1 files changed, 45 insertions, 11 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index f0177893840..53a6b699022 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -8706,6 +8706,39 @@ static bool check_interleaving_with_nj(JOIN_TAB *next_tab)
 /*
   Nested joins perspective: Remove the last table from the join order
 
+  The algorithm is the reciprocal of check_interleaving_with_nj(), hence
+  parent join nest nodes are updated only when the last table in its child
+  node is removed. The ASCII graphic below will clarify.
+
+  %A table nesting such as <tt> t1 x [ ( t2 x t3 ) x ( t4 x t5 ) ] </tt>is
+  represented by the below join nest tree.
+
+  @verbatim
+                     NJ1
+                  _/ /  \
+                _/  /    NJ2
+              _/   /     / \ 
+             /    /     /   \
+   t1 x [ (t2 x t3) x (t4 x t5) ]
+  @endverbatim
+
+  At the point in time when check_interleaving_with_nj() adds the table t5 to
+  the query execution plan, QEP, it also directs the node named NJ2 to mark
+  the table as covered. NJ2 does so by incrementing its @c counter
+  member. Since all of NJ2's tables are now covered by the QEP, the algorithm
+  proceeds up the tree to NJ1, incrementing its counter as well. All join
+  nests are now completely covered by the QEP.
+
+  restore_prev_nj_state() does the above in reverse. As seen above, the node
+  NJ1 contains the nodes t2, t3, and NJ2. Its counter being equal to 3 means
+  that the plan covers t2, t3, and NJ2, @e and that the sub-plan (t4 x t5)
+  completely covers NJ2. The removal of t5 from the partial plan will first
+  decrement NJ2's counter to 1. It will then detect that NJ2 went from being
+  completely to partially covered, and hence the algorithm must continue
+  upwards to NJ1 and decrement its counter to 2. %A subsequent removal of t4
+  will however not influence NJ1 since it did not un-cover the last table in
+  NJ2.
+
   SYNOPSIS
     restore_prev_nj_state()
       last  join table to remove, it is assumed to be the last in current 
@@ -8722,19 +8755,20 @@ static void restore_prev_nj_state(JOIN_TAB *last)
 {
   TABLE_LIST *last_emb= last->table->pos_in_table_list->embedding;
   JOIN *join= last->join;
-  while (last_emb)
+  for (;last_emb != NULL; last_emb= last_emb->embedding)
   {
-    if (!(--last_emb->nested_join->counter))
-      join->cur_embedding_map&= ~last_emb->nested_join->nj_map;
-    else if (last_emb->nested_join->join_list.elements-1 ==
-             last_emb->nested_join->counter) 
-    {
-      join->cur_embedding_map|= last_emb->nested_join->nj_map;
-      break;
-    }
-    else
+    NESTED_JOIN *nest= last_emb->nested_join;
+    DBUG_ASSERT(nest->counter > 0);
+    
+    bool was_fully_covered= nest->is_fully_covered();
+    
+    if (--nest->counter == 0)
+      join->cur_embedding_map&= ~nest->nj_map;
+    
+    if (!was_fully_covered)
       break;
-    last_emb= last_emb->embedding;
+    
+    join->cur_embedding_map|= nest->nj_map;
   }
 }