Bug#32798: DISTINCT in GROUP_CONCAT clause fails when ordering by a column

with null values

For queries containing GROUP_CONCAT(DISTINCT fields ORDER BY fields), there 
was a limitation that the DISTINCT fields had to be the same as ORDER BY 
fields, owing to the fact that one single sorted tree was used for keeping 
track of tuples, ordering and uniqueness. Fixed by introducing a second 
structure to handle uniqueness so that the original structure has only to 
order the result.

mysql-test/r/func_gconcat.result

@@ -271,7 +271,7 @@ group_concat(distinct s1 order by s2)
 c,b,a
 select group_concat(distinct s1 order by s2) from t1;
 group_concat(distinct s1 order by s2)
-c,b,a,c
+c,b,a
 drop table t1;
 create table t1 (a int, c int);
 insert into t1 values (1, 2), (2, 3), (2, 4), (3, 5);
@@ -819,4 +819,65 @@ id	group_concat(b.name)
 1	ra,ra
 2	ra,ra
 drop table t1;
+CREATE TABLE t1 (a INT, b INT);
+INSERT INTO t1 VALUES (1, 1), (2, 2), (2, 3);
+SELECT GROUP_CONCAT(DISTINCT a ORDER BY b) FROM t1;
+GROUP_CONCAT(DISTINCT a ORDER BY b)
+1,2
+SELECT GROUP_CONCAT(DISTINCT a ORDER BY b DESC) FROM t1;
+GROUP_CONCAT(DISTINCT a ORDER BY b DESC)
+2,1
+SELECT GROUP_CONCAT(DISTINCT a) FROM t1;
+GROUP_CONCAT(DISTINCT a)
+1,2
+SELECT GROUP_CONCAT(DISTINCT a + 1 ORDER BY 3 - b) FROM t1;
+GROUP_CONCAT(DISTINCT a + 1 ORDER BY 3 - b)
+3,2
+SELECT GROUP_CONCAT(DISTINCT a + 1 ORDER BY b) FROM t1;
+GROUP_CONCAT(DISTINCT a + 1 ORDER BY b)
+2,3
+SELECT GROUP_CONCAT(a ORDER BY 3 - b) FROM t1;
+GROUP_CONCAT(a ORDER BY 3 - b)
+2,2,1
+CREATE TABLE t2 (a INT, b INT, c INT, d INT);
+INSERT INTO t2 VALUES (1,1, 1,1), (1,1, 2,2), (1,2, 2,1), (2,1, 1,2);
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY c, d) FROM t2;
+GROUP_CONCAT(DISTINCT a, b ORDER BY c, d)
+11,21,12
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY d, c) FROM t2;
+GROUP_CONCAT(DISTINCT a, b ORDER BY d, c)
+11,12,21
+CREATE TABLE t3 (a INT, b INT, c INT);
+INSERT INTO t3 VALUES (1, 1, 1), (2, 1, 2), (3, 2, 1);
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY b, c) FROM t3;
+GROUP_CONCAT(DISTINCT a, b ORDER BY b, c)
+11,21,32
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY c, b) FROM t3;
+GROUP_CONCAT(DISTINCT a, b ORDER BY c, b)
+11,32,21
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY a, b) FROM t1;
+GROUP_CONCAT(DISTINCT a, b ORDER BY a, b)
+11,22,23
+SELECT GROUP_CONCAT(DISTINCT b, a ORDER BY a, b) FROM t1;
+GROUP_CONCAT(DISTINCT b, a ORDER BY a, b)
+11,22,32
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY b, a) FROM t1;
+GROUP_CONCAT(DISTINCT a, b ORDER BY b, a)
+11,22,23
+SELECT GROUP_CONCAT(DISTINCT b, a ORDER BY a, b) FROM t1;
+GROUP_CONCAT(DISTINCT b, a ORDER BY a, b)
+11,22,32
+SELECT GROUP_CONCAT(DISTINCT a ORDER BY a, b) FROM t1;
+GROUP_CONCAT(DISTINCT a ORDER BY a, b)
+1,2
+SELECT GROUP_CONCAT(DISTINCT b ORDER BY b, a) FROM t1;
+GROUP_CONCAT(DISTINCT b ORDER BY b, a)
+1,2,3
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY a) FROM t1;
+GROUP_CONCAT(DISTINCT a, b ORDER BY a)
+11,23,22
+SELECT GROUP_CONCAT(DISTINCT b, a ORDER BY b) FROM t1;
+GROUP_CONCAT(DISTINCT b, a ORDER BY b)
+11,22,32
+DROP TABLE t1, t2, t3;
 End of 5.0 tests

mysql-test/t/func_gconcat.test

@@ -562,4 +562,47 @@ insert into t1 (id, name) values (2, "
 select b.id, group_concat(b.name) from t1 a, t1 b group by b.id;
 drop table t1;
 
