Fix for BUG#12882 - min/max inconsistent on empty table.

The problem was in that the MIN/MAX optimization in opt_sum_query was
replacing MIN/MAX functions with their constant argument without
taking into account that a query has no result rows.


mysql-test/r/func_group.result:
  Test for BUG#12882.
mysql-test/t/func_group.test:
  Test for BUG#12882.
sql/item_sum.cc:
  If it is known that a query has no result rows, do not call add()
  via the call to Item_sum::no_rows_in_result() which calls reset().
  Instead directly call clear() so that the MIN and MAX functions
  produce NULL when there are no result rows.
sql/opt_sum.cc:
  * Do not apply MIN/MAX optimization when the operand of MIN/MAX is
    a constant if it can't be determined whether the query has any
    result rows. The reason is that if the query has result rows,
    then the result of MIN/MAX is its constant argument, but if the
    query result is empty, then the result of MIN/MAX must be NULL
    irrespective of its argument.
  
  * The patch also simplifies a bit the branch that hadles COUNT().

mysql-test/r/func_group.result

@@ -780,3 +780,139 @@ SELECT MAX(id) FROM t1 WHERE id < 3 AND a=2 AND b=6;
 MAX(id)
 NULL
 DROP TABLE t1;
+create table t1m (a int) engine=myisam;
+create table t1i (a int) engine=innodb;
+create table t2m (a int) engine=myisam;
+create table t2i (a int) engine=innodb;
+insert into t2m values (5);
+insert into t2i values (5);
+select min(a) from t1m;
+min(a)
+NULL
+select min(7) from t1m;
+min(7)
+NULL
+select min(7) from DUAL;
+min(7)
+NULL
+explain select min(7) from t2m join t1m;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+select min(7) from t2m join t1m;
+min(7)
+NULL
+select max(a) from t1m;
+max(a)
+NULL
+select max(7) from t1m;
+max(7)
+NULL
+select max(7) from DUAL;
+max(7)
+NULL
+explain select max(7) from t2m join t1m;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+select max(7) from t2m join t1m;
+max(7)
+NULL
+select 1, min(a) from t1m where a=99;
+1	min(a)
+1	NULL
+select 1, min(a) from t1m where 1=99;
+1	min(a)
+1	NULL
+select 1, min(1) from t1m where a=99;
+1	min(1)
+select 1, min(1) from t1m where 1=99;
+1	min(1)
+1	NULL
+select 1, max(a) from t1m where a=99;
+1	max(a)
+1	NULL
+select 1, max(a) from t1m where 1=99;
+1	max(a)
+1	NULL
+select 1, max(1) from t1m where a=99;
+1	max(1)
+select 1, max(1) from t1m where 1=99;
+1	max(1)
+1	NULL
+select min(a) from t1i;
+min(a)
+NULL
+select min(7) from t1i;
+min(7)
+NULL
+select min(7) from DUAL;
+min(7)
+NULL
+explain select min(7) from t2i join t1i;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2i	ALL	NULL	NULL	NULL	NULL	1	
+1	SIMPLE	t1i	ALL	NULL	NULL	NULL	NULL	1	
+select min(7) from t2i join t1i;
+min(7)
+NULL
+select max(a) from t1i;
+max(a)
+NULL
+select max(7) from t1i;
+max(7)
+NULL
+select max(7) from DUAL;
+max(7)
+NULL
+explain select max(7) from t2i join t1i;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2i	ALL	NULL	NULL	NULL	NULL	1	
+1	SIMPLE	t1i	ALL	NULL	NULL	NULL	NULL	1	
+select max(7) from t2i join t1i;
+max(7)
+NULL
+select 1, min(a) from t1i where a=99;
+1	min(a)
+1	NULL
+select 1, min(a) from t1i where 1=99;
+1	min(a)
+1	NULL
+select 1, min(1) from t1i where a=99;
+1	min(1)
+1	NULL
+select 1, min(1) from t1i where 1=99;
+1	min(1)
+1	NULL
+select 1, max(a) from t1i where a=99;
+1	max(a)
+1	NULL
+select 1, max(a) from t1i where 1=99;
+1	max(a)
+1	NULL
+select 1, max(1) from t1i where a=99;
+1	max(1)
+1	NULL
+select 1, max(1) from t1i where 1=99;
+1	max(1)
+1	NULL
+explain select count(*), min(7), max(7) from t1m, t1i;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1m	system	NULL	NULL	NULL	NULL	0	const row not found
+1	SIMPLE	t1i	ALL	NULL	NULL	NULL	NULL	1	
+select count(*), min(7), max(7) from t1m, t1i;
+count(*)	min(7)	max(7)
+0	NULL	NULL
+explain select count(*), min(7), max(7) from t1m, t2i;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1m	system	NULL	NULL	NULL	NULL	0	const row not found
+1	SIMPLE	t2i	ALL	NULL	NULL	NULL	NULL	1	
+select count(*), min(7), max(7) from t1m, t2i;
+count(*)	min(7)	max(7)
+0	NULL	NULL
+explain select count(*), min(7), max(7) from t2m, t1i;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2m	system	NULL	NULL	NULL	NULL	1	
+1	SIMPLE	t1i	ALL	NULL	NULL	NULL	NULL	1	
+select count(*), min(7), max(7) from t2m, t1i;
+count(*)	min(7)	max(7)
+0	NULL	NULL
+drop table t1m, t1i, t2m, t2i;

