Why should you care?

After a deploy, one query suddenly slows — Query Store shows the new plan versus the old one.

See it live — copy this example

Run in SQL Server Management Studio (SSMS) or Azure Data Studio.

ALTER DATABASE DataVerse SET QUERY_STORE = ON (OPERATION_MODE = READ_WRITE);
ALTER DATABASE DataVerse SET QUERY_STORE CLEAR ALL; -- lab only
SELECT TOP (20) q.query_id, qt.query_sql_text, rs.avg_duration
FROM sys.query_store_query q
JOIN sys.query_store_query_text qt ON qt.query_text_id = q.query_text_id
JOIN sys.query_store_plan p ON p.query_id = q.query_id
JOIN sys.query_store_runtime_stats rs ON rs.plan_id = p.plan_id
ORDER BY rs.avg_duration DESC;

What happened?

• Enabling Query Store starts capturing.
• The SELECT finds recent heavy queries by average duration — start of a regression hunt.

Practice next

1. Turn Query Store on for your database.
2. Run a few ordinary queries.
3. Open Query Store reports in SSMS or run the DMV query.
4. Set MAX_STORAGE_SIZE_MB thoughtfully.
5. Compare wait stats categories in Query Store UI.

Remember

Query Store history of plans + stats. Use it to diagnose regressions. Force plans sparingly.

Post-release regression

DataVerse checkout query flipped plans overnight.

Outcome: Team forced the prior plan, then fixed sniffing properly.