Oracle to Postgres Migration: Complete Guide with Tools, Steps & Zero-Downtime Strategy

When to Migrate from Oracle to PostgreSQL (and When Not To)

Moving from an Oracle to Postgres is one of the most common digital transformation trends today. But deciding to move is based on technical fit and business needs. 

Signs You've Outgrown Oracle

• Licensing costs and maintenance costs: Oracle’s licensing costs have increased, and have increased financial anxiety. It becomes an obstacle to innovations and growth.
• Vendor lock-in, Flexibility, and Customization: Oracle heavily relies on specific features, and this creates a vendor lock-in and creates more control over the database. This can restrict flexibility and cloud portability. Organizations that require hybrid setups often prefer open-source databases such as PostgreSQL, which offers extensive extensions and customization options.
• Cloud-native: Nowadays, new trends are moving towards Docker, Kubernetes, and agile DevOps to reduce dependency on proprietary systems. Oracle’s footprint may feel like a bottleneck, whereas Postgres integrates well within modern DevOps and containerized environments. 
• Over-Provisioned Enterprise features: If your business needs modern applications that require specialized data types such as vector searches for AI, advanced JSON handling, or Geospatial data, Postgres offers a more flexible, plugin-friendly architecture without any expensive add-on packs. Moreover, Oracle provides many advanced features that may not be needed. But for that, the organizations need to pay the costs in that case. Postgres serves as a better alternative.

When Oracle Is Still the Right Choice

But Oracle may also be a dominant force for a reason. In that case, migrating from Oracle to Postgres might become a risk.

• Dependency on Oracle features: If the database contains thousands of complex business logic, packages, and triggers, the cost may exceed the savings. Applications heavily using PL/SQL, RAC, or proprietary tools may be costly to refactor.
• Strong Vendor Support Requirements: Organizations that rely on dedicated enterprise support may prefer Oracle’s service model. Postgres support is available, but often through third-party vendors.
• Compliant needs: Some organizations may need Oracle’s certification and long-standing compliance track record. Switching to Postgres may require validation again and auditing.
• No cost pressure: If one has a stable setup and costs are not a concern, an Oracle to Postgres migration might not be necessaryVendor-Locked Third-Party Software: Many enterprise ERP, CRM, and financial systems are certified only for Oracle. So moving from Oracle to Postgres will create a significant compliance risk.‍
• Vendor-Locked Third-Party Software: Many enterprise ERP, CRM, and financial systems are certified only for Oracle. So moving from Oracle to Postgres will create a significant compliance risk.

Oracle vs PostgreSQL - Key Differences

Choosing between Oracle and Postgres is not between “enterprise vs. open-source.” It depends on the features, cost, and architectural philosophy.

Pre-Migration Assessment - What to Audit Before You Start

Pre-migration assessment and planning are critical steps for a successful Oracle to Postgres migration. The critical areas to be concentrated on while doing an Oracle to Postgres migration are

Schema Analysis & Document-to-Table Mapping

• Schema and Data types conversion: Mapping database schemas, including tables, views, indexes, and constraints, is important to prevent data loss. Both Oracle and PostgreSQL have differences in their data type syntax. 
  For example, Oracle’s NUMBER datatype may get mapped to either NUMERIC, INTEGER, or BIGINT in Postgres, and Postgres separates DATE and TIMESTAMP; those need to be corrected manually. Create a mapping list to track how each data type in Oracle should be mapped in Postgres. Converting Oracle-specific features, like BLOB, CLOB, and LONG datatypes, requires careful planning. ‍
• Stored procedures and functions: Oracle uses PL/SQL, and it is not compatible with PostgreSQL‘s PL/pgSQL; its syntax and structures of stored procedures differ in PostgreSQL. We need to rewrite these parts manually according to Postgres.‍
• Document-to-Table Mapping: The Oracle system stores semi-structured or document-like data. If the document data has a constituent structure, you need high-speed joins and map the document attributes to standard Postgres columns. This is called the Flattening approach. Postgres is famous for its JSONB support. Then map the Oracle’s JSON or XML blobs directly into JSONB columns.

