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Does creating a unique constraint on a Postgres column remove the need to index it?

I expect that an index is automatically needed to maintain the constraint efficiently.

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Yes. A UNIQUE constraint is implemented with the help of a unique index - a b-tree index with default ascending sort ordering over all involved columns. The index is created and maintained automatically, and is used for all purposes like a plain unique index by Postgres.

There is no need to create another (redundant) unique index like it, that would be a waste of resources.

Detailed explanation:

Some rare exceptions apply for multicolumn indexes with special sort options:

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