PostgreSQL cannot determine which collation to use when comparing columns with conflicting implicit collations. Resolve by explicitly specifying a collation using the COLLATE clause.
PostgreSQL uses collations to determine how strings are sorted and compared, including language-specific rules for alphabetical ordering and case sensitivity. When the database encounters conflicting non-default implicit collations among input expressions, it cannot automatically determine which collation rules to apply. This happens most commonly in comparison operations, ORDER BY clauses, and string functions. PostgreSQL considers distinct collation objects to be incompatible even when they have identical properties, such as the C and POSIX collations.
First, determine which columns or expressions have conflicting collations. Use this query to check the collations in your table:
SELECT column_name, collation_name
FROM information_schema.columns
WHERE table_name = 'your_table_name'
AND collation_name IS NOT NULL;Also check your database's default collation:
SELECT datcollate
FROM pg_database
WHERE datname = 'your_database_name';Add a COLLATE clause to one of the conflicting expressions to tell PostgreSQL which collation to use. For comparison operations:
SELECT * FROM test1
WHERE column_a < column_b COLLATE "en_US";For ORDER BY:
SELECT * FROM test1
ORDER BY column_a || column_b COLLATE "en_US";When combining queries with UNION, ensure consistent collations by specifying a COLLATE clause:
SELECT name COLLATE "en_US" FROM table1
UNION
SELECT name COLLATE "en_US" FROM table2;To prevent this error in the future, ensure columns that will be compared have the same collation. When creating or altering tables:
CREATE TABLE test1 (
column_a text COLLATE "en_US",
column_b text COLLATE "en_US"
);If you need to change an existing column's collation:
ALTER TABLE test1
ALTER COLUMN column_a TYPE text COLLATE "en_US";PostgreSQL provides multiple collations including C, POSIX, and locale-specific collations (de_DE, en_US, etc.). The C and POSIX collations are identical and use byte-order comparison. Locale-specific collations follow language-specific rules. When working with databases that support multiple languages or were migrated from systems with different locales, you may need to use explicit COLLATE clauses frequently. In some cases, you can also cast strings to bytea and back to bypass collation requirements, but this is generally not recommended as it loses language-specific sorting semantics.
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