PostgreSQL Unique Constraint – Remove Extra Email Characters to Enforce Uniqueness

For a moment I thought one might be able to use a pre-existing text_pattern_ops index with the USING INDEX clause when adding a UNIQUE CONSTRAINT. But that fails, because:

ERROR: index "book2_name_like" does not have default sorting behavior

Per documentation:

The index cannot have expression columns nor be a partial index. Also, it must be a b-tree index with default sort ordering. These restrictions ensure that the index is equivalent to one that would be built by a regular ADD PRIMARY KEY or ADD UNIQUE command.

For instance, a unique index like that would allow a FK constraint referencing it, but perform terribly, because it does not support standard operators.
Per documentation:

Note that you should also create an index with the default operator class if you want queries involving ordinary <, <=, >, or >= comparisons to use an index.

So to answer the question:

If you need a UNIQUE CONSTRAINT (among other reasons: to reference it with a FK), your first variant with constraint and index is the only option. Additionally, the default operator class of the index created by the constraint supports more operations (like sorting in default sort order).

If you don't need any of that go with your second variant because, obviously, just one index is cheaper to maintain: just a UNIQUE text_pattern_ops index.

Differences between index and constraint:

• How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use?

Alternative with COLLATE "C"

Instead of creating two indexes, there is another alternative for xxx_pattern_ops indexes that may be preferable. The documentation:

The difference from the default operator classes is that the values are compared strictly character by character rather than according to the locale-specific collation rules. This makes these operator classes suitable for use by queries involving pattern matching expressions (LIKE or POSIX regular expressions) when the database does not use the standard "C" locale.

And:

The index automatically uses the collation of the underlying column.

You can create the column without collation (using COLLATE "C"). Then the default operator class behaves the same way as text_pattern_ops would - plus the index can be used with all the standard operators.

CREATE TABLE book2 (
   book_id serial PRIMARY KEY,
   book    text NOT NULL COLLATE "C" UNIQUE  -- that's all!
);

Now, LIKE can use the index:

SELECT * FROM book2 WHERE book LIKE 'foo%';

But ILIKE still can't:

SELECT * FROM book2 WHERE book ILIKE 'foo%';

SQL Fiddle.

Consider a trigram index using the additional module pg_trgm for a more general solution.:

• Pattern matching with LIKE, SIMILAR TO or regular expressions in PostgreSQL

To enforce unique email addresses, I would remove all competing email columns and store them in one central email table for all active emails. And another table for deleted emails:

CREATE TABLE users (
  user_id  serial PRIMARY KEY
, username text UNIQUE NOT NULL
, email    text UNIQUE -- FK added below  -- can also be NOT NULL
);

CREATE TABLE email (
  email    text PRIMARY KEY
, user_id  int NOT NULL REFERENCES users ON DELETE CASCADE
, UNIQUE (user_id, email)  -- seems redundant, but required for FK
);

ALTER TABLE users ADD CONSTRAINT users_primary_email_fkey
FOREIGN KEY (user_id, email) REFERENCES email (user_id, email);

CREATE TABLE email_deleted (
  email_id serial PRIMARY KEY
, email    text NOT NULL  -- not necessarily unique
, user_id  int NOT NULL REFERENCES users ON DELETE CASCADE
);

This way:

• Active emails are unique, enforced by the PK constraint of email.
• Each user can have any number of active and deleted emails, but ...
• Each user can only have one primary email.
• Every email is always owned by one user and is deleted with the user.
• To soft-delete an email (without losing it and its affiliation to its user, move the row from email to email_deleted.
  • The primary email of a user cannot be deleted this way, because the primary email must not be deleted.
• I designed the FK constraint users_primary_email_fkey to span (user_id, email), which seems redundant at first. But this way the primary email can only be an email that is actually owned by the same user.
  Due to the default MATCH SIMPLE behavior of FK constraints, you can still enter a user without primary email, because the FK constraint is not enforced if any of the columns is null.
  Details:
  • Two-column foreign key constraint only when third column is NOT NULL

The UNIQUE constraint on users.email is redundant for this solution, but it may be useful for other reasons. The automatically created index should come in handy (for instance for the last query in this answer).

The only thing that's not enforced this way is that every user has a primary email. You can do this, too. Add NOT NULL constraint to users.email

UNIQUE (user_id, email) is required for the FK constraint:

• How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use?

You have doubtless spotted the circular reference in the above model. Contrary to what one might expect, this just works.

As long as users.email can be NULL, it's trivial:

1. INSERT user without email.
2. INSERT email referencing the owning user_id.
3. UPDATE user to set it's primary email if applicable.

It even works with users.email set to NOT NULL. You have to insert user and email at the same time though:

WITH u AS (
   INSERT INTO users(username, email)
   VALUES ('user_foo', 'foo@mail.com')
   RETURNING email, user_id
   )
INSERT INTO email (email, user_id)
SELECT email, user_id
FROM   u;

IMMEDIATE FK constraints (the default) are checked at the end of each statement. The above is one statement. That's why it works where two separate statements would fail. Detailed explanation:

• How to deal with mutually recursive inserts

To get all emails of a user as array, with the primary email first:

SELECT u.*, e.emails
FROM   users u
     , LATERAL (
      SELECT ARRAY (
      SELECT email
      FROM   email
      WHERE  user_id = u.user_id
      ORDER  BY (email <> u.email)  -- sort primary email first
      ) AS emails
   ) e
WHERE  user_id = 1;

You could create a VIEW with this for ease of use.
LATERAL requires Postgres 9.3. use a correlated subquery in pg 9.2:

SELECT *, ARRAY (
             SELECT email
             FROM   email
             WHERE  user_id = u.user_id
             ORDER  BY (email <> u.email)  -- sort primary email first
             ) AS emails
FROM   users u
WHERE  user_id = 1;

To soft-delete an email:

WITH del AS (
   DELETE FROM email
   WHERE  email = 'spam@mail.com'
   RETURNING email, user_id
   )
INSERT INTO email_deleted (email, user_id)
SELECT email, user_id FROM del;

To soft-delete the primary email of a given user:

WITH upd AS (
   UPDATE users u
   SET    email = NULL
   FROM   (SELECT user_id, email FROM users WHERE user_id = 123 FOR UPDATE) old
   WHERE  old.user_id = u.user_id
   AND    u.user_id = 1
   RETURNING old.*
   )
,    del AS (
   DELETE FROM email
   USING  upd
   WHERE  email.email = upd.email
   )
INSERT INTO email_deleted (email, user_id)
SELECT email, user_id FROM upd;

Details:

• Return pre-UPDATE Column Values Using SQL Only - PostgreSQL Version

Quick test for all of the above: SQL Fiddle.