Backport of subquery optimizations to 5.3.

There are still test failures because of:
- Wrong query results in outer join + semi join
- EXPLAIN output differences

sql/sql_array.h

@@ -66,3 +66,75 @@ public:
   }
 };
 
+
+/* 
+  Array of pointers to Elem that uses memory from MEM_ROOT
+
+  MEM_ROOT has no realloc() so this is supposed to be used for cases when
+  reallocations are rare.
+*/
+
+template <class Elem> class Array
+{
+  enum {alloc_increment = 16};
+  Elem **buffer;
+  uint n_elements, max_element;
+public:
+  Array(MEM_ROOT *mem_root, uint prealloc=16)
+  {
+    buffer= (Elem**)alloc_root(mem_root, prealloc * sizeof(Elem**));
+    max_element = buffer? prealloc : 0;
+    n_elements= 0;
+  }
+
+  Elem& at(int idx)
+  {
+    return *(((Elem*)buffer) + idx);
+  }
+
+  Elem **front()
+  {
+    return buffer;
+  }
+
+  Elem **back()
+  {
+    return buffer + n_elements;
+  }
+
+  bool append(MEM_ROOT *mem_root, Elem *el)
+  {
+    if (n_elements == max_element)
+    {
+      Elem **newbuf;
+      if (!(newbuf= (Elem**)alloc_root(mem_root, (n_elements + alloc_increment)*
+                                                  sizeof(Elem**))))
+      {
+        return FALSE;
+      }
+      memcpy(newbuf, buffer, n_elements*sizeof(Elem*));
+      buffer= newbuf;
+    }
+    buffer[n_elements++]= el;
+    return FALSE;
+  }
+
+  int elements()
+  {
+    return n_elements;
+  }
+
+  void clear()
+  {
+    n_elements= 0;
+  }
+
+  typedef int (*CMP_FUNC)(Elem * const *el1, Elem *const *el2);
+
+  void sort(CMP_FUNC cmp_func)
+  {
+    my_qsort(buffer, n_elements, sizeof(Elem*), (qsort_cmp)cmp_func);
+  }
+};
+
+