Does Postgresql Support "Accent Insensitive" Collations

Does PostgreSQL support accent insensitive collations?

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/<version>/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

This approach is more efficient as other solutions floating around, and safer.

Create an IMMUTABLE SQL wrapper function executing the two-parameter form with hard-wired schema-qualified function and dictionary.

Since nesting a non-immutable function would disable function inlining, base it on a copy of the C-function, (fake) declared IMMUTABLE as well. Its only purpose is to be used in the SQL function wrapper. Not meant to be used on its own.

The sophistication is needed as there is no way to hard-wire the dictionary in the declaration of the C function. (Would require to hack the C code itself.) The SQL wrapper function does that and allows both function inlining and expression indexes.

CREATE OR REPLACE FUNCTION public.immutable_unaccent(regdictionary, text)
  RETURNS text LANGUAGE c IMMUTABLE PARALLEL SAFE STRICT AS
'$libdir/unaccent', 'unaccent_dict';

CREATE OR REPLACE FUNCTION public.f_unaccent(text)
  RETURNS text LANGUAGE sql IMMUTABLE PARALLEL SAFE STRICT AS
$func$
SELECT public.immutable_unaccent(regdictionary 'public.unaccent', $1)
$func$;

Drop PARALLEL SAFE from both functions for Postgres 9.5 or older.

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

The explicit type declaration (regdictionary) defends against hypothetical attacks with overloaded variants of the function by malicious users.

<sub>Previously, I advocated a wrapper function based on the STABLE function unaccent() shipped with the unaccent module. That disabled function inlining. This version executes ten times faster than the simple wrapper function I had here earlier.

And that was already twice as fast as the first version which added SET search_path = public, pg_temp to the function - until I discovered that the dictionary can be schema-qualified, too. Still (Postgres 12) not too obvious from documentation.</sub>

If you lack the necessary privileges to create C functions, you are back to the second best implementation: An IMMUTABLE function wrapper around the STABLE unaccent() function provided by the module:

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 PARALLEL SAFE STRICT;

Finally, the expression index to make queries fast:

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

Remember to recreate indexes involving this function after any change to function or dictionary, like an in-place major release upgrade that would not recreate indexes. Recent major releases all had updates for the unaccent module.

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

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

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

The faster function does not translate to much faster queries using the expression index. That operates on pre-computed values and is very fast already. But index maintenance and queries not using the index benefit.

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

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

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

PostgreSQL accent + case insensitive search

If you need to "combine with case insensitive", there are a number of options, depending on your exact requirements.

Maybe simplest, make the expression index case insensitive.

Building on the function f_unaccent() laid out in the referenced answer:

• Does PostgreSQL support "accent insensitive" collations?

CREATE INDEX users_lower_unaccent_name_idx ON users(lower(f_unaccent(name)));

Then:

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

Or you could build the lower() into the function f_unaccent(), to derive something like f_lower_unaccent().

Or (especially if you need to do fuzzy pattern matching anyways) you can use a trigram index provided by the additional module pg_trgm building on above function, which also supports ILIKE. Details:

• LOWER LIKE vs iLIKE

I added a note to the referenced answer.

Or you could use the additional module citext (but I rather avoid it):

• Deferrable, case-insensitive unique constraint

PostgreSQL case-insensitive and accent-insensitive search

Creating case and accent insensitive ICU collations is pretty simple:

CREATE COLLATION english_ci_ai (
   PROVIDER = icu,
   DETERMINISTIC = FALSE,
   LOCALE = "en-US-u-ks-level1"
);

Or, equivalently (that syntax also works wil old ICU versions:

CREATE COLLATION english_ci_ai (
   PROVIDER = icu,
   DETERMINISTIC = FALSE,
   LOCALE = "en-US@colStrength=primary"
);

See the ICU documentation for details and my article for a detailed discussion.