+#
+# Bug#32798: DISTINCT in GROUP_CONCAT clause fails when ordering by a column
+# with null values
+#'
+CREATE TABLE t1 (a INT, b INT);
+
+INSERT INTO t1 VALUES (1, 1), (2, 2), (2, 3);
+
+SELECT GROUP_CONCAT(DISTINCT a ORDER BY b) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT a ORDER BY b DESC) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT a) FROM t1;
+
+SELECT GROUP_CONCAT(DISTINCT a + 1 ORDER BY 3 - b) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT a + 1 ORDER BY b) FROM t1;
+SELECT GROUP_CONCAT(a ORDER BY 3 - b) FROM t1;
+
+CREATE TABLE t2 (a INT, b INT, c INT, d INT);
+
+# There is one duplicate in the expression list: 1,10
+# There is one duplicate in ORDER BY list, but that shouldnt matter: 1,10
+INSERT INTO t2 VALUES (1,1, 1,1), (1,1, 2,2), (1,2, 2,1), (2,1, 1,2);
+
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY c, d) FROM t2;
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY d, c) FROM t2;
+
+CREATE TABLE t3 (a INT, b INT, c INT);
+
+INSERT INTO t3 VALUES (1, 1, 1), (2, 1, 2), (3, 2, 1);
+
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY b, c) FROM t3;
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY c, b) FROM t3;
+
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY a, b) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT b, a ORDER BY a, b) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY b, a) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT b, a ORDER BY a, b) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT a ORDER BY a, b) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT b ORDER BY b, a) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT a, b ORDER BY a) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT b, a ORDER BY b) FROM t1;
+
+DROP TABLE t1, t2, t3;
+
 --echo End of 5.0 tests

sql/item_sum.cc

@@ -2831,44 +2831,51 @@ String *Item_sum_udf_str::val_str(String *str)
  concat of values from "group by" operation
 
  BUGS
-   DISTINCT and ORDER BY only works if ORDER BY uses all fields and only fields
-   in expression list
    Blobs doesn't work with DISTINCT or ORDER BY
 *****************************************************************************/
 
-/*
-  function of sort for syntax:
-  GROUP_CONCAT(DISTINCT expr,...)
+
+
+/** 
+  Compares the values for fields in expr list of GROUP_CONCAT.
+  @note
+       
+     GROUP_CONCAT([DISTINCT] expr [,expr ...]
+              [ORDER BY {unsigned_integer | col_name | expr}
+                  [ASC | DESC] [,col_name ...]]
+              [SEPARATOR str_val])
+ 
+  @return
+  @retval -1 : key1 < key2 
+  @retval  0 : key1 = key2
+  @retval  1 : key1 > key2 
 */
 
-int group_concat_key_cmp_with_distinct(void* arg, byte* key1,
-				       byte* key2)
+int group_concat_key_cmp_with_distinct(void* arg, const void* key1, 
+                                       const void* key2)
 {
-  Item_func_group_concat* grp_item= (Item_func_group_concat*)arg;
-  TABLE *table= grp_item->table;
-  Item **field_item, **end;
+  Item_func_group_concat *item_func= (Item_func_group_concat*)arg;
+  TABLE *table= item_func->table;
 
-  for (field_item= grp_item->args, end= field_item + grp_item->arg_count_field;
-       field_item < end;
-       field_item++)
+  for (uint i= 0; i < item_func->arg_count_field; i++)
   {
+    Item *item= item_func->args[i];
+    /* 
+      If field_item is a const item then either get_tp_table_field returns 0
+      or it is an item over a const table. 
+    */
+    if (item->const_item())
+      continue;
     /*
       We have to use get_tmp_table_field() instead of
       real_item()->get_tmp_table_field() because we want the field in
       the temporary table, not the original field
     */
-    Field *field= (*field_item)->get_tmp_table_field();
-    /* 
-      If field_item is a const item then either get_tp_table_field returns 0
-      or it is an item over a const table. 
-    */
-    if (field && !(*field_item)->const_item())
-    {
-      int res;
-      uint offset= field->offset() - table->s->null_bytes;
-      if ((res= field->cmp((char *) key1 + offset, (char *) key2 + offset)))
-	return res;
-    }
+    Field *field= item->get_tmp_table_field();
+    int res;
+    uint offset= field->offset()-table->s->null_bytes;
+    if((res= field->cmp((char*)key1 + offset, (char*)key2 + offset)))
+      return res;
   }
   return 0;
 }
@@ -2879,7 +2886,8 @@ int group_concat_key_cmp_with_distinct(void* arg, byte* key1,
   GROUP_CONCAT(expr,... ORDER BY col,... )
 */
 
