BUG#26881 - Large MERGE tables report incorrect specification when no

            differences in tables
Certain merge tables were wrongly reported as having incorrect definition:
- Some fields that are 1 byte long (e.g. TINYINT, CHAR(1)), might
  be internally casted (in certain cases) to a different type on a
  storage engine layer. (affects 4.1 and up)
- If tables in a merge (and a MERGE table itself) had short VARCHAR column (less
  than 4 bytes) and at least one (but not all) tables were ALTER'ed (even to an
  identical table: ALTER TABLE xxx ENGINE=yyy), table definitions went ouf of
  sync. (affects 4.1 only)

This is fixed by relaxing a check for underlying conformance and setting
field type to FIELD_TYPE_STRING in case varchar is shorter than 4
when a table is created.

5 files changed, 77 insertions, 2 deletions

diff --git a/myisam/mi_create.c b/myisam/mi_create.c
index da9e0887b00..5e363a4e670 100644
--- a/myisam/mi_create.c
+++ b/myisam/mi_create.c
@@ -158,6 +158,10 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
       rec--;
       if (rec->type == (int) FIELD_SKIP_ZERO && rec->length == 1)
       {
+        /*
+          NOTE1: here we change a field type FIELD_SKIP_ZERO ->
+          FIELD_NORMAL
+        */
 	rec->type=(int) FIELD_NORMAL;
 	packed--;
 	min_pack_length++;
diff --git a/mysql-test/r/merge.result b/mysql-test/r/merge.result
index 33de735d714..00d8aa3d586 100644
--- a/mysql-test/r/merge.result
+++ b/mysql-test/r/merge.result
@@ -806,3 +806,16 @@ CREATE TABLE t2(c1 INT) ENGINE=MERGE UNION=(t1);
 INSERT DELAYED INTO t2 VALUES(1);
 ERROR HY000: Table storage engine for 't2' doesn't have this option
 DROP TABLE t1, t2;
+CREATE TABLE t1(c1 VARCHAR(1));
+CREATE TABLE m1 LIKE t1;
+ALTER TABLE m1 ENGINE=MERGE UNION=(t1);
+SELECT * FROM m1;
+c1
+DROP TABLE t1, m1;
+CREATE TABLE t1(c1 VARCHAR(4), c2 TINYINT, c3 TINYINT, c4 TINYINT,
+c5 TINYINT, c6 TINYINT, c7 TINYINT, c8 TINYINT, c9 TINYINT);
+CREATE TABLE m1 LIKE t1;
+ALTER TABLE m1 ENGINE=MERGE UNION=(t1);
+SELECT * FROM m1;
+c1	c2	c3	c4	c5	c6	c7	c8	c9
+DROP TABLE t1, m1;
diff --git a/mysql-test/t/merge.test b/mysql-test/t/merge.test
index f759d20dd80..032e80ecc93 100644
--- a/mysql-test/t/merge.test
+++ b/mysql-test/t/merge.test
@@ -437,4 +437,21 @@ CREATE TABLE t2(c1 INT) ENGINE=MERGE UNION=(t1);
 INSERT DELAYED INTO t2 VALUES(1);
 DROP TABLE t1, t2;
 
+#
+# BUG#26881 - Large MERGE tables report incorrect specification when no
+#             differences in tables
+#
+CREATE TABLE t1(c1 VARCHAR(1));
+CREATE TABLE m1 LIKE t1;
+ALTER TABLE m1 ENGINE=MERGE UNION=(t1);
+SELECT * FROM m1;
+DROP TABLE t1, m1;
+
+CREATE TABLE t1(c1 VARCHAR(4), c2 TINYINT, c3 TINYINT, c4 TINYINT,
+                c5 TINYINT, c6 TINYINT, c7 TINYINT, c8 TINYINT, c9 TINYINT);
+CREATE TABLE m1 LIKE t1;
+ALTER TABLE m1 ENGINE=MERGE UNION=(t1);
+SELECT * FROM m1;
+DROP TABLE t1, m1;
+
 # End of 4.1 tests
diff --git a/sql/ha_myisam.cc b/sql/ha_myisam.cc
index 2cec03a51db..c6802ffe53c 100644
--- a/sql/ha_myisam.cc
+++ b/sql/ha_myisam.cc
@@ -304,6 +304,12 @@ int table2myisam(TABLE *table_arg, MI_KEYDEF **keydef_out,
   RETURN VALUE
     0 - Equal definitions.
     1 - Different definitions.
+
+  TODO
+    - compare FULLTEXT keys;
+    - compare SPATIAL keys;
+    - compare FIELD_SKIP_ZERO which is converted to FIELD_NORMAL correctly
+      (should be corretly detected in table2myisam).
 */
 
 int check_definition(MI_KEYDEF *t1_keyinfo, MI_COLUMNDEF *t1_recinfo,
@@ -329,6 +335,28 @@ int check_definition(MI_KEYDEF *t1_keyinfo, MI_COLUMNDEF *t1_recinfo,
   {
     HA_KEYSEG *t1_keysegs= t1_keyinfo[i].seg;
     HA_KEYSEG *t2_keysegs= t2_keyinfo[i].seg;
+    if (t1_keyinfo[i].flag & HA_FULLTEXT && t2_keyinfo[i].flag & HA_FULLTEXT)
+      continue;
+    else if (t1_keyinfo[i].flag & HA_FULLTEXT ||
+             t2_keyinfo[i].flag & HA_FULLTEXT)
+    {
+       DBUG_PRINT("error", ("Key %d has different definition", i));
+       DBUG_PRINT("error", ("t1_fulltext= %d, t2_fulltext=%d",
+                            test(t1_keyinfo[i].flag & HA_FULLTEXT),
+                            test(t2_keyinfo[i].flag & HA_FULLTEXT)));
+       DBUG_RETURN(1);
+    }
+    if (t1_keyinfo[i].flag & HA_SPATIAL && t2_keyinfo[i].flag & HA_SPATIAL)
+      continue;
+    else if (t1_keyinfo[i].flag & HA_SPATIAL ||
+             t2_keyinfo[i].flag & HA_SPATIAL)
+    {
+       DBUG_PRINT("error", ("Key %d has different definition", i));
+       DBUG_PRINT("error", ("t1_spatial= %d, t2_spatial=%d",
+                            test(t1_keyinfo[i].flag & HA_SPATIAL),
+                            test(t2_keyinfo[i].flag & HA_SPATIAL)));
+       DBUG_RETURN(1);
+    }
     if (t1_keyinfo[i].keysegs != t2_keyinfo[i].keysegs ||
         t1_keyinfo[i].key_alg != t2_keyinfo[i].key_alg)
     {
@@ -365,7 +393,14 @@ int check_definition(MI_KEYDEF *t1_keyinfo, MI_COLUMNDEF *t1_recinfo,
   {
     MI_COLUMNDEF *t1_rec= &t1_recinfo[i];
     MI_COLUMNDEF *t2_rec= &t2_recinfo[i];
-    if (t1_rec->type != t2_rec->type ||
+    /*
+      FIELD_SKIP_ZERO can be changed to FIELD_NORMAL in mi_create,
+      see NOTE1 in mi_create.c
+    */
+    if ((t1_rec->type != t2_rec->type &&
+         !(t1_rec->type == (int) FIELD_SKIP_ZERO &&
+           t1_rec->length == 1 &&
+           t2_rec->type == (int) FIELD_NORMAL)) ||
         t1_rec->length != t2_rec->length ||
         t1_rec->null_bit != t2_rec->null_bit)
     {
diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index d537da7080b..66b68cfc2f1 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -4653,8 +4653,14 @@ bool add_field_to_list(THD *thd, char *field_name, enum_field_types type,
 	new_field->length++;
     }
     break;
-  case FIELD_TYPE_STRING:
   case FIELD_TYPE_VAR_STRING:
+    if (new_field->length < 4)
+    {
+      new_field->sql_type= FIELD_TYPE_STRING;
+      break;
+    }
+    /* fall through */
+  case FIELD_TYPE_STRING:
     if (new_field->length <= MAX_FIELD_CHARLENGTH || default_value)
       break;
     /* Convert long CHAR() and VARCHAR columns to TEXT or BLOB */