How to Handle Oracle Use Cases in PostgreSQL

Galaxy Glossary

How do I replicate common Oracle features in PostgreSQL?

Oracle-style analytics, functions, and data can be reproduced in PostgreSQL by using built-in equivalents, extensions, and the oracle_fdw foreign data wrapper.

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Description

Table of Contents

What Oracle features do developers most often need in PostgreSQL?

Most migrations require hierarchical queries (CONNECT BY), analytic functions, DECODE/NVL style expressions, and querying live Oracle data without moving it.

How can I run hierarchical queries like CONNECT BY?

Use the ANSI-standard WITH RECURSIVE clause. It delivers the same parent-child traversal without proprietary syntax.

Example

WITH RECURSIVE product_tree AS ( SELECT id, name, price, 1 AS level FROM products WHERE id = 1 UNION ALL SELECT p.id, p.name, p.price, pt.level + 1 FROM products p JOIN product_tree pt ON p.parent_id = pt.id) SELECT * FROM product_tree;

How do I replace DECODE and NVL?

Use CASE, COALESCE, or NULLIF. They are ANSI compliant and index-friendly.

Example

SELECT id, COALESCE(email,'no-email@example.com') AS email_fallback FROM customers;

How can I query Oracle tables directly?

Install oracle_fdw, create a foreign server, and import the desired schemas. This avoids a bulk migration when you only need read-through access.

Setup steps

1. Install Oracle client libs
2. CREATE EXTENSION oracle_fdw;
3. Define server, user mapping, and foreign tables.

Why use Oracle analytic functions in PostgreSQL?

PostgreSQL supports the full SQL:2003 window specification, matching functions like SUM() OVER(), LAG(), and RANK() without extra extensions.

Example

SELECT id, total_amount, RANK() OVER(ORDER BY total_amount DESC) AS sales_rank FROM orders;

Best practices for Oracle-to-PostgreSQL migrations

Translate proprietary syntax early, freeze DDL on both systems, write exhaustive tests, and use CI pipelines to validate rewritten SQL.

Which tools help automate the conversion?

Use ora2pg for schema/data migration and oracle_fdw for coexistence. Combine with Galaxy’s AI copilot to speed up query rewrites.

What are common pitfalls?

Avoid relying on implicit data type casts and unbounded sequences. Explicitly cast and reset sequence values to align with Oracle’s NUMBER defaults.

Why How to Handle Oracle Use Cases in PostgreSQL is important

How to Handle Oracle Use Cases in PostgreSQL Example Usage


-- Compute last-month sales per customer using imported Oracle tables
WITH monthly_sales AS (
  SELECT customer_id, SUM(total_amount) AS total
  FROM oracle_ecom.orders
  WHERE order_date >= CURRENT_DATE - INTERVAL '30 days'
  GROUP BY customer_id)
SELECT c.name, ms.total
FROM monthly_sales ms
JOIN oracle_ecom.customers c ON c.id = ms.customer_id
ORDER BY ms.total DESC;

How to Handle Oracle Use Cases in PostgreSQL Syntax


-- Install and configure oracle_fdw
CREATE EXTENSION oracle_fdw;

-- Create a foreign server pointing to Oracle
CREATE SERVER oracle_ecom
  FOREIGN DATA WRAPPER oracle_fdw
  OPTIONS (dbserver '//ora-host:1521/ORCL');

-- Map PostgreSQL role to Oracle credentials
CREATE USER MAPPING FOR postgres
  SERVER oracle_ecom
  OPTIONS (user 'ora_user', password 'secret');

-- Import Oracle 'ECOM' schema into Postgres 'oracle_ecom'
IMPORT FOREIGN SCHEMA ECOM
  LIMIT TO (CUSTOMERS, ORDERS, PRODUCTS, ORDERITEMS)
  FROM SERVER oracle_ecom INTO oracle_ecom;

-- Query Oracle data as if local
SELECT c.name, o.order_date, o.total_amount
FROM oracle_ecom.orders o
JOIN oracle_ecom.customers c ON c.id = o.customer_id
WHERE o.order_date >= CURRENT_DATE - INTERVAL '30 days';

Common Mistakes

Frequently Asked Questions (FAQs)

Can I write to Oracle tables from PostgreSQL?

Yes. oracle_fdw supports INSERT, UPDATE, and DELETE if your Oracle user has the rights.

Is ora2pg mandatory for migrations?

No, but it accelerates schema conversion and data export, saving weeks of manual work.

Are Oracle sequences auto-converted?

ora2pg can map sequences to PostgreSQL SERIAL or GENERATED AS IDENTITY, but verify max values to avoid collisions.

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