Basically, you create an index like
CREATE INDEX index_name ON table_name ( column(s) ) INVISIBLE;
What this has the effect of doing is to only allow use of this index if it's specified in an INDEX hint for the query.
This means that the developer / DBA can issue indexes on systems which will not be utilised by any other queries, and, therefore, can be used to test the performance impact of an index without affecting the main system.
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