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MySQL generated columns: https://dev.mysql.com/doc/refman/5.7/en/create-table-generated-columns.html
MariaDB virtual columns: https://mariadb.com/kb/en/mariadb/virtual-computed-columns/
Declare virtual columns with `t.virtual name, type: …, as: "expression"`.
Pass `stored: true` to persist the generated value (false by default).
Example:
create_table :generated_columns do |t|
t.string :name
t.virtual :upper_name, type: :string, as: "UPPER(name)"
t.virtual :name_length, type: :integer, as: "LENGTH(name)", stored: true
t.index :name_length # May be indexed, too!
end
Closes #22589
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