How to diagnose and fix the 42P22 indeterminate_collation error code in Postgres.

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The 42P22 indeterminate_collation error in PostgreSQL indicates that there is an issue with collation, which refers to a set of rules for comparing characters in a character set. This error typically arises when you’re working with text data that has conflicting or unspecified collation settings. Here are several examples of how this error might occur and how to resolve it:

  1. Combining Columns with Different Collations:
    If you’re combining columns from different tables that have different collations, you might encounter the 42P22 error. To resolve this, you can explicitly set the collation for the operation.
   SELECT a.column1 COLLATE "en_US", b.column2 COLLATE "en_US"
   FROM table1 a
   JOIN table2 b ON a.id = b.id;

In this example, both columns are explicitly set to use the en_US collation, which should resolve the collation conflict.

  1. Using Functions with Indeterminate Collation:
    Some string functions might result in indeterminate collation if the input expressions have conflicting collations. You can fix this by specifying the collation for the result of the function.
   SELECT CONCAT(column1 COLLATE "en_US", column2 COLLATE "en_US")
   FROM table;

By using the COLLATE clause, you specify the collation for the concatenation result.

  1. Ordering or Grouping by a Collation-Aware Expression:
    When using ORDER BY or GROUP BY on a column with an indeterminate collation, you need to define the collation.
   SELECT column1
   FROM table
   ORDER BY column1 COLLATE "en_US";

This ensures that the sorting is done using the specified en_US collation.

  1. Creating a Database or Column with a Specific Collation:
    When creating a new database or column, you can specify the collation to avoid future conflicts.
   CREATE DATABASE mydb
   WITH LC_COLLATE = 'en_US.utf8' LC_CTYPE = 'en_US.utf8' TEMPLATE template0;
   CREATE TABLE mytable (
     id serial PRIMARY KEY,
     name text COLLATE "en_US"
   );

These commands create a database and a table with a specified collation, ensuring consistency for collation-aware operations.

  1. Adjusting the Collation of an Existing Database or Column:
    If you need to change the collation for an existing database or column, you can use the ALTER command.
   ALTER DATABASE mydb SET LC_COLLATE TO 'en_US.utf8';
   ALTER TABLE mytable ALTER COLUMN name TYPE text COLLATE "en_US";

These commands change the collation setting for the database and the column, respectively.

Remember, when dealing with collation issues, it’s essential to ensure that the collation is consistent across your database operations to avoid the 42P22 indeterminate_collation error. Always back up your data before making changes to the database schema or collation settings.

For more details on collation support in PostgreSQL, you can refer to the official PostgreSQL documentation on collation.

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