Shulou(Shulou.com)06/01 Report--

Document database (Document-orient Database) supports Schema-Free, his one-line record format is: {"name": "design", "value": "straight board"}, in fact, the mongo stored in KEY/VALUE uses BSON binary coding, so it should be more economical than JSON in space, parsing according to JSON. Each Document contains metadata information, and each document is not forced to use the same format. At the same time, they also support a variety of indexes, which is more convenient than MySQL. For the comparison between Mysql table design and Mongodb scheme free design, you can refer to http://blog.nosqlfan.com/html/2985.html, however, due to the large amount of metadata redundancy, the waste of space is very serious.

For a typical relational database like MySQL, the impact of DDL on io can not be ignored, which is often caused by the data storage format.

The Innodb storage engine has made efforts to modify the secondary index, that is, it is possible not to copy data during the ddl process, but the table is still locked during this period. This is quite convenient for the use of ONLINE schema change OSC.

The general practice is to do the DDL operation on the slave database now, and then switch between master and slave, and then restore the original state after all the operations are completed.

The reason is that the relational storage engine stores metadata and data separately, and some changes to metadata require all data to be modified. One way to avoid this problem is to put metadata and data together, which is Schema-free, which is popular in the NoSQL field.

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