SQL Server to PostgreSQL Migration: What to Audit Before You Move

Meerako — Dallas-based backend architects with deep PostgreSQL expertise.

Migrating from SQL Server to PostgreSQL is a common move — usually driven by licensing cost, a desire to avoid vendor lock-in, or a broader move to open-source infrastructure. The teams that migrate smoothly are the ones who audit the specific SQL Server features their application actually depends on before starting, rather than assuming the migration is a straightforward schema-and-data transfer.

• What actually makes SQL Server and PostgreSQL meaningfully different, beyond SQL syntax.
• The specific audit steps that catch migration risk before it becomes a production problem.
• How to handle stored procedures, which are the most common migration blocker.
• How Meerako approaches database migration projects.

SQL Server and PostgreSQL differ in more than SQL dialect — data types don't map one-to-one (SQL Server's DATETIME and PostgreSQL's TIMESTAMP behave differently in edge cases), transaction isolation defaults differ, and PostgreSQL's approach to indexing and query planning has real behavioral differences that can affect application performance even after a technically successful data migration.

• Stored procedures and functions. T-SQL and PL/pgSQL are not directly compatible — every stored procedure needs to be rewritten, not translated mechanically, and this is consistently the largest source of underestimated migration effort.
• Data type mapping, reviewed field by field for anything beyond basic types — dates, decimals with specific precision requirements, and any SQL Server-specific types need explicit mapping decisions.
• Application-level SQL, including any ORM-generated queries or raw SQL embedded in application code that may rely on SQL Server-specific syntax or behavior.
• Indexing strategy, since a direct copy of SQL Server's index structure doesn't automatically produce equivalent PostgreSQL performance — see our PostgreSQL indexing deep dive for how PostgreSQL's index types differ meaningfully from SQL Server's.
• Third-party integrations and reporting tools that connect directly to the database, which need to be verified against PostgreSQL, not assumed to work identically.

Teams consistently underestimate stored procedure migration because it looks like syntax translation but is actually a rewrite requiring genuine understanding of the original procedure's business logic — logic that's sometimes as undocumented as any other legacy system's business rules. Budget real time for this specifically, and start it early in the migration timeline rather than treating it as a final cleanup step.

1. Audit and inventory every stored procedure, custom function, and SQL Server-specific feature in use.
2. Migrate schema and data types first, validated against a full data comparison, before touching application logic.
3. Rewrite stored procedures individually, testing each against real historical data and expected outputs.
4. Run both databases in parallel for a validation period, comparing outputs before full cutover — similar to the phased approach we recommend for any business-critical migration.

We start with a full audit of SQL Server-specific dependencies — especially stored procedures — before committing to a migration timeline, because that audit is what determines whether the migration is a straightforward few weeks or a genuinely substantial rewrite effort.

How long does a SQL Server to PostgreSQL migration typically take? Highly dependent on stored procedure complexity — a database with minimal procedural logic might migrate in 4-6 weeks; one with extensive stored procedures can take several months.

Are there tools that automate this migration? Tools exist to assist with schema and data migration, but stored procedure conversion generally requires manual rewriting and testing — automated conversion tools produce a starting point, not a finished, trustworthy result.

Will our application code need changes after migration? Likely yes, for any SQL Server-specific syntax or behavior the application relies on directly — auditing this before migration avoids discovering it in production.

Can we migrate incrementally, or does it have to be all at once? Running both databases in parallel during a validation period, with a full cutover once confidence is established, is generally safer than an all-at-once switch for business-critical systems.

A SQL Server to PostgreSQL migration's real complexity lives in stored procedures and behavioral differences, not basic schema translation. Audit these dependencies thoroughly before committing to a timeline, and the migration becomes a manageable, validated project instead of a source of post-cutover surprises.

If you're planning a SQL Server to PostgreSQL migration, Meerako can help you audit the real scope before you commit.

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