A key is a column or expression on which you can build an index. Follow these guidelines for choosing keys to index:
Oracle automatically creates indexes, or uses existing indexes, on the keys and expressions of unique and primary keys that you define with integrity constraints.
Indexing low selectivity columns can be helpful if the data distribution is skewed so that one or two values occur much less often than other values.
UPDATEstatements that modify indexed columns and
DELETEstatements that modify indexed tables take longer than if there were no index. Such SQL statements must modify data in indexes as well as data in tables. They also generate additional undo and redo.
WHEREclauses with functions or operators. A
WHEREclause that uses a function, other than
MAX, or an operator with an indexed key does not make available the access path that uses the index except with function-based indexes.
DELETEstatements access the parent and child tables. Such an index allows
DELETEs on the parent table without share locking the child table.
DELETEs and the use of the space required to store the index. You might want to experiment by comparing the processing times of the SQL statements with and without indexes. You can measure processing time with the SQL trace facility.
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