MDEV-415: Back-port of the WL task #1393 from the mysql-5.6 code line.

The task adds a more efficient handling of the queries with 
ORDER BY order LIMIT n, such that n is small enough and
no indexes are used for order.

2 files changed, 6 insertions, 1 deletions

diff --git a/mysys/mf_radix.c b/mysys/mf_radix.c
index 582ca76b8f8..2df1220acdd 100644
--- a/mysys/mf_radix.c
+++ b/mysys/mf_radix.c
@@ -25,6 +25,11 @@
 
 	/* Radixsort */
 
+my_bool radixsort_is_appliccable(uint n_items, size_t size_of_element)
+{
+  return size_of_element <= 20 && n_items >= 1000 && n_items < 100000;
+}
+
 void radixsort_for_str_ptr(uchar **base, uint number_of_elements, size_t size_of_element, uchar **buffer)
 {
   uchar **end,**ptr,**buffer_ptr;
diff --git a/mysys/mf_sort.c b/mysys/mf_sort.c
index 9ef02787716..b2c58c26624 100644
--- a/mysys/mf_sort.c
+++ b/mysys/mf_sort.c
@@ -23,7 +23,7 @@ void my_string_ptr_sort(uchar *base, uint items, size_t size)
 #if INT_MAX > 65536L
   uchar **ptr=0;
 
-  if (size <= 20 && items >= 1000 && items < 100000 &&
+  if (radixsort_is_appliccable(items, size) &&
       (ptr= (uchar**) my_malloc(items*sizeof(char*),MYF(0))))
   {
     radixsort_for_str_ptr((uchar**) base,items,size,ptr);