mysql-test/t/func_group.test

@@ -527,4 +527,73 @@ INSERT INTO t1 VALUES
 SELECT MAX(id) FROM t1 WHERE id < 3 AND a=2 AND b=6;
 DROP TABLE t1;
 
+#
+# Bug #12882  	min/max inconsistent on empty table
+#
+
+create table t1m (a int) engine=myisam;
+create table t1i (a int) engine=innodb;
+create table t2m (a int) engine=myisam;
+create table t2i (a int) engine=innodb;
+insert into t2m values (5);
+insert into t2i values (5);
+
+# test with MyISAM
+select min(a) from t1m;
+select min(7) from t1m;
+select min(7) from DUAL;
+explain select min(7) from t2m join t1m;
+select min(7) from t2m join t1m;
+
+select max(a) from t1m;
+select max(7) from t1m;
+select max(7) from DUAL;
+explain select max(7) from t2m join t1m;
+select max(7) from t2m join t1m;
+
+select 1, min(a) from t1m where a=99;
+select 1, min(a) from t1m where 1=99;
+select 1, min(1) from t1m where a=99;
+select 1, min(1) from t1m where 1=99;
+
+select 1, max(a) from t1m where a=99;
+select 1, max(a) from t1m where 1=99;
+select 1, max(1) from t1m where a=99;
+select 1, max(1) from t1m where 1=99;
+
+# test with InnoDB
+select min(a) from t1i;
+select min(7) from t1i;
+select min(7) from DUAL;
+explain select min(7) from t2i join t1i;
+select min(7) from t2i join t1i;
+
+select max(a) from t1i;
+select max(7) from t1i;
+select max(7) from DUAL;
+explain select max(7) from t2i join t1i;
+select max(7) from t2i join t1i;
+
+select 1, min(a) from t1i where a=99;
+select 1, min(a) from t1i where 1=99;
+select 1, min(1) from t1i where a=99;
+select 1, min(1) from t1i where 1=99;
+
+select 1, max(a) from t1i where a=99;
+select 1, max(a) from t1i where 1=99;
+select 1, max(1) from t1i where a=99;
+select 1, max(1) from t1i where 1=99;
+
+# mixed MyISAM/InnoDB test
+explain select count(*), min(7), max(7) from t1m, t1i;
+select count(*), min(7), max(7) from t1m, t1i;
+
+explain select count(*), min(7), max(7) from t1m, t2i;
+select count(*), min(7), max(7) from t1m, t2i;
+
+explain select count(*), min(7), max(7) from t2m, t1i;
+select count(*), min(7), max(7) from t2m, t1i;
+
+drop table t1m, t1i, t2m, t2i;
+
 # End of 4.1 tests

sql/item_sum.cc

@@ -552,8 +552,8 @@ void Item_sum_hybrid::cleanup()
 
 void Item_sum_hybrid::no_rows_in_result()
 {
-  Item_sum::no_rows_in_result();
   was_values= FALSE;
+  clear();
 }
 
 

sql/opt_sum.cc

@@ -80,6 +80,8 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
   List_iterator_fast<Item> it(all_fields);
   int const_result= 1;
   bool recalc_const_item= 0;
+  longlong count= 1;
+  bool is_exact_count= TRUE;
   table_map removed_tables= 0, outer_tables= 0, used_tables= 0;
   table_map where_tables= 0;
   Item *item;
@@ -88,9 +90,13 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
   if (conds)
     where_tables= conds->used_tables();
 
