In our environment, we usually add and drop generated columns every now and then. There is a very large table in the database with more than 3 million rows and adding/removing column was extremely slow. We opted to change the generated column from STORED to VIRTUAL, so that it is instantaneous.

However, we hit another road block: When we try to add the VIRTUAL column after a certain column (I believe this is due to application logic) e.g. AFTER ColumnX, the speed is extremely slow again. It takes more than 4-5 hours to just add the generated VIRTUAL column now.

Why would adding the virtual column without AFTER clause be instantaneous while adding AFTER clause slows it to a crawl?

Database: MySQL version 5.7
Table rows: 32,636,254

ALTER TABLE mySchema.myTable
json_extract(`m_dt`,'$.refId'))) Virtual AFTER `ColumnX`;

1 Answer 1


As per the official MySQL documentation reordering the columns will rebuild the entire table thus making it an expensive operation. However, this can be performed online without locking the table.


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