Fix for bug #3874 (Group by field is not considered)

mysql-test/r/select.result:
  test result fixed
mysql-test/t/select.test:
  test case added
sql/sql_select.cc:
  do the same for nullable

mysql-test/r/select.result

@@ -2441,3 +2441,39 @@ a	c
 SELECT a, b AS c FROM t1 ORDER BY b+1;
 a	c
 drop table t1;
+DROP TABLE IF EXISTS t1, t2;
+CREATE TABLE t1 ( a INT NOT NULL, b INT NOT NULL, UNIQUE idx (a,b) );
+INSERT INTO t1 VALUES (1,1),(1,2),(1,3),(1,4);
+CREATE TABLE t2 ( a INT NOT NULL, b INT NOT NULL, c INT );
+INSERT INTO t2 VALUES ( 1,10,1), (1,10,2), (1,11,1), (1,11,2), (1,2,1), (1,2,2),
+(1,2,3);
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2 LEFT JOIN
+t1 ON t2.a = t1.a AND t2.b = t1.b GROUP BY a, b, c;
+a	b	c	d
+1	2	1	1
+1	2	2	1
+1	2	3	1
+1	10		2
+1	11		2
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2 LEFT JOIN
+t1 ON t2.a = t1.a AND t2.b = t1.b GROUP BY t1.a, t1.b, c;
+a	b	c	d
+1	10		4
+1	2	1	1
+1	2	2	1
+1	2	3	1
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2 LEFT JOIN
+t1 ON t2.a = t1.a AND t2.b = t1.b GROUP BY t2.a, t2.b, c;
+a	b	c	d
+1	2	1	1
+1	2	2	1
+1	2	3	1
+1	10		2
+1	11		2
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2,t1
+WHERE t2.a = t1.a AND t2.b = t1.b GROUP BY a, b, c;
+a	b	c	d
+1	2	1	1
+1	2	2	1
+1	2	3	1
+DROP TABLE IF EXISTS t1, t2;

mysql-test/t/select.test

@@ -1992,3 +1992,24 @@ CREATE TABLE t1 (a INT, b INT);
 SELECT a, b AS c FROM t1 ORDER BY c+1;
 SELECT a, b AS c FROM t1 ORDER BY b+1;
 drop table t1;
+
+#
+# Bug #3874 (function in GROUP and LEFT JOIN)
+#
+
+DROP TABLE IF EXISTS t1, t2;
+CREATE TABLE t1 ( a INT NOT NULL, b INT NOT NULL, UNIQUE idx (a,b) );
+INSERT INTO t1 VALUES (1,1),(1,2),(1,3),(1,4);
+CREATE TABLE t2 ( a INT NOT NULL, b INT NOT NULL, c INT );
+INSERT INTO t2 VALUES ( 1,10,1), (1,10,2), (1,11,1), (1,11,2), (1,2,1), (1,2,2),
+ (1,2,3);
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2 LEFT JOIN
+t1 ON t2.a = t1.a AND t2.b = t1.b GROUP BY a, b, c;
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2 LEFT JOIN
+t1 ON t2.a = t1.a AND t2.b = t1.b GROUP BY t1.a, t1.b, c;
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2 LEFT JOIN
+t1 ON t2.a = t1.a AND t2.b = t1.b GROUP BY t2.a, t2.b, c;
+SELECT t2.a, t2.b, IF(t1.b IS NULL,'',c) AS c, COUNT(*) AS d FROM t2,t1
+WHERE t2.a = t1.a AND t2.b = t1.b GROUP BY a, b, c;
+DROP TABLE IF EXISTS t1, t2;
+

sql/sql_select.cc

@@ -3115,7 +3115,7 @@ eq_ref_table(JOIN *join, ORDER *start_order, JOIN_TAB *tab)
   tab->cached_eq_ref_table=1;
   if (tab->type == JT_CONST)			// We can skip const tables
     return (tab->eq_ref_table=1);		/* purecov: inspected */
-  if (tab->type != JT_EQ_REF)
+  if (tab->type != JT_EQ_REF || tab->table->maybe_null)
     return (tab->eq_ref_table=0);		// We must use this
   Item **ref_item=tab->ref.items;
   Item **end=ref_item+tab->ref.key_parts;