S3 select scan order

S3 Select does not guarantee any specific order for scanning CSV rows. It is designed to work on objects stored in Amazon S3, which is a distributed storage system, and it may process data in parallel across multiple nodes, which can cause the order of scanned rows to be non-deterministic.

Therefore, if you want to retrieve specific rows from a CSV file stored in S3 using S3 Select, you should not rely on any particular order of scanning the rows. S3 Select is optimized for performance, and it may choose to scan the entire file or only a subset of it, depending on factors such as the query filters and the size of the file.

In your example, if the rows you want to select are close to the bottom of the file, S3 Select may still need to scan the entire file to find them, depending on the query filters you are using. The same would be true if those rows were in the first rows of the file.

To improve the performance of S3 Select, you can consider partitioning your CSV file into smaller files or using a columnar data format like Apache Parquet, which can improve query performance by reducing the amount of data that needs to be scanned.