ERROR 1932 on a table with recently added VIRTUAL column
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Answer
Virtual columns should not be unstable - they've been present since MariaDB 5.2. You are using MariaDB 10.1.0 though, which was an alpha version, and definitely not recommended for production use. The latest 10.1 release is currently 10.1.6 (in beta), and the latest stable release is 10.0.20, so try upgrading to a more stable release. If the problem persists, you can report it as a bug.
MySQL does not have the virtual columns feature in its stable releases, although it's recently added it to its milestone release.