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When an object (a table, a column, a view, or some other object) is invalid within the Query Builder, it will turn red to indicate that an action should be taken to modify or remove it. Hovering over invalid objects presents tooltips with more information on why they are invalid and offer appropriate suggestions for how to resolve issues.
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Join Lines become red when any of their associated tables, columns, or views are invalid.
If your connection is in an invalid state (if the connection was lost because of a firewall or OS severing idle connections) it's often possible to perform a Reconnect to resolve connection issues. See Reconnecting for more.
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