How do I reference a unique index that uses a function in ON CONFLICT?

Question

I'm using postgres 9.5.3, and I have a table like this:

CREATE TABLE packages (
  id   SERIAL PRIMARY KEY,
  name VARCHAR NOT NULL
);

I have defined a function, canonical_name, like this:

CREATE FUNCTION canonical_name(text) RETURNS text AS $$
    SELECT replace(lower($1), '-', '_')
$$ LANGUAGE SQL;

I've added a unique index to this table that uses the function:

CREATE UNIQUE INDEX index_package_name
ON packages (canonical_name(name));
CREATE INDEX
# \d+ packages
                                                  Table "public.packages"
 Column |       Type        |                       Modifiers                       | Storage  | Stats target | Description
--------+-------------------+-------------------------------------------------------+----------+--------------+-------------
 id     | integer           | not null default nextval('packages_id_seq'::regclass) | plain    |              |
 name   | character varying | not null                                              | extended |              |
Indexes:
    "packages_pkey" PRIMARY KEY, btree (id)
    "index_package_name" UNIQUE, btree (canonical_name(name::text))

And this unique index is working as I expect; it prevents insertion of duplicates:

INSERT INTO packages (name) 
VALUES ('Foo-bar');

INSERT INTO packages (name) 
VALUES ('foo_bar');

ERROR:  duplicate key value violates unique constraint "index_package_name"
DETAIL:  Key (canonical_name(name::text))=(foo_bar) already exists.

My problem is that I want to use this unique index to do an upsert, and I can't figure out how I need to specify the conflict target. The documentation seems to say I can specify an index expression:

where conflict_target can be one of:

    ( { index_column_name | ( index_expression ) } [ COLLATE collation ] [ opclass ] [, ...] ) [ WHERE index_predicate ]
    ON CONSTRAINT constraint_name

But all of these things below that I've tried produce errors as shown, instead of a working upsert.

I've tried matching the index expression as I specified it:

INSERT INTO packages (name)
VALUES ('foo_bar')
ON CONFLICT (canonical_name(name)) 
DO UPDATE SET name = EXCLUDED.name;

ERROR:  there is no unique or exclusion constraint matching the ON CONFLICT specification

Matching the index expression as \d+ showed it:

INSERT INTO packages (name)
VALUES ('foo_bar')
ON CONFLICT (canonical_name(name::text)) 
DO UPDATE SET name = EXCLUDED.name;


ERROR:  there is no unique or exclusion constraint matching the ON CONFLICT specification

Just naming the column that the unique index is on:

INSERT INTO packages (name)
VALUES ('foo_bar')
ON CONFLICT (name) 
DO UPDATE SET name = EXCLUDED.name;

ERROR:  there is no unique or exclusion constraint matching the ON CONFLICT specification

Using the index name instead:

INSERT INTO packages (name)
VALUES ('foo_bar')
ON CONFLICT (index_package_name) 
DO UPDATE SET name = EXCLUDED.name;

ERROR:  column "index_package_name" does not exist
LINE 3: ON CONFLICT (index_package_name)

So how do I specify that I want to use this index? Or is this a bug?

Answer 1

Important note: This behavior can only be observed on versions before 9.5.4. This is a bug that was fixed in 9.5.4. The rest of the answer describes the buggy behavior:

As you found out, you can only specify the expression for a unique constraint and not the one for a unique index. This is somewhat confusing because under the hood a unique constraint is just a unique index (but that is considered an implementation detail).

To make matters worse for you, you cannot define a unique constraint over a unique index that contains expressions – I am not certain what the reason is, but suspect the SQL standard.

One way you can do this would be to add an artificial column, filled with the “canonical name” by a trigger and define the constraint on that column. I admit that that is not nice.

The correct solution, however, is to upgrade to the latest minor release for PostgreSQL 9.5.