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A unique index ensures that no two rows of a table have duplicate values in the indexed column (or columns). A non-unique index does not impose this restriction on the indexed column’s values. To create a unique index, you use the CREATE UNIQUE INDEX statement: CREATE UNIQUE INDEX index_name ON table_name(column1[,column2,...]);
3 dni temu · A PL/SQL Unique Index is a powerful database object used to ensure the uniqueness of values in one or more columns of a table. In Oracle databases, indexes play an important role in enhancing query performance by enabling quick retrieval of rows.. The unique index specifically enforces a rule that no two rows in a table can have the same values for the indexed column(s).
A unique constraint is enforced by an index though it is possible (and sometimes necessary) to enforce a unique constraint using a non-unique index. A deferrable unique constraint, for example, is enforced using a non-unique index.
3 cze 2020 · the idea is to have uniqueness between name and source_id, but only in cases where source_id is not null, like the first and second insert. Creating a unique index on test (name,source_id), will prevent to do the last insert, since will be a duplicate value.
1 lis 2013 · If you are defining uniqueness for purposes of query performance, then Oracle recommends that you instead create the unique index explicitly using a CREATE UNIQUE INDEX statement. I went through the following test cases.
There is a (unique) function-based index on trunc (calendar_date). Normally to use a function-based index the where clause has to match the index.
10 sie 2017 · When you create a primary key or unique constraint, Oracle Database will automatically create a unique index for you (assuming there isn't an index already available). In most cases you'll add the constraint to the table and let the database build the index for you.
