Postgres unique constraint vs index

Question

As I can understand documentation the following definitions are equivalent:

create table foo (
    id serial primary key,
    code integer,
    label text,
    constraint foo_uq unique (code, label));

create table foo (
    id serial primary key,
    code integer,
    label text);
create unique index foo_idx on foo using btree (code, label);    

However, a note in the manual for Postgres 9.4 says:

The preferred way to add a unique constraint to a table is ALTER TABLE ... ADD CONSTRAINT. The use of indexes to enforce unique constraints could be considered an implementation detail that should not be accessed directly.

(Edit: this note was removed from the manual with Postgres 9.5.)

Is it only a matter of good style? What are practical consequences of choice one of these variants (e.g. in performance)?

Answer 1

I had some doubts about this basic but important issue, so I decided to learn by example.

Let's create test table master with two columns, con_id with unique constraint and ind_id indexed by unique index.

create table master (
    con_id integer unique,
    ind_id integer
);
create unique index master_unique_idx on master (ind_id);

    Table "public.master"
 Column |  Type   | Modifiers
--------+---------+-----------
 con_id | integer |
 ind_id | integer |
Indexes:
    "master_con_id_key" UNIQUE CONSTRAINT, btree (con_id)
    "master_unique_idx" UNIQUE, btree (ind_id)

In table description (\d in psql) you can tell unique constraint from unique index.

Uniqueness

Let's check uniqueness, just in case.

test=# insert into master values (0, 0);
INSERT 0 1
test=# insert into master values (0, 1);
ERROR:  duplicate key value violates unique constraint "master_con_id_key"
DETAIL:  Key (con_id)=(0) already exists.
test=# insert into master values (1, 0);
ERROR:  duplicate key value violates unique constraint "master_unique_idx"
DETAIL:  Key (ind_id)=(0) already exists.
test=#

It works as expected!

Foreign keys

Now we'll define detail table with two foreign keys referencing to our two columns in master.

create table detail (
    con_id integer,
    ind_id integer,
    constraint detail_fk1 foreign key (con_id) references master(con_id),
    constraint detail_fk2 foreign key (ind_id) references master(ind_id)
);

    Table "public.detail"
 Column |  Type   | Modifiers
--------+---------+-----------
 con_id | integer |
 ind_id | integer |
Foreign-key constraints:
    "detail_fk1" FOREIGN KEY (con_id) REFERENCES master(con_id)
    "detail_fk2" FOREIGN KEY (ind_id) REFERENCES master(ind_id)

Well, no errors. Let's make sure it works.

test=# insert into detail values (0, 0);
INSERT 0 1
test=# insert into detail values (1, 0);
ERROR:  insert or update on table "detail" violates foreign key constraint "detail_fk1"
DETAIL:  Key (con_id)=(1) is not present in table "master".
test=# insert into detail values (0, 1);
ERROR:  insert or update on table "detail" violates foreign key constraint "detail_fk2"
DETAIL:  Key (ind_id)=(1) is not present in table "master".
test=#

Both columns can be referenced in foreign keys.

Constraint using index

You can add table constraint using existing unique index.

alter table master add constraint master_ind_id_key unique using index master_unique_idx;

    Table "public.master"
 Column |  Type   | Modifiers
--------+---------+-----------
 con_id | integer |
 ind_id | integer |
Indexes:
    "master_con_id_key" UNIQUE CONSTRAINT, btree (con_id)
    "master_ind_id_key" UNIQUE CONSTRAINT, btree (ind_id)
Referenced by:
    TABLE "detail" CONSTRAINT "detail_fk1" FOREIGN KEY (con_id) REFERENCES master(con_id)
    TABLE "detail" CONSTRAINT "detail_fk2" FOREIGN KEY (ind_id) REFERENCES master(ind_id)

Now there is no difference between column constraints description.

Partial indexes

In table constraint declaration you cannot create partial indexes. It comes directly from the definition of create table .... In unique index declaration you can set WHERE clause to create partial index. You can also create index on expression (not only on column) and define some other parameters (collation, sort order, NULLs placement).

You cannot add table constraint using partial index.

alter table master add column part_id integer;
create unique index master_partial_idx on master (part_id) where part_id is not null;

alter table master add constraint master_part_id_key unique using index master_partial_idx;
ERROR:  "master_partial_idx" is a partial index
LINE 1: alter table master add constraint master_part_id_key unique ...
                               ^
DETAIL:  Cannot create a primary key or unique constraint using such an index.

Answer 2

One more advantage of using UNIQUE INDEX vs. UNIQUE CONSTRAINT is that you can easily DROP/CREATE an index CONCURRENTLY, whereas with a constraint you can't.

