Compare Showplan shows where two plans differ, but not why. Here is a small before and after example you can run yourself.
Estimated vs Actual Rows: The First Thing to Read in a Plan
Read estimated vs actual rows in execution plans, find where the first gap appears, and investigate statistics, parameters, and predicates.
Sample Databases for Practicing Performance Tuning
Use sample databases for practicing performance tuning by checking data volume and skew, then enlarging the right tables carefully.
The Last Actual Execution Plan: LAST_QUERY_PLAN_STATS in Practice
Capture the last actual execution plan with LAST_QUERY_PLAN_STATS, inspect runtime rows, and understand what cache retention cannot provide.
Why Queries Recompile
Why queries recompile becomes clearer when you capture Extended Events causes and check schema changes, statistics, SET options, and temp tables.
Query Store Hints: Fixing a Query Without Touching Its Code
Apply query store hints to a vendor query without code changes, verify failures and plan behavior, then clear the hint after the real fix.
SQL SERVER – Introduction of Showplan Warning
SQL Server can be your performance tuning teacher. Vinod Kumar explains the Showplan Warning in execution plans and what each warning is telling you.