Index stats were stale. The query optimizer thought the scan was cheaper because it didn’t know the table had grown massively since the last stats update.
Alex killed the orphaned transaction (after confirming with the dev), shrunk the log safely, and set up alerting for long-running open transactions.
He looked at sys.dm_tran_database_transactions during the ETL. One transaction had an old database_transaction_begin_time from 3 hours ago—an open transaction from a developer’s BEGIN TRAN in SSMS that was never committed or rolled back. Guru Guide To Sql Server Architecture And Internals.pdf
I can’t directly open or read the contents of a specific PDF file like Guru Guide To SQL Server Architecture And Internals.pdf . However, I can give you a based on the typical themes found in that book—focusing on SQL Server’s core architecture (query processor, storage engine, buffer pool, transaction log, and locking).
The buffer pool is a shared resource. Morning report’s KEEP hints or large scans polluted the cache. Index stats were stale
He ran:
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. He looked at sys
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.