Risk Assessment and Mitigation: 

• Selecting the migration tool: Several migration tools are available, such as Ora2pg, SQLines, pgLoader, and AWS Database Migration Service (DMS). Ora2Pg is a free tool for migrating data from Oracle to PostgreSQL. It connects to Oracle DB, extracts the data, and generates SQL scripts that can be imported into Postgres DB. SQLines translates SQL code across the databases, and pgLoader handles the data migrations. Choose the migration tool based on database size, downtime tolerance, and real-time needs accordingly. 
• Performance tuning and query optimization: After the Oracle to Postgres migration, there will be some performance degradation in some of the queries due to the changes and incompatibility in the indexes. Carefully look into the SQL queries and tune them accordingly so that their performance is not affected.
• Mapping access and its permissions: Analyze the existing users, roles, and their privileges in the Oracle database. Using Postgres Role-based Access Control (RBAC), Row-level Security (RLS), and other authentication features, make sure to replicate the users’ access and align it with Oracle’s security policies.
• Minimizing the downtime: As Oracle can handle large volumes of data, migrating it requires a significant amount of downtime and might affect the business. Ensure that the business operations are monitored 24/7. Utilizing tools like AWS Database Migration Service (DMS) for replication can minimize downtime.

Oracle to Postgres Migration Tools - ETL & CDC Options

Choosing the right migration tool depends on database size, the complexity of the logic, and downtime in the migration process. The tools and methods for moving from Oracle to PostgreSQL are

Built-In Utilities (Manual / DIY)

This traditional method is useful if the database size is smaller and has a highly skilled DBA team. These are basically open-source or native tools that give full control of the Oracle to Postgres migration.

• Ora2Pg: It extracts schema, converts Oracle objects, and performs bulk data migration. 
• PgLoader: High-performance data loader with parallel processing.
• CSV Export/Import (SQL*Plus +COPY): In this method, it exports data from Oracle to flat files and uses the Postgres COPY command for high-speed ingestion.

When to use

• Small or medium datasets with flexible maintenance windows.
• Teams with strong DBA expertise.

No-Code ETL Platforms 

No-Code or low-code ETL tools simplify the data migration process without complex scripts. They give faster results with less manual effort.

• Fivetran: Cloud-based connectors for continuous data sync. It requires volume-based pricing.
• Airbyte: It is an open-source alternative that offers over 600 connectors and allows you to run your migration both on-premises and in the cloud. It uses usage-based pricing.
• AWS Database Migration Services (DMS): Managed service with schema conversion and continuous replication.

When to use

• One needs a simple interface and faster execution.

CDC (Change Data Capture) Tools for Zero-Downtime

CDC tools are essential when one needs a zero-downtime migration. These tools read Oracle’s Redo Logs or Archive Logs and “replay” those transactions on the Postgres target in real-time.

• Debezium: It is an open-source and highly flexible CDC tool. It streams real-time changes via log-based CDC and is ideal for custom, event-driven architectures.
• AWS DMS: It is a highly popular cloud-native tool. It handles both the initial “Full Load” and the ongoing CDC phase. It has built-in monitoring and validation with minimal downtime.
• BryteFlow: It is a no-code CDC tool with automated reconciliation. It can handle both initial load and incremental sync.

When to use

• Choose CDC when downtime is not to be considered.

How to choose the Migration Tool

Choosing a migration tool is an important decision. It can be based on 

• Data volume: For small datasets, choose Built-in utilities, and for medium to large datasets, choose CDC tools or an ETL platform.
• Downtime tolerance: If zero downtime is preferred, choose the CDC approach; if downtime can be tolerated, the ETL (Extract, Transform, and Load) approach can be chosen.
• Complexity: For a simple schema, choose DIY or no-code ETL, and for a complex transformation, choose advanced ETL or CDC pipelines.

Best Practices for Selecting a Tool

Before sticking with a tool, it is better to follow the Oracle to Postgres best practices.

• Test tools with a sample project with a minimal dataset before full migration.
• Ensure that the tool is compatible with both Oracle and Postgres.
• Evaluate scalability, monitoring, and support features.
• Consider all the costs associated with the tool, including setup, maintenance, and licensing.

Step-by-Step Oracle to Postgres Migration Process

Migrating from Oracle to Postgres needs a structured approach to ensure data accuracy, minimal downtime, and optimal performance. The Oracle to Postgres migration step-by-step process is

Step 1: Schema Design & Data Mapping

Before writing any code, do an audit assessment and identify risks and constraints. Then start designing your target schema. Perform an audit on the Oracle database and collect the details about schemas, objects, tables, data types, Oracle-specific features, stored procedures, functions, and application dependencies. Identify nested arrays and objects in your documents that should become separate, linked tables in PostgreSQL. 

Be clear in the migration objectives on what needs to be migrated and whether it is required in the Postgres database or not. This step will help us identify potential risks and complexities involved in the Oracle to Postgres migration. Use tools such as Ora2Pg to convert Oracle-specific objects into Postgres. Map Oracle-specific types such as Number to Numeric, VARCHAR2 to VARCHAR, and CLOB to TEXT.

Step 2: Set Up PostgreSQL Environment

Install Postgres on-premises, cloud, or managed like AWS RDS, Azure, or GCP. Adjust configurations such as max_connections, shared buffers, and work_mem based on Oracle AWR reports. Create users, roles, and configure them.

