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.
Alex killed the orphaned transaction (after confirming with the dev), shrunk the log safely, and set up alerting for long-running open transactions.
Alex updated stats:
Here’s a story that teaches a real-world lesson from those internals. The Case of the Midnight Slowdown
The buffer pool is a shared resource. Morning report’s KEEP hints or large scans polluted the cache. Guru Guide To Sql Server Architecture And Internals.pdf
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:
SELECT last_user_seek, last_user_scan, modifications FROM sys.dm_db_index_usage_stats WHERE database_id = DB_ID('SalesDB') AND object_id = OBJECT_ID('Orders'); The result: last_user_seek was yesterday. modifications was over 50,000. The transaction log is a circular log
SELECT name, log_reuse_wait_desc FROM sys.databases WHERE name = 'SalesDB'; Result: LOG_BACKUP . Wait—backups were running fine. But why?
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 updated stats: Here’s a story that teaches
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.
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).