Tuesday, October 11, 2016

OTN appreciation day : MySQL 8.0 data dictionary

About one month ago, the MySQL team at Oracle released MySQL 8.0, with a large list of changes. One of the most interesting features in the new release is also one that does not show up much, also because the team has gone to great length to keep most of its implementation hidden: the data dictionary.


What makes the data dictionary so interesting, despite its scarce visibility, is the effect that it has on performance. Up to MySQL 5.7, searching the information_schema was an onerous operation, potentially crippling the system. In MySQL 8.0, the same operations are 100 times faster. This would be reason enough to be excited, as I know many users who have had terrible problems with the inefficiency of information_schema.

But there are several other visible changes: the various files that were needed to identify database objects (.frm for tables, .trg for triggers, .par for partitions) are now gone. When the database server wants to know metadata about such objects, it doesn't have to open files anymore. All the information is stored in the hidden InnoDB tables of the data dictionary. If the hidden tables bother you, you can use a not so difficult hack to discover them, although most operations can be performed without inner knowledge of how the dictionary is organized. The idea is that the public interface (views in information_schema) should be considered reliable and used for future compatibility. Perhaps the hidden status will be lifted in one of the next releases: many in the community have given feedback in this direction.

What else is the data dictionary good for? It allows atomic DDL operations, which were not guaranteed when data for the dictionary was stored in files and MyISAM tables. A nice complement to the data dictionary is the elimination of all MyISAM tables from the system. Now the grant privileges are stored in InnoDB tables, which, probably for historical compatibility, were left visible.

I am sure users will come up with other clever usages of the data dictionary when the system is wider adopted and understood. For now, it's a wonderful toy to explore!

This post was suggested by Tim Hall, a well known community champion among Oracle users, who maintains a rich web site of news and free technical info. According to his suggestion, this post wants to add to the OTN appreciation day, a distributed community effort to show something useful, or pleasant, or both related to the Oracle world.

For those not used to the Oracle Technology Network (OTN), it is the center of Oracle technology, the place where users can get all software (proprietary or open source) and other resources related to Oracle products. In the image below you may find several familiar names.

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