diff --git a/mysql-test/r/innodb_mysql.result b/mysql-test/r/innodb_mysql.result
index 11c7c2aedc9..df1c2165c45 100644
--- a/mysql-test/r/innodb_mysql.result
+++ b/mysql-test/r/innodb_mysql.result
@@ -676,4 +676,55 @@ INSERT INTO t1 VALUES (1);
 switch to connection default
 SET AUTOCOMMIT=default;
 DROP TABLE t1,t2;
+CREATE TABLE t1 (
+id int NOT NULL auto_increment PRIMARY KEY,
+b int NOT NULL,
+c datetime NOT NULL,
+INDEX idx_b(b),
+INDEX idx_c(c)
+) ENGINE=InnoDB;
+CREATE TABLE t2 (
+b int NOT NULL auto_increment PRIMARY KEY,
+c datetime NOT NULL
+) ENGINE= MyISAM;
+INSERT INTO t2(c) VALUES ('2007-01-01');
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t1(b,c) SELECT b,c FROM t2;
+UPDATE t2 SET c='2007-01-02';
+INSERT INTO t1(b,c) SELECT b,c FROM t2;
+UPDATE t2 SET c='2007-01-03';
+INSERT INTO t1(b,c) SELECT b,c FROM t2;
+set @@sort_buffer_size=8192;
+SELECT COUNT(*) FROM t1;
+COUNT(*)
+3072
+EXPLAIN 
+SELECT COUNT(*) FROM t1 
+WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	idx_b,idx_c	NULL	NULL	NULL	3263	Using where
+SELECT COUNT(*) FROM t1 
+WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+COUNT(*)
+3072
+EXPLAIN 
+SELECT COUNT(*) FROM t1 FORCE INDEX(idx_b, idx_c) 
+WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index_merge	idx_b,idx_c	idx_c,idx_b	8,4	NULL	3262	Using sort_union(idx_c,idx_b); Using where
+SELECT COUNT(*) FROM t1 FORCE INDEX(idx_b, idx_c)
+WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+COUNT(*)
+3072
+set @@sort_buffer_size=default;
+DROP TABLE t1,t2;
 End of 5.0 tests
diff --git a/mysql-test/t/innodb_mysql.test b/mysql-test/t/innodb_mysql.test
index 25ba9a258ff..eb995a748a5 100644
--- a/mysql-test/t/innodb_mysql.test
+++ b/mysql-test/t/innodb_mysql.test
@@ -670,5 +670,59 @@ DISCONNECT c1;
 DISCONNECT c2;
 DROP TABLE t1,t2;
 
+#
+# Bug #25798: a query with forced index merge returns wrong result 
+#
+
+CREATE TABLE t1 (
+  id int NOT NULL auto_increment PRIMARY KEY,
+  b int NOT NULL,
+  c datetime NOT NULL,
+  INDEX idx_b(b),
+  INDEX idx_c(c)
+) ENGINE=InnoDB;
+
+CREATE TABLE t2 (
+  b int NOT NULL auto_increment PRIMARY KEY,
+  c datetime NOT NULL
+) ENGINE= MyISAM;
+
+INSERT INTO t2(c) VALUES ('2007-01-01');
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+INSERT INTO t2(c) SELECT c FROM t2;
+
+INSERT INTO t1(b,c) SELECT b,c FROM t2;
+UPDATE t2 SET c='2007-01-02';
+INSERT INTO t1(b,c) SELECT b,c FROM t2;
+UPDATE t2 SET c='2007-01-03';
+INSERT INTO t1(b,c) SELECT b,c FROM t2;
+
+set @@sort_buffer_size=8192;
+
+SELECT COUNT(*) FROM t1;
+
+EXPLAIN 
+SELECT COUNT(*) FROM t1 
+  WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+SELECT COUNT(*) FROM t1 
+  WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+
+EXPLAIN 
+SELECT COUNT(*) FROM t1 FORCE INDEX(idx_b, idx_c) 
+  WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+SELECT COUNT(*) FROM t1 FORCE INDEX(idx_b, idx_c)
+  WHERE (c >= '2007-01-02' AND c <= '2007-01-03') OR b >= 1;
+
+set @@sort_buffer_size=default;
+
+DROP TABLE t1,t2;
 
 --echo End of 5.0 tests
diff --git a/sql/filesort.cc b/sql/filesort.cc
index d518ddbb117..f8868ed6927 100644
--- a/sql/filesort.cc
+++ b/sql/filesort.cc
@@ -1052,6 +1052,7 @@ int merge_buffers(SORTPARAM *param, IO_CACHE *from_file,
   BUFFPEK *buffpek;
   QUEUE queue;
   qsort2_cmp cmp;
+  void *first_cmp_arg;
   volatile THD::killed_state *killed= &current_thd->killed;
   THD::killed_state not_killable;
   DBUG_ENTER("merge_buffers");
@@ -1077,9 +1078,18 @@ int merge_buffers(SORTPARAM *param, IO_CACHE *from_file,
   /* The following will fire if there is not enough space in sort_buffer */
   DBUG_ASSERT(maxcount!=0);
   
