How to Change Collation for a Column in PostgreSQL: 6-Step Guide with Performance and Indexing Considerations

Published On - February 19, 2026

Olivia Brown Blog

Databases often need to support multiple languages, regional rules, and case-sensitivity requirements. In PostgreSQL, collation controls how text is sorted and compared, which directly affects query results, indexing behavior, and overall performance. Changing the collation of a column is not as simple as flipping a switch—it requires careful planning, especially in production environments.

TL;DR: To change the collation of a column in PostgreSQL, you must verify the target collation, assess dependencies, drop related indexes if necessary, alter the column using ALTER TABLE ... ALTER COLUMN ... TYPE ... COLLATE, and rebuild indexes. Collation changes affect sorting, comparisons, and index usage, so performance testing is crucial. Always perform backups and test in staging before applying changes in production.

Understanding Collation in PostgreSQL

Collation defines the rules used to compare and sort textual data. These rules determine whether sorting is case-sensitive, accent-sensitive, and how locale-specific characters are ordered.

PostgreSQL supports:

  • Operating system collations (provided by libc)
  • ICU collations (from PostgreSQL 10+)
  • Database default collation
  • Column-level collation

Each text-type column (text, varchar, char) can have its own collation, overriding the database default.

When Should a Column’s Collation Be Changed?

Common scenarios include:

  • Adding multilingual support to an application
  • Switching from case-sensitive to case-insensitive sorting
  • Correcting incorrect alphabetical order for regional languages
  • Migrating databases between environments with different locale settings

Because collation influences how indexes store and compare values, changes can impact both query results and performance.

6-Step Guide to Changing Column Collation in PostgreSQL

Step 1: Check the Current Collation

Before making changes, it is essential to confirm the current collation of the column:


SELECT column_name, collation_name
FROM information_schema.columns
WHERE table_name = 'your_table';

To list available collations:


SELECT * FROM pg_collation;

This ensures clarity about the starting point and identifies compatible alternatives.

Step 2: Verify Dependencies (Indexes and Constraints)

Collation affects indexes that depend on the column. Attempting to change collation without addressing these dependencies will result in errors.

To find dependent indexes:


SELECT indexname, indexdef
FROM pg_indexes
WHERE tablename = 'your_table';

Pay close attention to:

  • B-tree indexes on text columns
  • Unique constraints
  • Foreign key constraints involving text columns

Indexes must typically be dropped and recreated after the collation change.

Step 3: Drop or Disable Dependent Indexes

If a column has indexes applied, they must be removed:


DROP INDEX index_name;

For mission-critical systems, consider:

  • Using DROP INDEX CONCURRENTLY to reduce locking
  • Scheduling downtime if necessary
  • Testing the operation in a staging environment first

Warning: Dropping unique indexes temporarily removes uniqueness enforcement.

Step 4: Alter the Column’s Collation

PostgreSQL does not allow changing only the collation directly. Instead, the column type must be redefined with the new collation:


ALTER TABLE your_table
ALTER COLUMN your_column
TYPE text COLLATE "en_US"
USING your_column::text;

Key considerations:

  • The USING clause ensures proper casting
  • The new collation must exist in pg_collation
  • The operation may rewrite the table, depending on PostgreSQL version

Step 5: Recreate Indexes

After successfully altering the column, indexes must be rebuilt:


CREATE INDEX index_name
ON your_table (your_column);

Or for a unique index:


CREATE UNIQUE INDEX index_name
ON your_table (your_column);

Since collation affects comparison logic, rebuilding ensures index consistency with the new sorting rules.

Where possible, use:

  • CREATE INDEX CONCURRENTLY in production
  • Maintenance windows for large datasets

Step 6: Test and Validate Performance

Changing collation can alter execution plans. Query behavior involving:

  • ORDER BY
  • GROUP BY
  • LIKE comparisons
  • Equality comparisons

should be evaluated using:


EXPLAIN ANALYZE
SELECT * FROM your_table
ORDER BY your_column;

Compare performance metrics before and after the modification.

Performance Considerations

Collation changes are not merely cosmetic. They influence CPU usage, index size, and sorting behavior.

1. Index Rebuild Cost

Rebuilding large indexes can consume significant I/O and CPU resources. On large datasets, this may take minutes or hours.

2. Query Planning Differences

Different collations may produce slightly different sort orders. PostgreSQL may choose different execution plans depending on selectivity and ordering behavior.

3. ICU vs libc Collations

  • ICU collations provide better multilingual support.
  • libc collations rely on the operating system and may vary between servers.

Inconsistent libc versions across environments can lead to index corruption or misaligned sorting. ICU collations are often preferred for consistency.

4. Case-Insensitive Alternatives

Instead of changing collation, sometimes performance is better served by:

  • Using the citext extension
  • Creating functional indexes like LOWER(column_name)

These options may avoid a full collation modification while achieving similar behavior.

Potential Pitfalls

  • Table locking: The ALTER TABLE command can lock the table.
  • Version-related differences: PostgreSQL versions handle ICU and collation versions differently.
  • Replication concerns: Ensure replicas use compatible collation providers.
  • Application dependencies: Sorting behavior visible to end-users may change.

Testing in a staging environment is strongly recommended before any production deployment.

Example Full Workflow

A practical example changing a column from default collation to en_US:


-- 1. Drop dependent index
DROP INDEX CONCURRENTLY idx_users_name;

-- 2. Alter column collation
ALTER TABLE users
ALTER COLUMN name
TYPE varchar(255) COLLATE "en_US"
USING name::varchar;

-- 3. Recreate index
CREATE INDEX CONCURRENTLY idx_users_name
ON users (name);

This structured workflow minimizes downtime while preserving data integrity.

FAQ

1. Can a database’s default collation be changed after creation?

No. PostgreSQL does not allow changing the database default collation directly. A new database must be created with the desired collation.

2. Does changing collation rewrite the entire table?

In many cases, yes. Depending on the PostgreSQL version and storage parameters, altering column type and collation can trigger a full table rewrite.

3. Will changing collation affect existing data?

The stored data remains the same, but how it is compared and sorted changes. This can alter uniqueness checks and ordering results.

4. Do all indexes need to be rebuilt?

Only indexes referencing the affected column must be rebuilt. Other indexes remain untouched.

5. Is ICU better than libc for production systems?

ICU is generally more consistent across environments and better suited for multilingual systems. libc collations depend on the operating system and may differ between servers.

6. Can collation be set per query instead of per column?

Yes. PostgreSQL allows specifying collation in queries using:


SELECT * FROM table
ORDER BY column COLLATE "en_US";

This can be useful when temporary sorting differences are needed.

7. Is downtime required to change column collation?

Not necessarily, but it depends on table size and indexing strategy. Using concurrent index operations can significantly reduce downtime.

By understanding the mechanics behind collations and carefully planning each step, database administrators can safely modify column behavior while maintaining performance, consistency, and data integrity.