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MDEV-22497 [ERROR] InnoDB: Unable to purge a record
The InnoDB insert buffer was upgraded in MySQL 5.5 into a change buffer that also covers delete-mark and delete (purge) operations. There is an important constraint for delete operations: a B-tree leaf page must not become empty unless the entire tree becomes empty, consisting of an empty root page. Because change buffer merges only occur on a single leaf page at a time, delete operations must not be buffered if it is possible that the last record of the page could be deleted. (In that case, we would refuse to use the change buffer, and if we really delete the last record, we would shrink the index tree.) The function ibuf_get_volume_buffered_hash() is part of our insurance that the page would not become empty. It is supposed to map each buffered INSERT or DELETE_MARK record payload into a hash value. We will only count each such record as a distinct key if there is no hash collision. DELETE operations will always decrement the predicted number fo records in the page. Due to a bug in the function, we would actually compute the hash value not only on the record payload, but also on some following bytes, in case the record contains NULL values. In MySQL Bug #61104, we had some examples of this dating back to 2012. But back then, we failed to reproduce the bug, and in commit d84c955 we simply demoted the hard assertion to a message printout and a debug assertion failure. ibuf_get_volume_buffered_hash(): Correctly compute the hash value of the payload bytes only. Note: we will consider ('foo','bar'),(NULL,'foobar'),('foob','ar') to be equal, but this is not a problem, because in case of a hash collision, we could also consider ('boo','far') to be equal, and underestimate the number of records in the page, leading to refusing to buffer a DELETE.
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