[SHOW] EXPLAIN UPDATE/DELETE, code re-structuring

- If a subquery is correlated wrt a const table, it will change from
  being a "DEPENDENT SUBQUERY" into "SUBQUERY", at the end of its parent's 
  JOIN::optimize() call.  Handle this, update the subquery's QPF.
- Make show_explain.test to work
   = "Query plan already deleted" does not happen anymore.
   = Handle special case of queries that don't have top-level selects, like 
     SET x = (SELECT ...)

mysql-test/r/show_explain.result

@@ -165,7 +165,11 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='+d,show_explain_probe_join_exec_end';
 select a, (select max(a) from t0 b where b.a+a.a<10) from t0 a where a<1;
 show explain for $thr2;
-ERROR HY000: Target is not running an EXPLAINable command
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	a	ALL	NULL	NULL	NULL	NULL	10	Using where
+2	DEPENDENT SUBQUERY	b	ALL	NULL	NULL	NULL	NULL	10	Using where
+Warnings:
+Note	1003	select a, (select max(a) from t0 b where b.a+a.a<10) from t0 a where a<1
 a	(select max(a) from t0 b where b.a+a.a<10)
 0	9
 set debug_dbug=@old_debug;
@@ -343,7 +347,11 @@ SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a;
 # FIXED by "conservative assumptions about when QEP is available" fix:
 #    NOTE: current code will not show "Using join buffer": 
 show explain for $thr2;
-ERROR HY000: Target is not running an EXPLAINable command
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using temporary; Using filesort
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using join buffer (flat, BNL join)
+Warnings:
+Note	1003	SELECT alias.a FROM t2, ( SELECT * FROM t2 ) AS alias GROUP BY alias.a
 a
 1
 2
@@ -428,7 +436,10 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='+d,show_explain_probe_join_exec_end';
 select * from t0 where 1>10;
 show explain for $thr2;
-ERROR HY000: Target is not running an EXPLAINable command
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+Warnings:
+Note	1003	select * from t0 where 1>10
 a
 set debug_dbug=@old_debug;
 #
@@ -440,7 +451,10 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='+d,show_explain_probe_join_exec_end';
 select * from t0,t3 where t3.a=112233;
 show explain for $thr2;
-ERROR HY000: Target is not running an EXPLAINable command
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+Warnings:
+Note	1003	select * from t0,t3 where t3.a=112233
 a	a
 set debug_dbug=@old_debug;
 drop table t3;
@@ -545,7 +559,12 @@ set @show_explain_probe_select_id=1;
 set debug_dbug='+d,show_explain_probe_join_exec_end';
 SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`);
 show explain for $thr2;
-ERROR HY000: Target is not running an EXPLAINable command
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<subquery2>	const	distinct_key	distinct_key	8	const,const	1	
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	20	Using join buffer (flat, BNL join)
+2	MATERIALIZED	t2	index	NULL	a1	4	NULL	20	Using index
+Warnings:
+Note	1003	SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`)
 pk	a1
 set debug_dbug=@old_debug;
 DROP TABLE t2;
@@ -651,7 +670,7 @@ SELECT a + 1 FROM v1;
 show explain for $thr2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	2	
-2	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
+2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	2	
 Warnings:
 Note	1003	SELECT a + 1 FROM v1
 a + 1
@@ -1061,7 +1080,7 @@ show explain for $thr2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	alias1	ALL	NULL	NULL	NULL	NULL	14	
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	20	
-3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Query plan already deleted
+3	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	20	Using where
 Warnings:
 Note	1003	SELECT max(a+b+c) FROM t1 AS alias1, ( SELECT * FROM t2 ) AS alias 
 WHERE EXISTS ( SELECT * FROM t3 WHERE b = c )  OR a <= 10

mysql-test/t/show_explain.test

@@ -202,7 +202,6 @@ set debug_dbug='+d,show_explain_probe_join_exec_end';
 send select a, (select max(a) from t0 b where b.a+a.a<10) from t0 a where a<1;
 connection default;
 --source include/wait_condition.inc
---error ER_TARGET_NOT_EXPLAINABLE
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -349,7 +348,7 @@ connection default;
 --source include/wait_condition.inc
 --echo # FIXED by "conservative assumptions about when QEP is available" fix:
 --echo #    NOTE: current code will not show "Using join buffer": 
---error ER_TARGET_NOT_EXPLAINABLE
+#--error ER_TARGET_NOT_EXPLAINABLE
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -428,7 +427,7 @@ set debug_dbug='+d,show_explain_probe_join_exec_end';
 send select * from t0 where 1>10;
 connection default;
 --source include/wait_condition.inc
---error ER_TARGET_NOT_EXPLAINABLE
+#--error ER_TARGET_NOT_EXPLAINABLE
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -444,7 +443,7 @@ set debug_dbug='+d,show_explain_probe_join_exec_end';
 send select * from t0,t3 where t3.a=112233;
 connection default;
 --source include/wait_condition.inc
---error ER_TARGET_NOT_EXPLAINABLE
+# --error ER_TARGET_NOT_EXPLAINABLE
 evalp show explain for $thr2;
 connection con1;
 reap;
@@ -540,7 +539,7 @@ send
   SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`);
 connection default;
 --source include/wait_condition.inc
---error ER_TARGET_NOT_EXPLAINABLE
+# --error ER_TARGET_NOT_EXPLAINABLE
 evalp show explain for $thr2;
 connection con1;
 reap;

sql/opt_qpf.h

@@ -216,7 +216,9 @@ public:
 
   /* Produce a tabular EXPLAIN output */
   int print_explain(select_result_sink *output, uint8 explain_flags);
-
+  
+  /* If true, at least part of EXPLAIN can be printed */
+  bool have_query_plan() { return upd_del_plan!= NULL || get_node(1) != NULL; }
   MEM_ROOT *mem_root;
 private:
   Dynamic_array<QPF_union*> unions;

sql/sql_lex.cc

@@ -3495,6 +3495,18 @@ bool st_select_lex::optimize_unflattened_subqueries(bool const_only)
         is_correlated_unit|= sl->is_correlated;
         inner_join->select_options= save_options;
         un->thd->lex->current_select= save_select;
+        /// psergey:
+        QPF_query *qpf;
+        if ((qpf= inner_join->thd->lex->query_plan_footprint))
+        {
+          QPF_select *qp_sel;
+          if ((qp_sel= qpf->get_select(inner_join->select_lex->select_number)))
+          {
+            sl->set_explain_type(TRUE);
+            qp_sel->select_type= sl->type;
+          }
+        }
+        ///
         if (empty_union_result)
         {
           /*
@@ -4182,7 +4194,7 @@ int LEX::print_explain(select_result_sink *output, uint8 explain_flags,
                        bool *printed_anything) //TODO: remove printed_anything
 {
   int res;
-  if (query_plan_footprint)
+  if (query_plan_footprint && query_plan_footprint->have_query_plan())
   {
     res= query_plan_footprint->print_explain(output, explain_flags);
     *printed_anything= true;