Internals.pdf: Guru Guide To Sql Server Architecture And

Here’s a story that teaches a real-world lesson from those internals. The Case of the Midnight Slowdown

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

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. Here’s a story that teaches a real-world lesson

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. Index stats were stale

SELECT name, log_reuse_wait_desc FROM sys.databases WHERE name = 'SalesDB'; Result: LOG_BACKUP . Wait—backups were running fine. But why?

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.

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.