Bug#47280 - strange results from count(*) with order by multiple

columns without where/group
                     
Simple SELECT with implicit grouping used to return many rows if
the query was ordered by the aggregated column in the SELECT
list. This was incorrect because queries with implicit grouping
should only return a single record.
                              
The problem was that when JOIN:exec() decided if execution needed
to handle grouping, it was assumed that sum_func_count==0 meant
that there were no aggregate functions in the query. This
assumption was not correct in JOIN::exec() because the aggregate
functions might have been optimized away during JOIN::optimize().
                  
The reason why queries without ordering behaved correctly was
that sum_func_count is only recalculated if the optimizer chooses
to use temporary tables (which it does in the ordered case).
Hence, non-ordered queries were correctly treated as grouped.
                  
The fix for this bug was to remove the assumption that
sum_func_count==0 means that there is no need for grouping. This
was done by introducing variable "bool implicit_grouping" in the
JOIN object.

mysql-test/r/func_group.result

@@ -1477,3 +1477,47 @@ COUNT(*)
 SET SQL_MODE=default;
 DROP TABLE t1;
 End of 5.0 tests
+#
+# BUG#47280 - strange results from count(*) with order by multiple 
+#             columns without where/group
+# 
+# 
+# Initialize test
+# 
+CREATE TABLE t1 (
+pk INT NOT NULL,
+i INT,
+PRIMARY KEY (pk)
+);
+INSERT INTO t1 VALUES (1,11),(2,12),(3,13);
+#
+# Start test
+# All the following queries shall return 1 record
+#
+
+# Masking all correct values {11...13} for column i in this result. 
+SELECT MAX(pk) as max, i
+FROM t1
+ORDER BY max;
+max	i
+3	#
+
+EXPLAIN
+SELECT MAX(pk) as max, i
+FROM t1
+ORDER BY max;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using temporary
+
+# Only 11 is correct for collumn i in this result
+SELECT MAX(pk) as max, i
+FROM t1
+WHERE pk<2
+ORDER BY max;
+max	i
+1	11
+#
+# Cleanup
+#
+DROP TABLE t1;
+End of 5.1 tests

mysql-test/t/func_group.test

@@ -1006,3 +1006,51 @@ DROP TABLE t1;
 
 ###
 --echo End of 5.0 tests
+
+--echo #
+--echo # BUG#47280 - strange results from count(*) with order by multiple 
+--echo #             columns without where/group
+--echo # 
+
+--echo # 
+--echo # Initialize test
+--echo # 
+
+CREATE TABLE t1 (
+  pk INT NOT NULL,
+  i INT,
+  PRIMARY KEY (pk)
+);
+INSERT INTO t1 VALUES (1,11),(2,12),(3,13);
+
+--echo #
+--echo # Start test
+--echo # All the following queries shall return 1 record
+--echo #
+
+--echo
+--echo # Masking all correct values {11...13} for column i in this result. 
+--replace_column 2 #
+SELECT MAX(pk) as max, i
+FROM t1
+ORDER BY max;
+
+--echo
+EXPLAIN
+SELECT MAX(pk) as max, i
+FROM t1
+ORDER BY max;
+
+--echo
+--echo # Only 11 is correct for collumn i in this result
+SELECT MAX(pk) as max, i
+FROM t1
+WHERE pk<2
+ORDER BY max;
+
+--echo #
+--echo # Cleanup
+--echo #
+DROP TABLE t1;
+
+--echo End of 5.1 tests

sql/opt_sum.cc

@@ -97,7 +97,8 @@ static ulonglong get_exact_record_count(TABLE_LIST *tables)
 
   @note
     This function is only called for queries with sum functions and no
-    GROUP BY part.
+    GROUP BY part. This means that the result set shall contain a single
+    row only
 