+  if (param->unique_buff)
+  {
+    cmp= param->compare;
+    first_cmp_arg= (void *) &param->cmp_context;
+  }
+  else
+  {
+    cmp= get_ptr_compare(sort_length);
+    first_cmp_arg= (void*) &sort_length;
+  }
   if (init_queue(&queue, (uint) (Tb-Fb)+1, offsetof(BUFFPEK,key), 0,
-                 (queue_compare) (cmp= get_ptr_compare(sort_length)),
-                 (void*) &sort_length))
+                 (queue_compare) cmp, first_cmp_arg))
     DBUG_RETURN(1);                                /* purecov: inspected */
   for (buffpek= Fb ; buffpek <= Tb ; buffpek++)
   {
@@ -1132,7 +1142,7 @@ int merge_buffers(SORTPARAM *param, IO_CACHE *from_file,
       buffpek= (BUFFPEK*) queue_top(&queue);
       if (cmp)                                        // Remove duplicates
       {
-        if (!(*cmp)(&sort_length, &(param->unique_buff),
+        if (!(*cmp)(first_cmp_arg, &(param->unique_buff),
                     (uchar**) &buffpek->key))
               goto skip_duplicate;
             memcpy(param->unique_buff, (uchar*) buffpek->key, rec_length);
@@ -1184,7 +1194,7 @@ int merge_buffers(SORTPARAM *param, IO_CACHE *from_file,
   */
   if (cmp)
   {
-    if (!(*cmp)(&sort_length, &(param->unique_buff), (uchar**) &buffpek->key))
+    if (!(*cmp)(first_cmp_arg, &(param->unique_buff), (uchar**) &buffpek->key))
     {
       buffpek->key+= rec_length;         // Remove duplicate
       --buffpek->mem_count;
diff --git a/sql/sql_sort.h b/sql/sql_sort.h
index da28ca07e2c..1e9322f7f5b 100644
--- a/sql/sql_sort.h
+++ b/sql/sql_sort.h
@@ -50,6 +50,12 @@ typedef struct st_buffpek {		/* Struktur om sorteringsbuffrarna */
   ulong max_keys;			/* Max keys in buffert */
 } BUFFPEK;
 
+struct BUFFPEK_COMPARE_CONTEXT
+{
+  qsort_cmp2 key_compare;
+  void *key_compare_arg;
+};
+
 typedef struct st_sort_param {
   uint rec_length;          /* Length of sorted records */
   uint sort_length;			/* Length of sorted columns */
@@ -65,6 +71,9 @@ typedef struct st_sort_param {
   uchar *unique_buff;
   bool not_killable;
   char* tmp_buffer;
+  /* The fields below are used only by Unique class */
+  qsort2_cmp compare;
+  BUFFPEK_COMPARE_CONTEXT cmp_context;
 } SORTPARAM;
 
 
diff --git a/sql/uniques.cc b/sql/uniques.cc
index 9eb827f62a3..7c197d2a2e9 100644
--- a/sql/uniques.cc
+++ b/sql/uniques.cc
@@ -361,17 +361,12 @@ Unique::reset()
 }
 
 /*
-  The comparison function, passed to queue_init() in merge_walk() must
+  The comparison function, passed to queue_init() in merge_walk() and in
+  merge_buffers() when the latter is called from Uniques::get() must
   use comparison function of Uniques::tree, but compare members of struct
   BUFFPEK.
 */
 
-struct BUFFPEK_COMPARE_CONTEXT
-{
-  qsort_cmp2 key_compare;
-  void *key_compare_arg;
-};
-
 C_MODE_START
 
 static int buffpek_compare(void *arg, byte *key_ptr1, byte *key_ptr2)
@@ -630,6 +625,10 @@ bool Unique::get(TABLE *table)
   sort_param.unique_buff= sort_buffer+(sort_param.keys*
 				       sort_param.sort_length);
 
+  sort_param.compare= (qsort2_cmp) buffpek_compare;
+  sort_param.cmp_context.key_compare= tree.compare;
+  sort_param.cmp_context.key_compare_arg= tree.custom_arg;
+
   /* Merge the buffers to one file, removing duplicates */
   if (merge_many_buff(&sort_param,sort_buffer,file_ptr,&maxbuffer,&file))
     goto err;