collecting

The PostgreSQL community has developed tools like AQO and sr_plan extensions to enhance migration paths from Oracle, making PostgreSQL more appealing for transitions. PostgreSQL has been noted to outperform Oracle in certain scenarios, particularly in automatic re-optimization. Migrations from Oracle to PostgreSQL are generally smooth, aided by session variable extensions. However, migrating from SQL Server to PostgreSQL presents challenges, including significant query slowdowns. For example, a query that executed in 20 milliseconds on SQL Server took weeks on PostgreSQL due to an inefficient query plan. A specific JOIN operation involving GROUP BY on small tables showed a drastic difference in execution time: PostgreSQL took 4000 seconds while SQL Server completed it in 300 seconds. SQL Server employs a Hash Join and parallelizes execution across eight threads, enhancing performance. PostgreSQL's excessive time on hash calculations and tuple comparisons during this operation was noted, as well as its limitations in parallel processing due to temporary tables not being visible to parallel workers. Adjusting PostgreSQL settings for parallel workers improved performance, allowing it to match SQL Server's execution time. PostgreSQL's selectivity estimation for JOINs with multiple conditions often leads to underestimations, while SQL Server excels in this area by collecting extensive statistics. PostgreSQL's current extended statistics capabilities are limited, but the community is working on improvements. SQL Server's advanced parameterization and caching further enhance its optimization capabilities compared to PostgreSQL.