When an enterprise decides to migrate from SQL Server to PostgreSQL, the first conversation is almost always about data movement. How many terabytes need to transfer? Which replication tool should we use? What is the cutover window? These are necessary questions, but they obscure a harder truth: moving the data is the straightforward part. Keeping the application working is where migrations stall.SQL Server applications were built on assumptions that PostgreSQL does not share. The T-SQL dialect handles NULL comparisons, temporary table scoping, date arithmetic, and cursor behavior differently than PL/pgSQL. The Tabular Data Stream (TDS) wire protocol has no native equivalent in the PostgreSQL ecosystem. Application drivers, connection strings and ORM configurations frequently contain SQL Server-specific parameters that either fail silently or produce different behavior when pointed at a PostgreSQL endpoint.