Fix for bug#49897: crash in ptr_compare when char(0) NOT NULL

column is used for ORDER BY

Problem: filesort isn't meant for null length sort data
(e.g. char(0)), that leads to a server crash.

Fix: disregard sort order if sort data record length is 0 (nothing
to sort).

2 files changed, 15 insertions, 0 deletions

diff --git a/sql/filesort.cc b/sql/filesort.cc
index f56e5b3a771..11be5d7f672 100644
--- a/sql/filesort.cc
+++ b/sql/filesort.cc
@@ -134,6 +134,8 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   error= 1;
   bzero((char*) &param,sizeof(param));
   param.sort_length= sortlength(thd, sortorder, s_length, &multi_byte_charset);
+  /* filesort cannot handle zero-length records. */
+  DBUG_ASSERT(param.sort_length);
   param.ref_length= table->file->ref_length;
   param.addon_field= 0;
   param.addon_length= 0;
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index d8ec5eff5c1..239809f1d4c 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -521,13 +521,26 @@ JOIN::prepare(Item ***rref_pointer_array,
 
   if (order)
   {
+    bool real_order= FALSE;
     ORDER *ord;
     for (ord= order; ord; ord= ord->next)
     {
       Item *item= *ord->item;
+      /*
+        Disregard sort order if there's only "{VAR}CHAR(0) NOT NULL" fields
+        there. Such fields don't contain any data to sort.
+      */
+      if (!real_order &&
+          (item->type() != Item::Item::FIELD_ITEM ||
+           ((Item_field *) item)->field->maybe_null() ||
+           ((Item_field *) item)->field->sort_length()))
+        real_order= TRUE;
+
       if (item->with_sum_func && item->type() != Item::SUM_FUNC_ITEM)
         item->split_sum_func(thd, ref_pointer_array, all_fields);
     }
+    if (!real_order)
+      order= NULL;
   }
 
   if (having && having->with_sum_func)