Indexes can also be used to enforce uniqueness of a column's value, or the uniqueness of the combined values of more than one column.
CREATE UNIQUE INDEX
column[, ...]) [ INCLUDE (
column[, ...]) ];
Currently, only B-tree indexes can be declared unique.
When an index is declared unique, multiple table rows with
equal indexed values are not allowed. Null values are not
considered equal. A multicolumn unique index will only reject
cases where all indexed columns are equal. Columns listed in the
INCLUDE clause, if any, aren't
considered when determining whether index entries are equal.
PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.
There's no need to manually create indexes on unique columns; doing so would just duplicate the automatically-created index.
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