Bug#34773: query with explain extended and derived table / other table

crashes server

When creating temporary table that contains aggregate functions a 
non-reversible source transformation was performed to redirect aggregate
function arguments towards temporary table columns.
This caused EXPLAIN EXTENDED to fail because it was trying to resolve
references to the (freed) temporary table.
Fixed by preserving the original aggregate function arguments and
using them (instead of the transformed ones) for EXPLAIN EXTENDED.

mysql-test/r/explain.result:
  Bug#34773: test case
mysql-test/t/explain.test:
  Bug#34773: test case
sql/item.cc:
  Bug#34773: use accessor functions instead of public members
sql/item_sum.cc:
  Bug#34773: 
   - Encapsulate the arguments into Item_sum and
     provide accessor and mutator methods 
   - print the orginal arguments (if present)
     in EXPLAIN EXTENDED
   - preserve the original arguments list.
sql/item_sum.h:
  Bug#34773: 
   - Encapsulate the arguments into Item_sum and
     provide accessor and mutator methods 
   - print the orginal arguments (if present)
     in EXPLAIN EXTENDED
   - preserve the original arguments list.
sql/opt_range.cc:
  Bug#34773: use accessor functions instead of public members
sql/opt_sum.cc:
  Bug#34773: use accessor functions instead of public members
sql/sql_select.cc:
  Bug#34773: use accessor functions instead of public members

1 files changed, 3 insertions, 3 deletions

diff --git a/sql/opt_sum.cc b/sql/opt_sum.cc
index 42d36756845..3ccc1e5cf41 100644
--- a/sql/opt_sum.cc
+++ b/sql/opt_sum.cc
@@ -199,7 +199,7 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
           to the number of rows in the tables if this number is exact and
           there are no outer joins.
         */
-        if (!conds && !((Item_sum_count*) item)->args[0]->maybe_null &&
+        if (!conds && !((Item_sum_count*) item)->get_arg(0)->maybe_null &&
             !outer_tables && maybe_exact_count)
         {
           if (!is_exact_count)
@@ -225,7 +225,7 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
           parts of the key is found in the COND, then we can use
           indexes to find the key.
         */
-        Item *expr=item_sum->args[0];
+        Item *expr=item_sum->get_arg(0);
         if (expr->real_item()->type() == Item::FIELD_ITEM)
         {
           uchar key_buff[MAX_KEY_LENGTH];
@@ -373,7 +373,7 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
           parts of the key is found in the COND, then we can use
           indexes to find the key.
         */
-        Item *expr=item_sum->args[0];
+        Item *expr=item_sum->get_arg(0);
         if (expr->real_item()->type() == Item::FIELD_ITEM)
         {
           uchar key_buff[MAX_KEY_LENGTH];