-int group_concat_key_cmp_with_order(void* arg, byte* key1, byte* key2)
+int group_concat_key_cmp_with_order(void* arg, const void* key1, 
+                                    const void* key2)
 {
   Item_func_group_concat* grp_item= (Item_func_group_concat*) arg;
   ORDER **order_item, **end;
@@ -2917,25 +2925,6 @@ int group_concat_key_cmp_with_order(void* arg, byte* key1, byte* key2)
 }
 
 
-/*
-  function of sort for syntax:
-  GROUP_CONCAT(DISTINCT expr,... ORDER BY col,... )
-
-  BUG:
-    This doesn't work in the case when the order by contains data that
-    is not part of the field list because tree-insert will not notice
-    the duplicated values when inserting things sorted by ORDER BY
-*/
-
-int group_concat_key_cmp_with_distinct_and_order(void* arg,byte* key1,
-						 byte* key2)
-{
-  if (!group_concat_key_cmp_with_distinct(arg,key1,key2))
-    return 0;
-  return(group_concat_key_cmp_with_order(arg,key1,key2));
-}
-
-
 /*
   Append data from current leaf to item->result
 */
@@ -3020,7 +3009,7 @@ Item_func_group_concat(Name_resolution_context *context_arg,
                        bool distinct_arg, List<Item> *select_list,
                        SQL_LIST *order_list, String *separator_arg)
   :tmp_table_param(0), warning(0),
-   separator(separator_arg), tree(0), table(0),
+   separator(separator_arg), tree(0), unique_filter(NULL), table(0),
    order(0), context(context_arg),
    arg_count_order(order_list ? order_list->elements : 0),
    arg_count_field(select_list->elements),
@@ -3075,6 +3064,7 @@ Item_func_group_concat::Item_func_group_concat(THD *thd,
   warning(item->warning),
   separator(item->separator),
   tree(item->tree),
+  unique_filter(item->unique_filter),
   table(item->table),
   order(item->order),
   context(item->context),
@@ -3125,6 +3115,11 @@ void Item_func_group_concat::cleanup()
         delete_tree(tree);
         tree= 0;
       }
+      if (unique_filter)
+      {
+        delete unique_filter;
+        unique_filter= NULL;
+      }
       if (warning)
       {
         char warn_buff[MYSQL_ERRMSG_SIZE];
@@ -3154,6 +3149,8 @@ void Item_func_group_concat::clear()
   no_appended= TRUE;
   if (tree)
     reset_tree(tree);
+  if (distinct)
+    unique_filter->reset();
   /* No need to reset the table as we never call write_row */
 }
 
@@ -3177,9 +3174,19 @@ bool Item_func_group_concat::add()
   }
 
   null_value= FALSE;
+  bool row_eligible= TRUE;
+
+  if (distinct) 
+  {
+    /* Filter out duplicate rows. */
+    uint count= unique_filter->elements_in_tree();
+    unique_filter->unique_add(table->record[0] + table->s->null_bytes);
+    if (count == unique_filter->elements_in_tree())
+      row_eligible= FALSE;
+  }
 
   TREE_ELEMENT *el= 0;                          // Only for safety
-  if (tree)
+  if (row_eligible && tree)
     el= tree_insert(tree, table->record[0] + table->s->null_bytes, 0,
                     tree->custom_arg);
   /*
@@ -3187,7 +3194,7 @@ bool Item_func_group_concat::add()
     we can dump the row here in case of GROUP_CONCAT(DISTINCT...)
     instead of doing tree traverse later.
   */
-  if (!warning_for_row &&
+  if (row_eligible && !warning_for_row &&
       (!tree || (el->count == 1 && distinct && !arg_count_order)))
     dump_leaf_key(table->record[0] + table->s->null_bytes, 1, this);
 
@@ -3263,7 +3270,6 @@ bool Item_func_group_concat::setup(THD *thd)
 {
   List<Item> list;
   SELECT_LEX *select_lex= thd->lex->current_select;
-  qsort_cmp2 compare_key;
   DBUG_ENTER("Item_func_group_concat::setup");
 