   @retval
     0                    no errors

sql/sql_class.h

@@ -2647,7 +2647,32 @@ public:
   MI_COLUMNDEF *recinfo,*start_recinfo;
   KEY *keyinfo;
   ha_rows end_write_records;
-  uint	field_count,sum_func_count,func_count;
+  /**
+    Number of normal fields in the query, including those referred to
+    from aggregate functions. Hence, "SELECT `field1`,
+    SUM(`field2`) from t1" sets this counter to 2.
+
+    @see count_field_types
+  */
+  uint	field_count; 
+  /**
+    Number of fields in the query that have functions. Includes both
+    aggregate functions (e.g., SUM) and non-aggregates (e.g., RAND).
+    Also counts functions referred to from aggregate functions, i.e.,
+    "SELECT SUM(RAND())" sets this counter to 2.
+
+    @see count_field_types
+  */
+  uint  func_count;  
+  /**
+    Number of fields in the query that have aggregate functions. Note
+    that the optimizer may choose to optimize away these fields by
+    replacing them with constants, in which case sum_func_count will
+    need to be updated.
+
+    @see opt_sum_query, count_field_types
+  */
+  uint  sum_func_count;   
   uint  hidden_field_count;
   uint	group_parts,group_length,group_null_parts;
   uint	quick_group;

sql/sql_select.cc

@@ -644,8 +644,11 @@ JOIN::prepare(Item ***rref_pointer_array,
   this->group= group_list != 0;
   unit= unit_arg;
 
+  if (tmp_table_param.sum_func_count && !group_list)
+    implicit_grouping= TRUE;
+
 #ifdef RESTRICTED_GROUP
-  if (sum_func_count && !group_list && (func_count || field_count))
+  if (implicit_grouping)
   {
     my_message(ER_WRONG_SUM_SELECT,ER(ER_WRONG_SUM_SELECT),MYF(0));
     goto err;
@@ -881,15 +884,23 @@ JOIN::optimize()
   }
 #endif
 
-  /* Optimize count(*), min() and max() */
+  /* 
-  if (tables_list && tmp_table_param.sum_func_count && ! group_list)
+     Try to optimize count(*), min() and max() to const fields if
+     there is implicit grouping (aggregate functions but no
+     group_list). In this case, the result set shall only contain one
+     row. 
+  */
+  if (tables_list && implicit_grouping)
   {
     int res;
     /*
       opt_sum_query() returns HA_ERR_KEY_NOT_FOUND if no rows match
       to the WHERE conditions,
-      or 1 if all items were resolved,
+      or 1 if all items were resolved (optimized away),
       or 0, or an error number HA_ERR_...
+
+      If all items were resolved by opt_sum_query, there is no need to
+      open any tables.
     */
     if ((res=opt_sum_query(select_lex->leaf_tables, all_fields, conds)))
     {
@@ -2024,7 +2035,7 @@ JOIN::exec()
     count_field_types(select_lex, &curr_join->tmp_table_param, 
                       *curr_all_fields, 0);
   
-  if (curr_join->group || curr_join->tmp_table_param.sum_func_count ||
+  if (curr_join->group || curr_join->implicit_grouping ||
       (procedure && (procedure->flags & PROC_GROUP)))
   {
     if (make_group_fields(this, curr_join))
@@ -10811,6 +10822,12 @@ Next_select_func setup_end_select_func(JOIN *join)
   }
   else
   {
+    /* 
+       Choose method for presenting result to user. Use end_send_group
+       if the query requires grouping (has a GROUP BY clause and/or one or
+       more aggregate functions). Use end_send if the query should not
+       be grouped.
+     */
     if ((join->sort_and_group ||
          (join->procedure && join->procedure->flags & PROC_GROUP)) &&
         !tmp_tbl->precomputed_group_by)

sql/sql_select.h

@@ -278,7 +278,14 @@ public:
   TABLE    **table,**all_tables,*sort_by_table;
   uint	   tables,const_tables;
   uint	   send_group_parts;
-  bool	   sort_and_group,first_record,full_join,group, no_field_update;
+  /**
+    Indicates that grouping will be performed on the result set during
+    query execution. This field belongs to query execution.
+
+    @see make_group_fields, alloc_group_fields, JOIN::exec
+  */
+  bool     sort_and_group; 
+  bool     first_record,full_join,group, no_field_update;
   bool	   do_send_rows;
   /**
     TRUE when we want to resume nested loop iterations when
@@ -428,6 +435,7 @@ public:
     tables= 0;
     const_tables= 0;
     join_list= 0;
+    implicit_grouping= FALSE;
     sort_and_group= 0;
     first_record= 0;
     do_send_rows= 1;
@@ -533,6 +541,11 @@ public:
                                         select_lex == unit->fake_select_lex));
   }
 private:
+  /**
+    TRUE if the query contains an aggregate function but has no GROUP
+    BY clause. 
+  */
+  bool implicit_grouping; 
   bool make_simple_join(JOIN *join, TABLE *tmp_table);
 };