Step 3: Choose Your Migration Method

Select the migration strategy that aligns with your uptime requirements. For smaller and non-critical DBs, choose Offline Migration. For mission-critical systems, choose CDC migrations to sync changes in real-time until the final cutover.

Step 4: Export & Transform Data

Start extracting data from the source by ensuring the Postgres environment is ready. Export data using tools such as Ora2Pg or native utilities. Transform data types and formats. This will reduce the migration errors and ensure compatibility. 

Step 5: Load Data in Postgres

Once the data (objects, tables) is exported, load it into PostgreSQL using the COPY command, even if you are using Ora2Pg.

Step 6: Validate data

Validate row counts, checksums, and constraints. Validate Oracle and Postgres to ensure no data was lost or corrupted during the migration process. Compare sample queries between the Oracle and Postgres databases. If any mismatches occur, resolve them immediately. Run validation scripts to compare the data integrity between the two databases.

Step 7: Application code Updates

Checking application complexity is one of the most time-consuming steps. Modify application logic to work with Postgres by replacing Oracle-specific SQL syntax. Update database drivers and connection strings. Refactor PL/SQL logic into PL/pgDQL or application code.

Step 8: Test Application and Cutover Planning

Run parallel testing on both Oracle and Postgres environments. Compare the results of queries from both databases. Testing is essential to identify any migration-related issues before going live. Set up a separate test environment and test the data migration process. We should check the consistency of the transferred data and verify that the migrated data behaves in the same way as it did in the Oracle database. Do functional testing, performance testing, and regression testing to ensure that there are no data losses.

Ensure all services and APIs work seamlessly with PostgreSQL. Schedule a downtime period for the final cutover of the Oracle. Stop writing to Oracle and decommission the old database only after a sufficient observation period. Closely monitor the Postgres and application after the cutover to identify and address any issues.

Step 9: Monitor & Optimize Post-Migration

Redirect application traffic from Oracle to Postgres. Continuously monitor application and database performance. Track query performance, index utilization, and optimize the workload. For the slow query log, tune them using Postgres-specific indexing such as GIN or BRIN.

Common Data Mapping Challenges and How to Solve Them

Data mapping is one of the time-consuming parts of the Oracle to Postgres migration challenges. Some of the challenges faced during the Oracle to Postgres migration are

• Integer confusion: Oracle’s NUMBER type represents integers, decimals, and large values. But PostgreSQL separates them into different types. To solve this, use INTEGER or NUMERIC. For whole numbers, use INTEGER or BIGINT, and for precision-based values, use NUMERIC.
• VARCHAR2 vs TEXT Behaviour: Oracle’s VARCHAR has size limits and behaves differently compared to Postgres. To solve this, use TEXT in Postgres for flexibility and VARCHAR only when the length is constrained.
• DATE and TIMESTAMP differences: Oracle uses both date and time in its DATE type, while Postgres displays only date in the DATE datatype, and TIMESTAMP handles the time. To solve this, always map Oracle’s DATE columns to Postgres TIMESTAMP.
• EmptyString vs. NULL Debate: In Oracle, an empty string is treated as NULL. In PostgreSQL, an empty string is a valid, non-null value. To solve this, during the ETL process, decide whether to convert all empty strings to NULL or handle empty strings in your application code. Add CHECK constraints to ensure consistency if one wants to forbid empty strings.
• Boolean Data Type Differences: Oracle does not have a native BOOLEAN type in tables, whereas Postgres does. To solve this, map Oracle flags NUMBER, CHAR to BOOLEAN. Update queries and application logic accordingly.

Zero-Downtime Oracle to PostgreSQL Migration with CDC

Perfect planning in Oracle to Postgres migration ensures zero downtime or near-zero downtime cutover to minimize business disruption. The migration begins with a staging environment to validate data mappings, relational schemas, and application query behavior. Business critical applications often require continuous availability, which is where Change Data Capture (CDC) enables a zero-downtime transition from Oracle to Postgres.

How CDC Works for Oracle Migrations

Change Data Capture (CDC) is a software pattern that identifies and tracks changes in a source database so that action can be taken using change data. It captures the real-time changes for inserts, updates, and deletes in real time. These changes are streamed into Postgres, which ensures both databases stay in sync. Most commonly used tools are Debezium and Kafka.

CDC works in the following flow

• Do a full export of existing data from Oracle and load it into PostgreSQL using bulk migration tools.
• CDC tools read redo/transaction logs, capture their ongoing changes (INSERT, UPDATE, DELETE), and convert the stream into Postgres.
• Now apply the captured changes to Postgres in near real time, monitor for lag and consistency.