   /*
@@ -3334,38 +3340,33 @@ bool Item_func_group_concat::setup(THD *thd)
   table->file->extra(HA_EXTRA_NO_ROWS);
   table->no_rows= 1;
 
+  /*
+     Need sorting or uniqueness: init tree and choose a function to sort.
+     Don't reserve space for NULLs: if any of gconcat arguments is NULL,
+     the row is not added to the result.
+  */
+  uint tree_key_length= table->s->reclength - table->s->null_bytes;
 
-  if (distinct || arg_count_order)
+  if (arg_count_order)
   {
-    /*
-      Need sorting: init tree and choose a function to sort.
-      Don't reserve space for NULLs: if any of gconcat arguments is NULL,
-      the row is not added to the result.
-    */
-    uint tree_key_length= table->s->reclength - table->s->null_bytes;
-
     tree= &tree_base;
-    if (arg_count_order)
-    {
-      if (distinct)
-        compare_key= (qsort_cmp2) group_concat_key_cmp_with_distinct_and_order;
-      else
-        compare_key= (qsort_cmp2) group_concat_key_cmp_with_order;
-    }
-    else
-    {
-      compare_key= (qsort_cmp2) group_concat_key_cmp_with_distinct;
-    }
     /*
-      Create a tree for sorting. The tree is used to sort and to remove
-      duplicate values (according to the syntax of this function). If there
-      is no DISTINCT or ORDER BY clauses, we don't create this tree.
+      Create a tree for sorting. The tree is used to sort (according to the
+      syntax of this function). If there is no ORDER BY clause, we don't
+      create this tree.
     */
     init_tree(tree, (uint) min(thd->variables.max_heap_table_size,
                                thd->variables.sortbuff_size/16), 0,
-              tree_key_length, compare_key, 0, NULL, (void*) this);
+              tree_key_length, 
+              group_concat_key_cmp_with_order , 0, NULL, (void*) this);
   }
 
+  if (distinct)
+    unique_filter= new Unique(group_concat_key_cmp_with_distinct,
+                              (void*)this,
+                              tree_key_length,
+                              thd->variables.max_heap_table_size);
+  
   DBUG_RETURN(FALSE);
 }
 
@@ -3435,3 +3436,10 @@ void Item_func_group_concat::print(String *str)
   str->append(*separator);
   str->append(STRING_WITH_LEN("\')"));
 }
+
+
+Item_func_group_concat::~Item_func_group_concat()
+{
+  if (unique_filter) 
+    delete unique_filter;    
+}

sql/item_sum.h

@@ -1173,11 +1173,22 @@ class Item_func_group_concat : public Item_sum
   String *separator;
   TREE tree_base;
   TREE *tree;
+
+  /**
+     If DISTINCT is used with this GROUP_CONCAT, this member is used to filter
+     out duplicates. 
+     @see Item_func_group_concat::setup
+     @see Item_func_group_concat::add
+     @see Item_func_group_concat::clear
+   */
+  Unique *unique_filter;
   TABLE *table;
   ORDER **order;
   Name_resolution_context *context;
-  uint arg_count_order;               // total count of ORDER BY items
-  uint arg_count_field;               // count of arguments
+  /** The number of ORDER BY items. */
+  uint arg_count_order;
+  /** The number of selected items, aka the expr list. */
+  uint arg_count_field;
   uint count_cut_values;
   bool distinct;
   bool warning_for_row;
@@ -1190,13 +1201,10 @@ class Item_func_group_concat : public Item_sum
   */
   Item_func_group_concat *original;
 
-  friend int group_concat_key_cmp_with_distinct(void* arg, byte* key1,
-					      byte* key2);
-  friend int group_concat_key_cmp_with_order(void* arg, byte* key1,
-					     byte* key2);
-  friend int group_concat_key_cmp_with_distinct_and_order(void* arg,
-							byte* key1,
-							byte* key2);
+  friend int group_concat_key_cmp_with_distinct(void* arg, const void* key1,
+                                                const void* key2);
+  friend int group_concat_key_cmp_with_order(void* arg, const void* key1,
+					     const void* key2);
   friend int dump_leaf_key(byte* key,
                            element_count count __attribute__((unused)),
 			   Item_func_group_concat *group_concat_item);
@@ -1207,7 +1215,7 @@ public:
                          SQL_LIST *is_order, String *is_separator);
 
   Item_func_group_concat(THD *thd, Item_func_group_concat *item);
-  ~Item_func_group_concat() {}
+  ~Item_func_group_concat();
   void cleanup();
 
   enum Sumfunctype sum_func () const {return GROUP_CONCAT_FUNC;}