Fixed bug #28272: crash that occurs when running an EXPLAIN command

for a query over an empty table right after its creation. 
The crash is the result of an attempt made by JOIN::optimize to evaluate
the WHERE condition when no records have been actually read.
The added test case can reproduce the crash only with InnoDB tables and
only with 5.0.x.
 


mysql-test/r/innodb_mysql.result:
  Added a test case for bug #28272.
mysql-test/t/innodb_mysql.test:
  Added a test case for bug #28272.
sql/sql_select.cc:
  Fixed bug #28272: crash that occurs when running an EXPLAIN command
  for a query over an empty table right after its creation. 
  The crash is the result of an attempt made by JOIN::optimize to evaluate
  the WHERE condition when no records have been actually read.
  Such attempts could be observed only with EXPLAIN commands.
  Now at the optimization phase the WHERE condition is never evaluated if 
  there is no record previously read from the table.

mysql-test/r/innodb_mysql.result

@@ -161,4 +161,25 @@ SELECT * FROM t1;
 c1	cnt
 1a	2
 DROP TABLE t1;
+CREATE TABLE t1 (
+a1 decimal(10,0) DEFAULT NULL,
+a2 blob,
+a3 time DEFAULT NULL,
+a4 blob,
+a5 char(175) DEFAULT NULL,
+a6 timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
+a7 tinyblob,
+INDEX idx (a6,a7(239),a5)
+) ENGINE=InnoDB;
+EXPLAIN SELECT a4 FROM t1 WHERE
+a6=NULL AND
+a4='UNcT5pIde4I6c2SheTo4gt92OV1jgJCVkXmzyf325R1DwLURkbYHwhydANIZMbKTgdcR5xS';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+EXPLAIN SELECT t1.a4 FROM t1, t1 t WHERE
+t.a6=t.a6 AND t1.a6=NULL AND
+t1.a4='UNcT5pIde4I6c2SheTo4gt92OV1jgJCVkXmzyf325R1DwLURkbYHwhydANIZMbKTgdcR5xS';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+DROP TABLE t1;
 End of 4.1 tests

mysql-test/t/innodb_mysql.test

@@ -191,4 +191,29 @@ INSERT INTO t1 (c1) VALUES ('1b') ON DUPLICATE KEY UPDATE cnt=cnt+1;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+#
+# Bug #28272: EXPLAIN for SELECT from an empty InnoDB table
+#
+
+CREATE TABLE t1 (
+  a1 decimal(10,0) DEFAULT NULL,
+  a2 blob,
+  a3 time DEFAULT NULL,
+  a4 blob,
+  a5 char(175) DEFAULT NULL,
+  a6 timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
+  a7 tinyblob,
+  INDEX idx (a6,a7(239),a5)
+) ENGINE=InnoDB;
+
+EXPLAIN SELECT a4 FROM t1 WHERE
+a6=NULL AND
+a4='UNcT5pIde4I6c2SheTo4gt92OV1jgJCVkXmzyf325R1DwLURkbYHwhydANIZMbKTgdcR5xS';
+
+EXPLAIN SELECT t1.a4 FROM t1, t1 t WHERE
+t.a6=t.a6 AND t1.a6=NULL AND
+t1.a4='UNcT5pIde4I6c2SheTo4gt92OV1jgJCVkXmzyf325R1DwLURkbYHwhydANIZMbKTgdcR5xS';
+
+DROP TABLE t1;
+
 --echo End of 4.1 tests

sql/sql_select.cc

@@ -634,6 +634,12 @@ JOIN::optimize()
     DBUG_PRINT("error",("Error: make_select() failed"));
     DBUG_RETURN(1);
   }
+  if (conds &&!outer_join && const_table_map != found_const_table_map && 
+      (select_options & SELECT_DESCRIBE) &&
+      select_lex->master_unit() == &thd->lex->unit) // upper level SELECT
+  {
+    conds=new Item_int((longlong) 0,1);	// Always false
+  }
   if (make_join_select(this, select, conds))
   {
     zero_result_cause=