How Can I Achieve Accent-Insensitive Searches in PostgreSQL?

PostgreSQL's Approach to Accent-Insensitive Searching

Unlike some databases (like Microsoft SQL Server), PostgreSQL doesn't natively support accent-insensitive collations. While PostgreSQL 12 introduced non-deterministic ICU collations offering case and accent insensitivity, these come with performance trade-offs and operational restrictions.

Strategies for Accent-Insensitive Queries in PostgreSQL

Several methods exist to achieve accent-insensitive searching in PostgreSQL:

1. The unaccent Module:

This module provides the unaccent() function, removing accents from strings. This allows queries like:

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

However, unaccent() isn't IMMUTABLE, preventing its use in expression indexes, and it doesn't expand ligatures (e.g., 'Œ').

2. Optimized C Function Wrapper:

To address unaccent()'s limitations, a more efficient solution involves creating an IMMUTABLE C function wrapper:

CREATE OR REPLACE FUNCTION public.f_unaccent(text)
  RETURNS text
  LANGUAGE sql IMMUTABLE PARALLEL SAFE STRICT
RETURN public.immutable_unaccent(regdictionary 'public.unaccent', );

This allows for the creation of expression indexes:

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

Queries then use the wrapped function:

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

3. Leveraging pg_trgm for Pattern Matching and Ligatures:

For more flexible pattern matching and ligature handling, the pg_trgm module with trigram indexes offers a powerful solution. A trigram GIN index enables case-insensitive searches and similarity detection:

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

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

Note that pg_trgm indexes are more resource-intensive than standard B-tree indexes.

Choosing the optimal approach depends on the specific needs of your application, balancing query performance with index maintenance costs and the requirement for ligature handling.

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