-  /* Don't replace expression on a table that is part of an outer join */
+  /*
+    Analyze outer join dependencies, and, if possible, compute the number
+    of returned rows.
+  */
   for (TABLE_LIST *tl=tables; tl ; tl= tl->next)
   {
+    /* Don't replace expression on a table that is part of an outer join */
     if (tl->on_expr)
     {
       outer_tables|= tl->table->map;
@@ -102,15 +108,31 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
           WHERE t2.field IS NULL;
       */
       if (tl->table->map & where_tables)
-        return 0;
+        const_result= 0;
     }
     else
       used_tables|= tl->table->map;
+
+    /*
+      If the storage manager of 'tl' gives exact row count, compute the total
+      number of rows. If there are no outer table dependencies, this count
+      may be used as the real count.
+    */
+    if (tl->table->file->table_flags() & HA_NOT_EXACT_COUNT)
+      is_exact_count= FALSE;
+    else
+    {
+      tl->table->file->info(HA_STATUS_VARIABLE | HA_STATUS_NO_LOCK);
+      count*= tl->table->file->records;
+    }
   }
 
+  if (!const_result)
+    return 0;
+
   /*
-    Iterate through item is select part and replace COUNT(), MIN() and MAX()
-    with constants (if possible)
+    Iterate through all items in the SELECT clause and replace
+    COUNT(), MIN() and MAX() with constants (if possible).
   */
 
   while ((item= it++))
@@ -122,29 +144,15 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
       case Item_sum::COUNT_FUNC:
         /*
           If the expr in count(expr) can never be null we can change this
-          to the number of rows in the tables
+          to the number of rows in the tables if this number is exact and
+          there are no outer joins.
         */
-        if (!conds && !((Item_sum_count*) item)->args[0]->maybe_null)
-        {
-          longlong count= 1;
-          TABLE_LIST *table;
-          for (table=tables ; table ; table=table->next)
-          {
-            if (outer_tables || (table->table->file->table_flags() &
-                                 HA_NOT_EXACT_COUNT))
-            {
-              const_result= 0;			// Can't optimize left join
-              break;
-            }
-            tables->table->file->info(HA_STATUS_VARIABLE | HA_STATUS_NO_LOCK);
-            count*= table->table->file->records;
-          }
-          if (!table)
+        if (!conds && !((Item_sum_count*) item)->args[0]->maybe_null &&
+            !outer_tables && is_exact_count)
         {
             ((Item_sum_count*) item)->make_const(count);
             recalc_const_item= 1;
         }
-        }
         else
           const_result= 0;
         break;
@@ -210,12 +218,27 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
 	  }
           removed_tables|= table->map;
         }
-        else if (!expr->const_item())		// This is VERY seldom false
+        else if (!expr->const_item() || !is_exact_count)
         {
+          /*
+            The optimization is not applicable in both cases:
+            (a) 'expr' is a non-constant expression. Then we can't
+            replace 'expr' by a constant.
+            (b) 'expr' is a costant. According to ANSI, MIN/MAX must return
+            NULL if the query does not return any rows. Thus, if we are not
+            able to determine if the query returns any rows, we can't apply
+            the optimization and replace MIN/MAX with a constant.
+          */
           const_result= 0;
           break;
         }
-        ((Item_sum_min*) item_sum)->reset();
+        if (!count)
+        {
+          /* If count != 1, then we know that is_exact_count == TRUE. */
+          ((Item_sum_min*) item_sum)->clear(); /* Set to NULL. */
+        }
+        else
+          ((Item_sum_min*) item_sum)->reset(); /* Set to the constant value. */
         ((Item_sum_min*) item_sum)->make_const();
         recalc_const_item= 1;
         break;
@@ -282,13 +305,28 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
 	  }
           removed_tables|= table->map;
         }
-        else if (!expr->const_item())		// This is VERY seldom false
+        else if (!expr->const_item() || !is_exact_count)
         {
+          /*
+            The optimization is not applicable in both cases:
+            (a) 'expr' is a non-constant expression. Then we can't
+            replace 'expr' by a constant.
+            (b) 'expr' is a costant. According to ANSI, MIN/MAX must return
+            NULL if the query does not return any rows. Thus, if we are not
+            able to determine if the query returns any rows, we can't apply
+            the optimization and replace MIN/MAX with a constant.
+          */
           const_result= 0;
           break;
         }
-        ((Item_sum_min*) item_sum)->reset();
-        ((Item_sum_min*) item_sum)->make_const();
+        if (!count)
+        {
+          /* If count != 1, then we know that is_exact_count == TRUE. */
+          ((Item_sum_max*) item_sum)->clear(); /* Set to NULL. */
+        }
+        else
+          ((Item_sum_max*) item_sum)->reset(); /* Set to the constant value. */
+        ((Item_sum_max*) item_sum)->make_const();
         recalc_const_item= 1;
         break;
       }