Full Table Scans
A full table scan occurs when an index is either not used or there is no index on the table(s) being used by the SQL statement. Full table scans usually return data much slower than when an index is used. The larger the table, the slower that data is returned when a full table scan is performed. The query optimizer decides whether to use an index when executing the SQL statement. The index is used—if it exists—in most cases.
Some implementations have sophisticated query optimizers that can decide whether an index should be used. Decisions such as this are based on statistics that are gathered on database objects, such as the size of an object and the estimated number of rows that are returned by a condition with an indexed column. ...
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