Benefits of CDC for Migration

• Minimal zero downtime
• Reduced Business risk
• Parallel Testing
• Scalable and Flexible

Dual-Write Strategy

The dual-write strategy involves writing data on both Oracle and Postgres simultaneously during migration. Both systems stay in sync at the application level and are used alongside or instead of CDC in some cases. Choose it when CDC is not feasible.

Migration Window vs. Live Replication

Choosing between a migration window and live replication depends on business needs. When the cutover is planned, we must decide between traditional migration and live replication.

Best Practices for ensuring Zero-downtime migration

• Start with a pilot project with a small dataset to validate the pipeline.
• Monitor replication lag and system performance.
• Ensure to follow the data validation mechanism.
• Use feature flags or routing layers for controlled cutover.
• Always have a backup plan to rollover.
• Implement a rollback strategy even with CDC in place.

Testing, Validation, and Post-Migration Performance Tuning

The success of the Oracle to Postgres migration depends on how thoroughly you test, validate, and optimize the Postgres environment. Without rigorous validation and strategic tuning, it leads to decreased performance. 

Continuous monitoring of the new PostgreSQL environment is needed. Check for error logs and tune the Postgres according to workload requirements. Update the applications and systems to connect to the newly configured PostgreSQL database by modifying the connection strings, updating configuration files, or rewriting SQL queries. Differences in database behaviour, query execution, and indexing can introduce subtle issues that only appear under real workloads. 

Confirm data migrated is accurate and complete. Use checksums or hashes for data verification. This step ensures no data loss or corruption.

Post-migration validation checklist:

• The priority is analyzing query performance, reviewing executing plans, and tuning indexes based on real usage patterns. Continuously monitor applications for bottlenecks and anomalies. Utilize the cloud-native features for better scalability. Ensure row counts match between Oracle and Postgres.
• Critical applications workflows are tested with proper security roles and permissions.
• Postgres settings are generic. Database administrators should review foreign key constraints, update statistics, and enable routine vacuuming. Do a checksum on critical tables. Connection pooling configured. Indexes created for high-traffic query patterns.
• Regular backup and snapshot policies must be updated to reflect the new environment, along with automated alerting for query latency. Continuously monitoring ensures long-term performance stability and data reliability.

Proper testing and validation delivers its promise of scalability, reliability, and cost efficiency.

Common Oracle to PostgreSQL Migration Mistakes to Avoid

The most critical mistakes to avoid when doing an Oracle to Postgres migration are

• PL/SQL to PL/pgSQL: Oracle’s PL/SQL is incredibly deep with features such as autonomous transactions and structures that don’t exist natively in Postgres. To overcome this, don’t use automated tools, manually rewrite complex logic, and convert PL/SQL to PL/pgSQL carefully. Refactor complex logic into the application layer.
• No “Lift and Shift”: Copying the schema and data directly without redesigning for Postgres. To solve this, redesign the schema wherever needed, optimize accordingly to Postgres features.
• Ignoring Case Sensitive and Quoting: Oracle does not identify the identifiers whereas Postgres identifiers are folded to lowercase by default. To solve this problem, convert all tables and columns to lowercase during the Oracle to Postgres migration. This makes SQL remain clean and standard-compliant.
• Data Type Mapping Decisions: Using incorrect data type mappings impacts performance and storage. It causes data precision issues. To solve this problem, analyze actual data usage before mapping. Use appropriate Postgres type for INTEGER, BIGINT, NUMERIC, TEXT, etc., for Oracle’s NUMERIC data type.
• Application changes: One should also check the application-level impact, not only considering databases. To mitigate this, audit the environment beforehand before starting the migration and test for compatibility thoroughly.
• Validating data: Not doing the data verification once the migration has happened. To avoid this, validate row counts, checksums, and key datasets and reconciliation.
• Downtime: Downtime should be considered when planning for migration. To avoid this use, CDC for near-zero downtime and do a test cutover scenario in advance.

Why Choose Entrans for Your Oracle to Postgres Migration

Though Oracle is solid, the future now belongs to Postgres. Migrating is not just copying data to another database; it is about performance tuning and application-level changes. Oracle to Postgres migration is a complex process that demands substantial investment in skilled personnel and dedicated development time, and Entrans provides all of that. 

By meticulously planning, perfect execution, and optimizing each phase, Entrans unlocks the full potential of the Postgres database, thereby giving a strong foundation for future growth. Hiring a team of migration experts like the team from Entrans takes away the risks that come along with it; we optimize your data integration seamlessly.

Planning to migrate from Oracle to Postgres, Entrans is here to help make the transition smoother and more predictable. Book a 20-minute Consultation today.