awesome-copilot/skills/reviewing-oracle-to-postgres-migration/references/oracle-to-postgres-sorting.md

Oracle to PostgreSQL Sorting Migration Guide

Purpose: Preserve Oracle-like sorting semantics when moving queries to PostgreSQL.

Key points

• Oracle often treats plain ORDER BY as binary/byte-wise, giving case-insensitive ordering for ASCII.
• PostgreSQL defaults differ; to match Oracle behavior, use COLLATE "C" on sort expressions.

1) Standard SELECT … ORDER BY

Goal: Keep Oracle-style ordering.

Pattern:

SELECT col1
FROM your_table
ORDER BY col1 COLLATE "C";

Notes:

• Apply COLLATE "C" to each sort expression that must mimic Oracle.
• Works with ascending/descending and multi-column sorts, e.g. ORDER BY col1 COLLATE "C", col2 COLLATE "C" DESC.

2) SELECT DISTINCT … ORDER BY

Issue: PostgreSQL enforces that ORDER BY expressions appear in the SELECT list for DISTINCT, raising: Npgsql.PostgresException: 42P10: for SELECT DISTINCT, ORDER BY expressions must appear in select list

Oracle difference: Oracle allowed ordering by expressions not projected when using DISTINCT.

Recommended pattern (wrap and sort):

SELECT *
FROM (
  SELECT DISTINCT col1, col2
  FROM your_table
) AS distinct_results
ORDER BY col2 COLLATE "C";

Why:

• The inner query performs the DISTINCT projection.
• The outer query safely orders the result set and adds COLLATE "C" to align with Oracle sorting.

Tips:

• Ensure any columns used in the outer ORDER BY are included in the inner projection.
• For multi-column sorts, collate each relevant expression: ORDER BY col2 COLLATE "C", col3 COLLATE "C" DESC.

Validation checklist

• Added COLLATE "C" to every ORDER BY that should follow Oracle sorting rules.
• For DISTINCT queries, wrapped the projection and sorted in the outer query.
• Confirmed ordered columns are present in the inner projection.
• Re-ran tests or representative queries to verify ordering matches Oracle outputs.