That open transaction was preventing the transaction log from truncating. The log had grown to 200 GB. The ETL’s large update inside FactSales_Load had to wait for log space, causing log autogrowth events (zero-initialization → slow).
The transaction log is a circular log. It can’t reuse space if any active transaction holds onto a VLFL (virtual log file) even if it’s old. Guru Guide To Sql Server Architecture And Internals.pdf
SELECT * FROM sys.dm_os_buffer_descriptors WHERE database_id = DB_ID('SalesDB'); He saw that 40 GB of the buffer pool was filled with old data from a morning report. The ETL’s needed pages (the clustered index of Orders ) were being paged in from disk— couldn’t save it because the scan had already caused random I/O earlier. That open transaction was preventing the transaction log
UPDATE STATISTICS Orders; The plan switched to an index seek. The ETL dropped to 12 minutes. Good, but not great. Why not 8 minutes? Alex dug deeper. During the ETL, he monitored: The transaction log is a circular log