MySQL – ERROR 1824 (HY000) While Importing Dump from 5.7 to 8.0.x

character-setmigrationMySQLmysqldump

I have a MySQL 5.7.x dump with about 12GB that I try to import into a newly installed MySQL 8.0.x local instance.

mysql -u username -p test < DB-210508.sql

The import starts but fails with the following message:

ERROR 1824 (HY000) at line 518: Failed to open the referenced table 'table-name'

I am wondering why there is a problem with foreign keys and if it is not more due to the fact that I am importing into 8.0.x

The MySQL 8.0.x instance is configured like this:

[mysqld]
default_authentication_plugin= mysql_native_password
innodb_buffer_pool_size = 8G
innodb_buffer_pool_instances = 32
innodb_buffer_pool_chunk_size = 134217728
collation_server        = utf8_unicode_ci
character_set_server    = utf8

What could be the reason for the failed import?

Best Answer

This error happens when you have a classic “chicken and egg” problem. The most common situations for this would be:

populating a self-referencing table where a lower ID requires the existence of a higher ID
⇢ a good example of this would be an Employee table where record 33 had a manager with an ID of 999. 33 could not be recorded until 999 existed in the table.
populating a table that depends on another table that has not yet been created
⇢ this can happen when you have an Employee table that has references to User, but User isn’t created until after Employee.

The error you received was:

… Failed to open the referenced table 'table-name'

This points to the second item as being the most likely culprit.

The simplest way to resolve this would be to remove the foreign key checks when importing the data, then restore the checks after the import is complete. You can do this be editing your 12GB .sql file to have this at the very start:

SET @OLD_FOREIGN_KEY_CHECKS = @@FOREIGN_KEY_CHECKS;
SET FOREIGN_KEY_CHECKS = 0;

Then, at the very end of the import file:

SET FOREIGN_KEY_CHECKS = @OLD_FOREIGN_KEY_CHECKS;

Note: With the foreign key check disabled, there is a possibility of recording bad data. If this is something you need to avoid for regulatory purposes, do not follow this “quick” merhod.

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At first glance, I would say you have some corruption. But, this an import. That scares me. Take your mysqldump and run this:

head -20 SQLDump.sql

Does the dump contain DROP TABLE and CREATE TABLE ? My guess would be no.

Did you change the table structure, i.e.,

added columns to the table
removed columns from the table
changed charset/collation to utf8

after doing that mysqldump ? My guess would be yes.

If neither of these are the case, then your table has some corruption in the .frm.

Try installing MySQL on another DB Server and loading the SQLDump there. If nothing errors out, the mysqldump is fine. This verifies some corruption.

Try dropping the problem table, create it from scratch. Then, load it back. You could run myisamchk against the MyISAM table.

Mysql – How to debug a db memory-leak causing thesql to go before it’s own limits

...even surpassing it's theorically maximum possible allocation.

[OK] Maximum possible memory usage: 7.3G (46% of installed RAM)

There is not actually a way to calculate maximum possible memory usage for MySQL, because there is no cap on the memory it can request from the system.

The calculation done by mysqltuner.pl is only an estimate, based on a formula that doesn't take into account all possible variables, because if all possible variables were taken into account, the answer would always be "infinite." It's unfortunate that it's labeled this way.

Here is my theory on what's contributing to your excessive memory usage:

thread_cache_size       = 128

Given that your max_connections is set to 200, the value of 128 for thread_cache_size seems far too high. Here's what makes me think this might be contributing to your problem:

When a thread is no longer needed, the memory allocated to it is released and returned to the system unless the thread goes back into the thread cache. In that case, the memory remains allocated.

^{http://dev.mysql.com/doc/refman/5.6/en/memory-use.html}

If your workload causes even an occasional client thread to require a large amount of memory, those threads may be holding onto that memory, then going back to the pool and sitting around, continuing to hold on to memory they don't technically "need" any more, on the premise that holding on to the memory is less costly than releasing it if you're likely to need it again.

I think it's worth a try to do the following, after first making a note of how much memory MySQL is using at the moment.

Note how many threads are currently cached:

mysql> show status like 'Threads_cached';
+----------------+-------+
| Variable_name  | Value |
+----------------+-------+
| Threads_cached | 9     |
+----------------+-------+
1 row in set (0.00 sec)

Next, disable the thread cache.

mysql> SET GLOBAL thread_cache_size = 0;

This disables the thread cache, but the cached threads will stay in the pool until they're used one more time. Disconnect from the server, then reconnect and repeat.

mysql> show status like 'Threads_cached';

Continue disconnecting, reconnecting, and checking until the counter reaches 0.

Then, see how much memory MySQL is holding.

You may see a decrease, possibly significant, and then again you may not. I tested this on one of my systems, which had 9 threads in the cache. Once those threads had all been cleared out of the cache, the total memory held by MySQL did decrease... not by much, but it does illustrate that threads in the cache do release at least some memory when they are destroyed.

If you see a significant decrease, you may have found your problem. If you don't, then there's one more thing that needs to happen, and how quickly it can happen depends on your environment.

If the theory holds that the other threads -- the ones currently servicing active client connections -- have significant memory allocated to them, either because of recent work in their current client session or because of work requiring a lot of memory that was done by another connection prior to them languishing in the pool, then you won't see all of the potential reduction in memory consumption until those threads are allowed to die and be destroyed. Presumably your application doesn't hold them forever, but how long it will take to know for sure whether there's a difference will depend on whether you have the option of cycling your application (dropping and reconnecting the client threads) or if you'll have to just wait for them to be dropped and reconnected over time on their own.

But... it seems like a worthwhile test. You should not see a substantial performance penalty by setting thread_cache_size to 0. Fortunately, thread_cache_size is a dynamic variable, so you can freely change it with the server running.

Best Answer

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