Fixed bug #29205.

When a UNION statement forced conversion of an UTF8
charset value to a binary charset value, the byte
length of the result values was truncated to the
CHAR_LENGTH of the original UTF8 value.

mysql-test/r/ctype_utf8.result

@@ -1657,3 +1657,40 @@ colA	colB	colA	colB
 1	foo	1	foo
 2	foo bar	2	foo bar
 DROP TABLE t1, t2;
+SELECT 'н1234567890' UNION SELECT _binary '1';
+н1234567890
+н1234567890
+1
+SELECT 'н1234567890' UNION SELECT 1;
+н1234567890
+н1234567890
+1
+SELECT '1' UNION SELECT 'н1234567890';
+1
+1
+н1234567890
+SELECT 1 UNION SELECT 'н1234567890';
+1
+1
+н1234567890
+CREATE TABLE t1 (c VARCHAR(11)) CHARACTER SET utf8;
+CREATE TABLE t2 (b CHAR(1) CHARACTER SET binary, i INT);
+INSERT INTO t1 (c) VALUES ('н1234567890');
+INSERT INTO t2 (b, i) VALUES ('1', 1);
+SELECT c FROM t1 UNION SELECT b FROM t2;
+c
+н1234567890
+1
+SELECT c FROM t1 UNION SELECT i FROM t2;
+c
+н1234567890
+1
+SELECT b FROM t2 UNION SELECT c FROM t1;
+b
+1
+н1234567890
+SELECT i FROM t2 UNION SELECT c FROM t1;
+i
+1
+н1234567890
+DROP TABLE t1, t2;

mysql-test/t/ctype_utf8.test

@@ -1338,3 +1338,28 @@ INSERT INTO t2 (colA, colB) VALUES (1, 'foo'),(2, 'foo bar');
 SELECT * FROM t1 JOIN t2 ON t1.colA=t2.colA AND t1.colB=t2.colB
 WHERE t1.colA < 3;
 DROP TABLE t1, t2;
+
+#
+# Bug#29205: truncation of UTF8 values when the UNION statement
+# forces collation to the binary charset
+#
+
+SELECT 'н1234567890' UNION SELECT _binary '1';
+SELECT 'н1234567890' UNION SELECT 1;
+
+SELECT '1' UNION SELECT 'н1234567890';
+SELECT 1 UNION SELECT 'н1234567890';
+
+CREATE TABLE t1 (c VARCHAR(11)) CHARACTER SET utf8;
+CREATE TABLE t2 (b CHAR(1) CHARACTER SET binary, i INT);
+
+INSERT INTO t1 (c) VALUES ('н1234567890');
+INSERT INTO t2 (b, i) VALUES ('1', 1);
+
+SELECT c FROM t1 UNION SELECT b FROM t2;
+SELECT c FROM t1 UNION SELECT i FROM t2;
+
+SELECT b FROM t2 UNION SELECT c FROM t1;
+SELECT i FROM t2 UNION SELECT c FROM t1;
+
+DROP TABLE t1, t2;

sql/item.cc

@@ -6595,9 +6595,15 @@ bool Item_type_holder::join_types(THD *thd, Item *item)
       expansion of the size of the values because of character set
       conversions.
      */
-    max_length= max(old_max_chars * collation.collation->mbmaxlen,
-                    display_length(item) / item->collation.collation->mbmaxlen *
-                    collation.collation->mbmaxlen);
+    if (collation.collation != &my_charset_bin)
+    {
+      max_length= max(old_max_chars * collation.collation->mbmaxlen,
+                      display_length(item) /
+                      item->collation.collation->mbmaxlen *
+                      collation.collation->mbmaxlen);
+    }
+    else
+      set_if_bigger(max_length, display_length(item));
     break;
   }
   case REAL_RESULT: