BUG#28248 - mysqldump results with MERGE ... UNION=() cannot be executed

When there are no underlying tables specified for a merge table,
SHOW CREATE TABLE outputs a statement that cannot be executed. The
same is true for mysqldump (it generates dumps that cannot be
executed).

This happens because SQL parser does not accept empty UNION() clause.

This patch changes the following:
- it is now possible to execute CREATE/ALTER statement with
  empty UNION() clause.
- the same as above, but still worth noting: it is now possible to
  remove underlying tables mapping using ALTER TABLE ... UNION=().
- SHOW CREATE TABLE does not output UNION() clause if there are
  no underlying tables specified for a merge table. This makes
  mysqldump slightly smaller.


mysql-test/r/merge.result:
  A test case for BUG#28248.
mysql-test/t/merge.test:
  A test case for BUG#28248.
sql/ha_myisammrg.cc:
  Do not output UNION clause in SHOW CREATE TABLE, when there are
  no underlying tables defined.
sql/sql_yacc.yy:
  Make underlying table list for MERGE engine optional.
  
  As for MERGE engine empty underlying tables list is valid, it should
  be valid for the parser as well.
  
  This change is mostly needed to restore dumps made by earlier MySQL
  versions. Also with this fix it is possible to remove underlying
  tables mapping by using ALTER TABLE ... UNION=().

1 files changed, 22 insertions, 0 deletions

diff --git a/mysql-test/r/merge.result b/mysql-test/r/merge.result
index 2ceae95dfca..f8ef4f23180 100644
--- a/mysql-test/r/merge.result
+++ b/mysql-test/r/merge.result
@@ -918,4 +918,26 @@ id	ref
 3	2
 4	5
 DROP TABLE t1, t2, t3;
+CREATE TABLE t1(a INT);
+CREATE TABLE m1(a INT) ENGINE=MERGE;
+SHOW CREATE TABLE m1;
+Table	Create Table
+m1	CREATE TABLE `m1` (
+  `a` int(11) default NULL
+) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1
+DROP TABLE m1;
+CREATE TABLE m1(a INT) ENGINE=MERGE UNION=();
+SHOW CREATE TABLE m1;
+Table	Create Table
+m1	CREATE TABLE `m1` (
+  `a` int(11) default NULL
+) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1
+ALTER TABLE m1 UNION=(t1);
+ALTER TABLE m1 UNION=();
+SHOW CREATE TABLE m1;
+Table	Create Table
+m1	CREATE TABLE `m1` (
+  `a` int(11) default NULL
+) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1, m1;
 End of 5.0 tests