![]() Partial update is not limited to the JSON_SET function. Similarly, if the original JSON value didn’t have a name attribute at all, a full update would be performed to make room for the new attribute. In this case, the update will fall back to performing a full update, which means that it writes the whole new JSON value to the database. If the JSON value being updated is something like, there would not be enough room for the new name. ![]() Additionally, the JSON value being updated must already have a member called name, and there must be enough existing space in the JSON value to hold the new name. This update can be performed as a partial update because it takes a JSON column ( json_col), modifies one of its members, and then stores it into the same column as it reads from ( json_col). ![]() UPDATE t SET json_col = JSON_SET(json_col, '$.name', 'Knut') WHERE id = 123 Instead, MySQL analyzes each UPDATE statement and enables it when it can, so your application will benefit from the improvements automatically if the UPDATE statements already are on a form that is recognized as partial update.Ī simple example is the following statement, which changes the name attribute in a JSON value: There is no new syntax for partial update. ![]() So what do you have to do to get the benefit of partial update of JSON values in your application? In many cases nothing, apart from upgrading to MySQL 8.0. This can result in significantly reduced disk I/O and network I/O for update-intensive workloads. The patches are also used by row-based replication to reduce the amount of binary log shipped to the replication slave, if the proper option is enabled. The storage engine can use these patches to write the minimal amount of data. What’s new in MySQL 8.0, is that MySQL is able to recognize that certain UPDATE statements can modify JSON documents in place, and for these statements it will provide the storage engine with patches that describe the modifications. Before, in MySQL 5.7, whenever you made a change to a stored JSON document, the full new JSON document would be written to the database, even if the update just changed a few bytes in the document. It increases the complexity of any code you write to do it, and the developer who needs to take over maintenance of your code after you have moved on to another project will curse your name.MySQL 8.0 introduces partial update of JSON values, which is a nice performance improvement for applications that frequently update small portions of large JSON documents. Storing data in a JSON document when you eventually want to use SQL expressions to search or update individual fields within the JSON document is a costly mistake. Now compare how difficult that is to: UPDATE content SET status = 1 WHERE name = 'Jack' Using this in an UPDATE looks like this: mysql> update example set content = json_set(content, concat(json_unquote(json_search(json_extract(content, '$.name'), 'one', 'Jack')), '.status'), '1') Now use it in a JSON_SET() call: mysql> select json_pretty(Ĭoncat(json_unquote(json_search(json_extract(content, '$.name'), 'one', 'Jack')), '.status'), status and you can get the full path to the field you want to update: mysql> select concat(json_unquote(json_search(json_extract(content, '$.name'), 'one', 'Jack')), '.status') as path from example Then you can search that array to get the array position: mysql> select json_search(json_extract(content, '$.name'), 'one', 'Jack') as root from example Here's a trick: You can extract the name field, and that turns into an array of those values: mysql> select json_extract(content, '$.name') as a from example mysql> select j.* from example cross join json_table(content, '$' columns( And that doesn't tell you the path to the element, it only allows you to return a specific row out of the array. To search for a key/value pair, you need to use JSON_TABLE() and that requires you upgrade to MySQL 8.0. mysql> select json_unquote(json_search(content, 'one', 'Jack')) as path from example If "Jack" occurs in some other field, that would also be found. You can get the path to a JSON element with JSON_SEARCH(), but you can only search by value, not by a key/value pair. The solution in that case depends on you knowing which array element your pseudo-record exists in. The question like yours has come up before on Stack Overflow, for example JSON update single value in MySQL table. mysql> select json_pretty(json_replace(content, '$.status', '1')) as j ![]() So in this case, we can see that the array element is $ but if you didn't know that, you couldn't use this solution. You can use JSON_REPLACE() or JSON_SET(), but both require that you know the path to the field you want to change. This is very awkward, nearly impossible with MySQL's JSON functions. ![]()
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