BUG#779885: Crash in eliminate_item_equal with materialization=on in maria-5.3

- In eliminate_item_equal(), we could end up in a situation where:
  = The multiple equality has a constant C1 (and so it is the "head item")
  = The join order was such that we've generated "sj_inner_table1=C1" equality,
    and now are looking to generate "sj_inner_table2_=..." equality. 
  When looking for what should be the other member of equality, we run

      Item *head_item= current_sjm? current_sjm_head: head;
  
  which sees current_sjm!=NULL, and takes current_sjm_head (which is NULL because 
  the constant C1 is the head for all cases).

- Fixed in a trivial way: take current_sjm_head if we don't have constant.

1 files changed, 9 insertions, 1 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 5e84138bf9c..2d2f556ded7 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -10160,8 +10160,16 @@ Item *eliminate_item_equal(COND *cond, COND_EQUAL *upper_levels,
     {
       if (eq_item)
         eq_list.push_back(eq_item);
+      
+      /*
+        If we're inside an SJM-nest (current_sjm!=NULL), and the multi-equality
+        doesn't include a constant, we should produce equality with the first
+        of the equals in this SJM.
 
-      Item *head_item= current_sjm? current_sjm_head: head;
+        In other cases, get the "head" item, which is either first of the
+        equals on top level, or the constant.
+      */
+      Item *head_item= (!item_const && current_sjm)? current_sjm_head: head;
       Item *head_real_item=  head_item->real_item();
       if (head_real_item->type() == Item::FIELD_ITEM)
         head_item= head_real_item;