MDEV-3798: EXPLAIN UPDATE/DELETE

- Update test results after last few csets
- Generate correct value for `possible_keys` column for single table UPDATE/DELETE.

mysql-test/r/explain_non_select.result

@@ -13,7 +13,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 # DELETE without WHERE is a special case:
 explain delete from t0;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Deleting all rows
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	8	Deleting all rows
 create table t1 (a int, b int, filler char(100), key(a), key(b));
 insert into t1 
 select A.a+10*B.a + 10*C.a, A.a+10*B.a + 10*C.a, 'filler' 
@@ -65,10 +65,10 @@ select A.a+10*B.a + 10*C.a, A.a+10*B.a + 10*C.a, 'filler'
 from t0 A, t0 B, t0 C;
 explain update t1 set a=a+1 where 3>4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible where
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 explain update t1 set a=a+1 where a=3 and a=4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible where
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 # This should use an index,  possible_keys=NULL because there is no WHERE
 explain update t1 set a=a+1 order by a limit 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

sql/opt_range.cc

@@ -861,6 +861,14 @@ class PARAM : public RANGE_OPT_PARAM
 {
 public:
   ha_rows quick_rows[MAX_KEY];
+
+  /*
+    This will collect 'possible keys' based on the range optimization.
+    
+    Queries with a JOIN object actually use ref optimizer (see add_key_field)
+    to collect possible_keys. This is used by single table UPDATE/DELETE.
+  */
+  key_map possible_keys;
   longlong baseflag;
   uint max_key_part, range_count;
 
@@ -2955,6 +2963,8 @@ int SQL_SELECT::test_quick_select(THD *thd, key_map keys_to_use,
     read_time= (double) records + scan_time + 1; // Force to use index
   else if (read_time <= 2.0 && !force_quick_range)
     DBUG_RETURN(0);				/* No need for quick select */
+  
+  possible_keys.clear_all();
 
   DBUG_PRINT("info",("Time to scan table: %g", read_time));
 
@@ -2986,6 +2996,7 @@ int SQL_SELECT::test_quick_select(THD *thd, key_map keys_to_use,
     param.using_real_indexes= TRUE;
     param.remove_jump_scans= TRUE;
     param.force_default_mrr= ordered_output;
+    param.possible_keys.clear_all();
 
     thd->no_errors=1;				// Don't warn about NULL
     init_sql_alloc(&alloc, thd->variables.range_alloc_block_size, 0,
@@ -3197,6 +3208,7 @@ int SQL_SELECT::test_quick_select(THD *thd, key_map keys_to_use,
         quick= NULL;
       }
     }
+    possible_keys= param.possible_keys;
 
   free_mem:
     free_root(&alloc,MYF(0));			// Return memory & allocator
@@ -3204,6 +3216,7 @@ int SQL_SELECT::test_quick_select(THD *thd, key_map keys_to_use,
     thd->no_errors=0;
   }
 
+
   DBUG_EXECUTE("info", print_quick(quick, &needed_reg););
 
   /*
@@ -10467,6 +10480,7 @@ ha_rows check_quick_select(PARAM *param, uint idx, bool index_only,
   if (rows != HA_POS_ERROR)
   {
     param->quick_rows[keynr]= rows;
+    param->possible_keys.set_bit(keynr);
     if (update_tbl_stats)
     {
       param->table->quick_keys.set_bit(keynr);

sql/opt_range.h

@@ -982,6 +982,8 @@ class SQL_SELECT :public Sql_alloc {
   key_map quick_keys;		// Possible quick keys
   key_map needed_reg;		// Possible quick keys after prev tables.
   table_map const_tables,read_tables;
+  /* See PARAM::possible_keys */
+  key_map possible_keys;
   bool	free_cond; /* Currently not used and always FALSE */
 
   SQL_SELECT();

sql/sql_delete.cc

@@ -436,7 +436,7 @@ bool mysql_delete(THD *thd, TABLE_LIST *table_list, COND *conds,
   }
 
   query_plan.select= select;
-  query_plan.possible_keys= table->quick_keys;
+  query_plan.possible_keys= select? select->possible_keys: key_map(0);
   query_plan.table_rows= table->file->stats.records;
   
   /*

sql/sql_update.cc

@@ -492,8 +492,8 @@ int mysql_update(THD *thd,
      - if we're running EXPLAIN UPDATE, get out
   */
   query_plan.select= select;
-  query_plan.possible_keys= table->quick_keys;
   query_plan.table_rows= table->file->stats.records;
+  query_plan.possible_keys= select? select->possible_keys: key_map(0);
   
   /*
     Ok, we have generated a query plan for the UPDATE.