MDEV-3798: EXPLAIN UPDATE/DELETE

- Handle the case when EXPLAIN UPDATE/DELETE has pruned away all partitions.

mysql-test/r/explain_non_select.result

@@ -161,3 +161,23 @@ explain extended delete from t2 where a in (3,4);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 drop table t1,t2;
+#
+#  Check the special case where partition pruning removed all partitions
+#
+create table t1 (a int, b int) 
+partition by range (a) (
+partition p0 values less than (10),
+partition p1 values less than (20),
+partition p2 values less than (30)
+);
+insert into t1 values (9,9), (19,19), (29,29);
+explain partitions select * from t1 where a in (32,33);
+id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+explain partitions delete from t1 where a in (32,33);
+id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No matching rows after partition pruning
+explain partitions update t1 set b=12345 where a in (32,33);
+id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No matching rows after partition pruning
+drop table t1;

mysql-test/t/explain_non_select.test

@@ -136,3 +136,22 @@ explain extended delete from t2 where a in (3,4);
 
 drop table t1,t2;
 
+--echo #
+--echo #  Check the special case where partition pruning removed all partitions
+--echo #
+
+create table t1 (a int, b int) 
+partition by range (a) (
+ partition p0 values less than (10),
+ partition p1 values less than (20),
+ partition p2 values less than (30)
+);
+insert into t1 values (9,9), (19,19), (29,29);
+
+explain partitions select * from t1 where a in (32,33);
+
+explain partitions delete from t1 where a in (32,33);
+
+explain partitions update t1 set b=12345 where a in (32,33);
+
+drop table t1;

sql/sql_delete.cc

@@ -83,14 +83,22 @@ void Update_plan::save_explain_data_intern(Explain_query *query,
 {
   explain->select_type= "SIMPLE";
   explain->table_name.append(table->pos_in_table_list->alias);
+  
+  explain->impossible_where= false;
+  explain->no_partitions= false;
+
   if (impossible_where)
   {
     explain->impossible_where= true;
     return;
   }
-  
-  explain->impossible_where= false;
 
+  if (no_partitions)
+  {
+    explain->no_partitions= true;
+    return;
+  }
+  
   select_lex->set_explain_type(TRUE);
   explain->select_type= select_lex->type;
   /* Partitions */
@@ -139,7 +147,8 @@ void Update_plan::save_explain_data_intern(Explain_query *query,
   /* Calculate key_len */
   if (select && select->quick)
   {
-    select->quick->add_keys_and_lengths(&explain->key_str, &explain->key_len_str);
+    select->quick->add_keys_and_lengths(&explain->key_str, 
+                                        &explain->key_len_str);
   }
   else
   {
@@ -356,8 +365,11 @@ bool mysql_delete(THD *thd, TABLE_LIST *table_list, COND *conds,
   if (prune_partitions(thd, table, conds))
   {
     free_underlaid_joins(thd, select_lex);
-    // No matching record
-    //psergey-explain-todo: No-partitions used EXPLAIN here..
+
+    query_plan.set_no_partitions();
+    if (thd->lex->describe)
+      goto exit_without_my_ok;
+
     my_ok(thd, 0);
     DBUG_RETURN(0);
   }

sql/sql_explain.cc

@@ -766,9 +766,11 @@ int Explain_update::print_explain(Explain_query *query,
                                   uint8 explain_flags)
 {
   StringBuffer<64> extra_str;
-  if (impossible_where)
+  if (impossible_where || no_partitions)
   {
-    const char *msg= "Impossible where";
+    const char *msg= impossible_where ? 
+                     "Impossible where" : 
+                     "No matching rows after partition pruning";
     int res= print_explain_message_line(output, explain_flags,
                                         1 /*select number*/,
                                         select_type, msg);

sql/sql_explain.h

@@ -460,6 +460,7 @@ public:
   bool used_partitions_set;
 
   bool impossible_where;
+  bool no_partitions;
   StringBuffer<64> table_name;
 
   enum join_type jtype;

sql/sql_lex.h

@@ -2380,8 +2380,10 @@ class Update_plan
 {
 protected:
   bool impossible_where;
+  bool no_partitions;
 public:
   bool updating_a_view;
+
   TABLE *table;
   SQL_SELECT *select;
   uint index;
@@ -2395,14 +2397,17 @@ public:
   key_map possible_keys;
   bool using_filesort;
   
-  /* Set this plan to be a plan to do nothing because of impossible WHRE*/
+  /* Set this plan to be a plan to do nothing because of impossible WHERE */
   void set_impossible_where() { impossible_where= true; }
+  void set_no_partitions() { no_partitions= true; }
 
   void save_explain_data(Explain_query *query);
   void save_explain_data_intern(Explain_query *query, Explain_update *eu);
   virtual ~Update_plan() {}
 
-  Update_plan() : impossible_where(false), using_filesort(false) {}
+  Update_plan() : 
+    impossible_where(false), no_partitions(false), using_filesort(false) 
+  {}
 };
 
 

sql/sql_update.cc

@@ -403,6 +403,11 @@ int mysql_update(THD *thd,
   if (prune_partitions(thd, table, conds))
   {
     free_underlaid_joins(thd, select_lex);
+
+    query_plan.set_no_partitions();
+    if (thd->lex->describe)
+      goto exit_without_my_ok;
+
     my_ok(thd);				// No matching records
     DBUG_RETURN(0);
   }