How to diagnose and fix the 42P21 collation_mismatch error code in Postgres.

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The 42P21 error code in PostgreSQL indicates a collation_mismatch. This error occurs when there is an inconsistency in collation (i.e., the set of rules for comparing characters in a character set) between different text expressions that are being compared or combined. Collation issues can arise when you are joining tables, comparing strings, or using string functions where the operands have different collations.

Here are some common scenarios where a collation_mismatch error might occur, along with examples and sample code to diagnose and fix the issue:

Example 1: Joining Tables with Different Collations

When you perform a join on two tables where the columns being joined have different collations, you may encounter the 42P21 error.

Diagnosis:

SELECT *
FROM table1
JOIN table2 ON table1.text_column = table2.text_column;

If table1.text_column and table2.text_column have different collations, the above query might fail.

Fix:
You can explicitly specify the collation for the join condition using the COLLATE clause.

SELECT *
FROM table1
JOIN table2 ON table1.text_column COLLATE "desired_collation" = table2.text_column COLLATE "desired_collation";

Replace "desired_collation" with the collation that you want to use for the comparison.

Example 2: Concatenating Strings with Different Collations

When concatenating strings from different columns with different collations, you might get a collation_mismatch error.

Diagnosis:

SELECT text_column1 || text_column2 AS concatenated_string
FROM table;

If text_column1 and text_column2 have different collations, the above concatenation might cause an error.

Fix:
Explicitly set the collation for the concatenated parts.

SELECT text_column1 COLLATE "desired_collation" || text_column2 COLLATE "desired_collation" AS concatenated_string
FROM table;

Example 3: Using String Functions on Columns with Different Collations

Applying string functions to columns with different collations can also cause the 42P21 error.

Diagnosis:

SELECT UPPER(text_column1) FROM table1
UNION ALL
SELECT UPPER(text_column2) FROM table2;

If text_column1 and text_column2 have different collations, the UNION may fail.

Fix:
Standardize the collation before applying the string function.

SELECT UPPER(text_column1 COLLATE "desired_collation") FROM table1
UNION ALL
SELECT UPPER(text_column2 COLLATE "desired_collation") FROM table2;

General Tips:

  • Determine the collation of your columns using the pg_collation system catalog.
  • Use the COLLATE clause to explicitly define the collation for an operation.
  • Ensure consistency in collation across your database schema to prevent such errors.

In all cases, "desired_collation" should be replaced with the appropriate collation for your dataset and the specific collation rules you need to apply. Choose a collation that matches the requirements of your comparison or operation, and make sure that it is supported by PostgreSQL.

The PostgreSQL documentation provides a comprehensive list of PostgreSQL Error Codes, which can be helpful for understanding the specific error codes and their meanings.

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