Fixed a bug in optimiser with MERGE tables with non-unique values

with aggregating functions.

This consistently crashed Mysql

Docs/manual.texi

@@ -49456,7 +49456,12 @@ Our TODO section contains what we plan to have in 4.0. @xref{TODO MySQL 4.0}.
 
 @itemize @bullet
 @item
-Fixed a bug in optimiser when a range specified makes index grouping impossible 
+Fixed a bug in optimiser with merge tables when non-uniques values are 
+used in summing up. 
+This consistently crashed MySQL.
+@item
+Fixed a bug in optimiser when a range specified makes index grouping impossible. 
+This consistently crashed MySQL.
 @item
 Fixed a rare bug when fulltext index is present and no tables are used
 @item

mysql-test/r/merge.result

@@ -462,3 +462,12 @@ a	b
 6	1
 6	2
 drop table if exists t6, t5, t4, t3, t2, t1;
+CREATE TABLE t1 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t1 VALUES (1,1), (2,1);
+CREATE TABLE t2 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t2 VALUES (1,2), (2,2);
+CREATE TABLE t (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  KEY a (a,b)) TYPE=MRG_MyISAM UNION=(t1,t2);
+select max(b) from t where a = 2;
+max(b)
+NULL
+drop table if exists t,t1,t2;

mysql-test/t/merge.test

@@ -168,3 +168,10 @@ select * from t3 order by a,b;
 select * from t4 order by a,b;
 select * from t5 order by a,b;
 drop table if exists t6, t5, t4, t3, t2, t1;
+CREATE TABLE t1 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t1 VALUES (1,1), (2,1);
+CREATE TABLE t2 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) TYPE=MyISAM;
+INSERT INTO t2 VALUES (1,2), (2,2);
+CREATE TABLE t (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  KEY a (a,b)) TYPE=MRG_MyISAM UNION=(t1,t2);
+select max(b) from t where a = 2;
+drop table if exists t,t1,t2;

sql/opt_sum.cc

@@ -152,9 +152,10 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
 	    error=table->file->index_last(table->record[0]) !=0;
 	  else
 	  {
-	    (void) table->file->index_read(table->record[0], key_buff,
+	    error= table->file->index_read(table->record[0], key_buff,
 					   ref.key_length,
 					   HA_READ_AFTER_KEY);
+	    if (!error)
 	      error=table->file->index_prev(table->record[0]) ||
 		key_cmp(table,key_buff,ref.key,ref.key_length);
 	  }