What are Oracle hidden fields?

Question

ALL_TAB_COLS differs from ALL_TAB_COLUMNS in that hidden columns are not filtered out.

What are hidden fields?

This view differs from "ALL_TAB_COLUMNS" in that hidden columns are not filtered out.

Answer 1

Hidden columns are columns that exist in the table but that cannot be selected. There are various reasons that Oracle might create a hidden column-- two of the more common reasons are

• When a column is marked as unused but not dropped, it is hidden
• Oracle virtual columns create hidden columns in the table

Various other Oracle features may create hidden columns as well though those features tend to be more esoteric (i.e. if memory serves, interMedia creates a couple of hidden columns to track various bits of data).