, federated.t2, federated.t3, federated.t4;
 connection slave;
 DROP TABLE federated.t1, federated.t2;

mysql-test/suite/federated/federatedx_create_handlers.test

@@ -167,6 +167,40 @@ select name into outfile 'tmp.txt' from federated.t1;
 let $path=`select concat(@@datadir, 'test/tmp.txt')`;
 remove_file $path;
 
+--echo #
+--echo # MDEV-22993: Crash on EXPLAIN with PUSHED DOWN SELECT and subquery
+--echo #
+
+explain
+select * from federated.t1
+where name in (select name from federated.t2);
+
+explain format=json
+select * from federated.t1
+where name in (select name from federated.t2);
+
+--echo #
+--echo # MDEV-22993, testcase #2: EXPLAIN output doesn't make sense when
+--echo #   derived table pushdown is used.
+--echo #
+
+create table t5 (a int) engine=myisam;
+insert into t5 values (1),(2);
+
+--echo # Must not show lines with id=3
+explain 
+select * from t5, 
+(select id from federated.t1
+ where name in (select name from federated.t2) or name like 'foo%') as TQ;
+
+--echo # Must not show elements with select_id=3
+explain format=json
+select * from t5, 
+(select id from federated.t1
+ where name in (select name from federated.t2) or name like 'foo%') as TQ;
+
+drop table t5;
+
 DROP TABLE federated.t1, federated.t2, federated.t3, federated.t4;
 
 connection slave;

sql/sql_select.cc

@@ -26840,24 +26840,33 @@ int JOIN::save_explain_data_intern(Explain_query *output,
     output->add_node(xpl_sel);
   }
 
-  for (SELECT_LEX_UNIT *tmp_unit= join->select_lex->first_inner_unit();
-       tmp_unit;
-       tmp_unit= tmp_unit->next_unit())
+  /*
+    Don't try to add query plans for child selects if this select was pushed
+    down into a Smart Storage Engine:
+    - the entire statement was pushed down ("PUSHED SELECT"), or
+    - this derived table was pushed down ("PUSHED DERIVED")
+  */
+  if (!select_lex->pushdown_select && select_lex->type != pushed_derived_text)
   {
-    /* 
-      Display subqueries only if 
-      (1) they are not parts of ON clauses that were eliminated by table 
-          elimination.
-      (2) they are not merged derived tables
-      (3) they are not hanging CTEs (they are needed for execution)
-    */
-    if (!(tmp_unit->item && tmp_unit->item->eliminated) &&    // (1)
-        (!tmp_unit->derived ||
-         tmp_unit->derived->is_materialized_derived()) &&     // (2)
-        !(tmp_unit->with_element &&
-          (!tmp_unit->derived || !tmp_unit->derived->derived_result))) // (3)
-   {
-      explain->add_child(tmp_unit->first_select()->select_number);
+    for (SELECT_LEX_UNIT *tmp_unit= join->select_lex->first_inner_unit();
+         tmp_unit;
+         tmp_unit= tmp_unit->next_unit())
+    {
+      /*
+        Display subqueries only if
+        (1) they are not parts of ON clauses that were eliminated by table
+            elimination.
+        (2) they are not merged derived tables
+        (3) they are not hanging CTEs (they are needed for execution)
+      */
+      if (!(tmp_unit->item && tmp_unit->item->eliminated) &&    // (1)
+          (!tmp_unit->derived ||
+           tmp_unit->derived->is_materialized_derived()) &&     // (2)
+          !(tmp_unit->with_element &&
+            (!tmp_unit->derived || !tmp_unit->derived->derived_result))) // (3)
+     {
+        explain->add_child(tmp_unit->first_select()->select_number);
+      }
     }
   }
 
@@ -26890,7 +26899,16 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
   THD *thd=join->thd;
   select_result *result=join->result;
   DBUG_ENTER("select_describe");
-  
+
+  if (join->select_lex->pushdown_select)
+  {
+    /*
+      The whole statement was pushed down to a Smart Storage Engine. Do not
+      attempt to produce a query plan locally.
+    */
+    DBUG_VOID_RETURN;
+  }
+
   /* Update the QPF with latest values of using_temporary, using_filesort */
   for (SELECT_LEX_UNIT *unit= join->select_lex->first_inner_unit();
        unit;