Does PostgreSQL support “accent insensitive” colla

Question

In Microsoft SQL Server, it's possible to specify an "accent insensitive" collation (for a database, table or column), which means that it's possible for a query like

SELECT * FROM users WHERE name LIKE 'João'

to find a row with a Joao name.

I know that it's possible to strip accents from strings in PostgreSQL using the unaccent_string contrib function, but I'm wondering if PostgreSQL supports these "accent insensitive" collations so the SELECT above would work.

Answer 1

Use the unaccent module for that - which is completely different from what you are linking to.

unaccent is a text search dictionary that removes accents (diacritic signs) from lexemes.

Install once per database with:

CREATE EXTENSION unaccent;

If you get an error like:

ERROR: could not open extension control file "/usr/share/postgresql/9.x/extension/unaccent.control": No such file or directory

Install the contrib package on your database server like instructed in this related answer:

• Error when creating unaccent extension on PostgreSQL

Among other things, it provides the function unaccent() you can use with your example (where LIKE seems not needed).

SELECT *
FROM   users
WHERE  unaccent(name) = unaccent('João');

Index

To use an index for that kind of query, create an index on the expression. However, Postgres only accepts IMMUTABLE functions for indexes. If a function can return a different result for the same input, the index could silently break.

unaccent() only STABLE not IMMUTABLE

Unfortunately, unaccent() is only STABLE, not IMMUTABLE. According to this thread on pgsql-bugs, this is due to three reasons:

1. It depends on the behavior of a dictionary.
2. There is no hard-wired connection to this dictionary.
3. It therefore also depends on the current search_path, which can change easily.

Some tutorials on the web instruct to just alter the function volatility to IMMUTABLE. This brute-force method can break under certain conditions.

Others suggest a simple IMMUTABLE wrapper function (like I did myself in the past).

There is an ongoing debate whether to make the variant with two parameters IMMUTABLE which declares the used dictionary explicitly. Read here or here.

Another alternative would be this module with an IMMUTABLE unaccent() function by Musicbrainz, provided on Github. Haven't tested it myself. I think I have come up with a better idea:

Best for now

I propose an approach that is at least as efficient as other solutions floating around, but safer: Create a wrapper function with the two-parameter form and "hard-wire" the schema for function and dictionary:

CREATE OR REPLACE FUNCTION public.f_unaccent(text)
  RETURNS text AS
$func$
SELECT public.unaccent('public.unaccent', $1)  -- schema-qualify function and dictionary
$func$  LANGUAGE sql IMMUTABLE;

public being the schema where you installed the extension (public is the default).

Previously, I had added SET search_path = public, pg_temp to the function - until I discovered that the dictionary can be schema-qualified, too, which is currently (pg 10) not documented. This version is a bit shorter and around twice as fast in my tests on pg 9.5 and pg 10.

The updated version still doesn't allow function inlining because functions declared IMMUTABLE may not call non-immutable functions in the body to allow that. Hardly matters for performance while we make use of an expression index on this IMMUTABLE function:

CREATE INDEX users_unaccent_name_idx ON users(public.f_unaccent(name));

Security for client programs has been tightened with Postgres 10.3 / 9.6.8 etc. You need to schema-qualify function and dictionary as demonstrated when used in any indexes. See:

• 'text search dictionary “unaccent” does not exist' entries in postgres log, supposedly during automatic analyze

Adapt your queries to match the index (so the query planner can use it):

SELECT * FROM users
WHERE  f_unaccent(name) = f_unaccent('João');

You don't need the function in the right expression. You can supply unaccented strings like 'Joao' directly.

Ligatures

In Postgres 9.5 or older ligatures like 'Œ' or 'ß' have to be expanded manually (if you need that), since unaccent() always substitutes a single letter:

SELECT unaccent('Œ Æ œ æ ß');

unaccent
----------
E A e a S

You will love this update to unaccent in Postgres 9.6:

Extend contrib/unaccent's standard unaccent.rules file to handle all diacritics known to Unicode, and expand ligatures correctly (Thomas Munro, Léonard Benedetti)

Bold emphasis mine. Now we get:

SELECT unaccent('Œ Æ œ æ ß');

unaccent
----------
OE AE oe ae ss

Pattern matching

For LIKE or ILIKE with arbitrary patterns, combine this with the module pg_trgm in PostgreSQL 9.1 or later. Create a trigram GIN (typically preferable) or GIST expression index. Example for GIN:

CREATE INDEX users_unaccent_name_trgm_idx ON users
USING gin (f_unaccent(name) gin_trgm_ops);

Can be used for queries like:

SELECT * FROM users
WHERE  f_unaccent(name) LIKE ('%' || f_unaccent('João') || '%');

GIN and GIST indexes are more expensive to maintain than plain btree:

• Difference between GiST and GIN index

There are simpler solutions for just left-anchored patterns. More about pattern matching and performance:

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

pg_trgm also provides useful operators for "similarity" (%) and "distance" (<->).

Trigram indexes also support simple regular expressions with ~ et al. and case insensitive pattern matching with ILIKE:

• PostgreSQL accent + case insensitive search

Answer 2

No, PostgreSQL does not support collations in that sense

PostgreSQL does not support collations like that (accent insensitive or not) because no comparison can return equal unless things are binary-equal. This is because internally it would introduce a lot of complexities for things like a hash index. For this reason collations in their strictest sense only affect ordering and not equality.

Workarounds

Full-Text-Search Dictionary that Unaccents lexemes.

For FTS, you can define your own dictionary using unaccent,

CREATE EXTENSION unaccent;

CREATE TEXT SEARCH CONFIGURATION mydict ( COPY = simple );
ALTER TEXT SEARCH CONFIGURATION mydict
  ALTER MAPPING FOR hword, hword_part, word
  WITH unaccent, simple;

Which you can then index with a functional index,

-- Just some sample data...
CREATE TABLE myTable ( myCol )
  AS VALUES ('fóó bar baz'),('qux quz');

-- No index required, but feel free to create one
CREATE INDEX ON myTable
  USING GIST (to_tsvector('mydict', myCol));

You can now query it very simply

SELECT *
FROM myTable
WHERE to_tsvector('mydict', myCol) @@ 'foo & bar'

    mycol    
-------------
 fóó bar baz
(1 row)

See also

• Creating a case-insensitive and accent/diacritics insensitive search on a field

Unaccent by itself.

The unaccent module can also be used by itself without FTS-integration, for that check out Erwin's answer

Answer 3

I'm pretty sure PostgreSQL relies on the underlying operating system for collation. It does support creating new collations, and customizing collations. I'm not sure how much work that might be for you, though. (Could be quite a lot.)