Answer 3

Uniqueness is a constraint. It happens to be implemented via the creation of a unique index since an index is quickly able to search all existing values in order to determine if a given value already exists.

Conceptually the index is an implementation detail and uniqueness should be associated only with constraints.

The full text

So speed performance should be same

Answer 4

Since various people have provided advantages of unique indexes over unique constraints, here's a drawback: a unique constraint can be deferred (only checked at the end of the transaction), a unique index can not be.

Answer 5

Another thing I've encountered is that you can use sql expressions in unique indexes but not in constraints.

So, this does not work:

CREATE TABLE users (
    name text,
    UNIQUE (lower(name))
);

but following works.

CREATE TABLE users (
    name text
);
CREATE UNIQUE INDEX uq_name on users (lower(name));

Answer 6

A very minor thing that can be done with constraints only and not with indexes is using the ON CONFLICT ON CONSTRAINT clause (see also this question).

This doesn't work:

CREATE TABLE T (a INT PRIMARY KEY, b INT, c INT);
CREATE UNIQUE INDEX u ON t(b);

INSERT INTO T (a, b, c)
VALUES (1, 2, 3)
ON CONFLICT ON CONSTRAINT u
DO UPDATE SET c = 4
RETURNING *;

It produces:

[42704]: ERROR: constraint "u" for table "t" does not exist

Turn the index into a constraint:

DROP INDEX u;
ALTER TABLE t ADD CONSTRAINT u UNIQUE (b);

And the INSERT statement now works.

Answer 7

There is a difference in locking.
Adding an index does not block read access to the table.
Adding a constraint does put a table lock (so all selects are blocked) since it is added via ALTER TABLE.

Answer 8

I read this in the doc:

ADD table_constraint [ NOT VALID ]

This form adds a new constraint to a table using the same syntax as CREATE TABLE, plus the option NOT VALID, which is currently only allowed for foreign key constraints. If the constraint is marked NOT VALID, the potentially-lengthy initial check to verify that all rows in the table satisfy the constraint is skipped. The constraint will still be enforced against subsequent inserts or updates (that is, they'll fail unless there is a matching row in the referenced table). But the database will not assume that the constraint holds for all rows in the table, until it is validated by using the VALIDATE CONSTRAINT option.

So I think it is what you call "partial uniqueness" by adding a constraint.

And, about how to ensure the uniqueness:

Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. A uniqueness restriction covering only some rows cannot be written as a unique constraint, but it is possible to enforce such a restriction by creating a unique partial index.

Note: The preferred way to add a unique constraint to a table is ALTER TABLE … ADD CONSTRAINT. The use of indexes to enforce unique constraints could be considered an implementation detail that should not be accessed directly. One should, however, be aware that there’s no need to manually create indexes on unique columns; doing so would just duplicate the automatically-created index.

So we should add constraint, which creates an index, to ensure uniqueness.

How I see this problem?

A "constraint" aims to gramatically ensure that this column should be unique, it establishes a law, a rule; while "index" is semantical, about "how to implement, how to achieve the uniqueness, what does unique means when it comes to implementation". So, the way Postgresql implements it, is very logical: first, you declare that a column should be unique, then, Postgresql adds the implementation of adding an unique index for you.

Answer 9

In addition to the other answers, there's a topic of whether unique constraints are also used to speed-up queries as indexes are.

Apparently constraints are actually used for Index Scans as indicated by EXPLAIN:

ALTER TABLE mytable
    ADD CONSTRAINT mytable_uc UNIQUE (other_id, name);

explain select * from mytable
    where name = 'name' and other_id = 154

Result:

Index Scan using mytable_uc on mytable  (cost=0.28..2.29 rows=1 width=101)
  Index Cond: ((other_id = 154) AND ((name)::text = 'name'::text))

Answer 10

SELECT a.phone_number,count(*) FROM public.users a
Group BY phone_number Having count(*)>1;

SELECT a.phone_number,count(*) FROM public.retailers a
Group BY phone_number Having count(*)>1;

select a.phone_number from users a inner join users b
on a.id <> b.id and a.phone_number = b.phone_number order by a.id;


select a.phone_number from retailers a inner join retailers b
on a.id <> b.id and a.phone_number = b.phone_number order by a.id

DELETE FROM
    users a
        USING users b
WHERE
    a.id > b.id
    AND a.phone_number = b.phone_number;
    
DELETE FROM
    retailers a
        USING retailers b
WHERE
    a.id > b.id
    AND a.phone_number = b.phone_number;
    
CREATE UNIQUE INDEX CONCURRENTLY users_phone_number 
ON users (phone_number);

To Verify:

insert into users(name,phone_number,created_at,updated_at) select name,phone_number,created_at,updated_at from users