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The original description can be found further in this field
The problem is reproducible with MySQL 5.1-5.6 and filed as http://bugs.mysql.com/bug.php?id=68135.
A variable can be created with an empty name. Apparently, it's confusing (see the original request below) and potentially error-prone; but it also causes problems with binary logging and replication:
SET @`` = 1;
CREATE TABLE t1 AS SELECT @`` AS f;
SHOW BINLOG EVENTS;
ERROR HY000: Error when executing command SHOW BINLOG EVENTS: Wrong offset or I/O error
Consequently, replication fails with ER_SLAVE_RELAY_LOG_READ_FAILURE.
Currently it's only reproducible on maria/5.5, but not on maria/5.1, 5.2, 5.3 or 10.0-base. However, I suppose it's just a matter of time, since it came to maria/5.5 with the last mysql-5.1.67 merge, and it exists in all current trees of MySQL.
Test case:
--source include/master-slave.inc --source include/have_binlog_format_statement.inc SET @`` = 1; CREATE TABLE t1 AS SELECT @`` AS f; --sync_slave_with_master
==================================
Original description
I'm not sure if this is a bug or a feature request. Empty names seem to be allowed for variables. No warnings.
Empty names are not allowed for schemas, tables, columns, views, etc.
MariaDB [test]> SET @`` = 1;
Query OK, 0 rows affected (0.00 sec)
MariaDB [test]> SELECT @``;
+------+
| @`` |
+------+
| 1 |
+------+
1 row in set (0.07 sec)
MariaDB [test]>
The same happens with local vars in stored routines:
DROP FUNCTION IF EXISTS `test`; CREATE FUNCTION `test`() RETURNS BOOLEAN BEGIN DECLARE `` TINYINT UNSIGNED; SET `` = 1; RETURN ``; END;
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MDEV-4333 5.5.31 merge
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Hi Elena
I agree that it is serious, for this reason: I can hardly beilive that someone wants to use an empty name. I did it, but my SQL was generated by a buggy PHP script. In such cases, a warning would make debugging easier.
However, I admit that in some weird cases buggy code can provide the expected output, so I understand that breaking compatibility is a bad thing.
A comment from the corresponding MySQL bug report:
"
Fixed in 5.1+. Documented in the 5.1.69, 5.5.31, 5.6.11, and 5.7.1 changelogs as follows:
A zero-length name for a user variable was incorrectly considered to be a sign of data corruption and thus was not written to the binary log.
"
So I suppose they haven't made an empty name illegal.
fixed by the mysql-5.5.31 merge
Hi Federico,
I suppose as a feature request it wouldn't stand much of a chance, not because it's unreasonable, but because it would almost inevitably cause compatibility issues – replication from an old slave to a new master, stored procedures which use an empty name, etc., and of course for MariaDb also compatibility with the upstream.
However, it turns out there is a bug with empty names, most likely a regression, which causes replication problems. I've updated the description and refiled it to MySQL bug base, and mentioned your request there, too. I doubt, however, that the bug will make Oracle forbid empty names, on the same reason of breaking compatibility with older versions; and if they don't, we can't either.
Thanks for the report, it revealed a serious issue.