But your problem is that you want substring search. So you should create a trigram index:

CREATE EXTENSION IF NOT EXISTS pg_trgm;
CREATE EXTENSION IF NOT EXISTS unaccent;

CREATE INDEX ON tab USING gin (unaccent(doc) gin_trgm_ops);

Then you can search like this:

SELECT * FROM tab
WHERE unaccent(doc) ILIKE unaccent('%joh%');

Note that you have to force a minimal length of 4 or so on the search string if you want that to be efficient.

Postgres accent insensitive LIKE search in Rails 3.1 on Heroku

Proper solution

Since PostgreSQL 9.1 you can just:

CREATE EXTENSION unaccent;

Provides a function unaccent(), doing what you need (except for lower(), just use that additionally if needed). Read the manual about this extension.

More about unaccent and indexes:

• Does PostgreSQL support "accent insensitive" collations?

Poor man's solution

If you can't install unacccent, but are able to create a function. I compiled the list starting here and added to it over time. It is comprehensive, but hardly complete:

CREATE OR REPLACE FUNCTION lower_unaccent(text)
  RETURNS text
  LANGUAGE sql IMMUTABLE STRICT AS
$func$
SELECT lower(translate($1
     , '¹²³áàâãäåāăąÀÁÂÃÄÅĀĂĄÆćčç©ĆČÇĐÐèéêёëēĕėęěÈÊËЁĒĔĖĘĚ€ğĞıìíîïìĩīĭÌÍÎÏЇÌĨĪĬłŁńňñŃŇÑòóôõöōŏőøÒÓÔÕÖŌŎŐØŒř®ŘšşșߊŞȘùúûüũūŭůÙÚÛÜŨŪŬŮýÿÝŸžżźŽŻŹ'
     , '123Does Postgresql Support "Accent Insensitive" CollationsDoes Postgresql Support "Accent Insensitive" CollationsaaacccccccddDoes PostgreSQL support accent insensitive collations? PostgreSQL accent + case insensitive search PostgreSQL case-insensitive and accent-insensitive search Postgres acDoes PostgreSQL support accent insensitive collations? PostgreSQL accent + case insensitive search PostgreSQL case-insensitive and accent-insensitive search Postgres aceeeeggiiiiiiiiiiiiiiiiiillnnnnnnooooooooooooooooooorrrsssssssuuuuuuuuuuuuuuuuyyyyzzzzzz'
     ));
$func$;

Your query should work like that:

find(:all, :conditions => ["lower_unaccent(name) LIKE ?", "%#{search.downcase}%"])

For left-anchored searches, you can use an index on the function for very fast results:

CREATE INDEX tbl_name_lower_unaccent_idx
  ON fest (lower_unaccent(name) text_pattern_ops);

For queries like:

SELECT * FROM tbl WHERE (lower_unaccent(name)) LIKE 'bob%';

Or use COLLATE "C". See:

• PostgreSQL LIKE query performance variations
• Is there a difference between text_pattern_ops and COLLATE "C"?

Yii2: accent insensitive filter

I resolved it using the lower_unaccent PostgreSQL function and the ILIKE PostgreSQL operator:

$query->andWhere(new Expression(
    'lower_unaccent(name) ILIKE \'%\' || lower_unaccent(\'' . $this->name . '\') || \'%\''
));

In Django with Postgresql 9.6 how to sort case and accent insensitive?

It isn't related to Django itself, PostgreSQL's lc_collate configuration determines this. I'd suggest you to review its value:

SHOW lc_collate;

The right thing to do is fix this configuration. Don't forget to take a look on related settings too (lc_ctype, etc.).

But if you cannot create another database with the right setting, try to explicit collate on ORDER like the following test case:

CREATE TEMPORARY TABLE table1 (column1 TEXT); 

INSERT INTO table1 VALUES('Barn'),
('beef'),
('bémol'),
('Bœuf'),
('boulette'),
('Bubble');

SELECT * FROM table1 ORDER BY column1 COLLATE "en_US"; --Gives the expected order
SELECT * FROM table1 ORDER BY column1 COLLATE "C"; --Gives "wrong" order  (in your case)

It's important to remember that PostgreSQL relies on operating system locales. This test case was executed on CentOS 7. More info here and here.

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