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Career/Database - SQL/SQL Execution Plans.md

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+---
+note type:
+  - database
+  - sql
+  - note
+date: 2026-07-03
+done:
+link: https://bertwagner.com/posts/introduction-to-sql-server-execution-plans/
+---
+## What is an Execution Plan?
+
+- SQL Server is a **declarative language**:
+    - You specify _what_ data you want, not _how_ to retrieve it.
+- An **execution plan** shows:
+    - How SQL Server decides to retrieve the data.
+    - The sequence of operations (joins, scans, seeks, etc.).
+
+### Key Idea
+
+- The **Query Optimizer**:
+    - Evaluates multiple possible ways to execute a query.
+    - Chooses a plan that is **“good enough”**, not necessarily the absolute fastest.
+- Reason:
+    - Evaluating every possible plan would be too slow and impractical.
+
+### When Problems Occur
+
+- Poor performance happens when:
+    - The optimizer picks an inefficient plan.
+- Causes may include:
+    - Inaccurate statistics
+    - Incorrect assumptions about data distribution
+- Execution plans help diagnose these issues.
+
+---
+
+## Types of Execution Plan Views
+
+### 1. Text-Based Execution Plan
+
+To see an execution plan for your query, you can run `SET SHOWPLAN_ALL ON`. This will provide a text-based tree representation of the plan:
+
+`SET SHOWPLAN_ALL ON GO --Query 
+`GO SET SHOWPLAN_ALL OFF GO`
+
+![[Pasted image 20260703150027.png]]
+
+### 2. Graphical Execution Plan (Preferred)
+
+- Generated via:
+    - **“Display Estimated Execution Plan”** in SSMS
+- Features:
+    - Visual flow of query execution
+    - Easier to interpret than text-based plans
+![[Pasted image 20260703150047.png]]
+#### XML View
+
+- Access via:
+    - Right-click → “Show Execution Plan XML”
+- Contains:
+    - Detailed metadata and properties not always visible graphically
+
+---
+
+## Estimated vs Actual Execution Plans
+
+### Estimated Execution Plan
+
+- Created **before execution**
+- Based on:
+    - Table statistics
+    - Metadata
+- Does **NOT include runtime data**
+
+---
+
+### Actual Execution Plan
+
+- Created **after execution**
+- Includes:
+    - Actual number of rows processed
+    - Number of executions
+    - Runtime metrics
+
+### Key Difference
+
+|Feature|Estimated Plan|Actual Plan|
+|---|---|---|
+|When generated|Before execution|After execution|
+|Uses real data|No|Yes|
+|Structure|Same|Same|
+|Runtime metrics|No|Yes|
+
+**Important Insight:**
+
+- The plan structure is usually identical.
+- Only the **runtime statistics differ**.
+
+---
+
+## Live Query Statistics (SQL Server 2016+)
+
+### What It Does
+
+- Shows execution progress in **real time**
+- Combines:
+    - Estimated plan + live runtime data
+
+### Benefits
+
+- Helps identify **bottlenecks quickly**
+- Useful for:
+    - Long-running queries
+    - Beginners learning execution plans
+
+### Example Insight
+
+- If a **Clustered Index Scan** takes most time:
+    - Focus optimisation efforts there.
+
+---
+
+## Historical Execution Plans
+
+### 1. Plan Cache
+
+- SQL Server stores plans for reuse.
+- Access via DMV:
+    - `sys.dm_exec_query_plan`
+
+#### Key Notes
+
+- Contains:
+    - Cached (previously executed) query plans
+- Limitations:
+    - Plans may be removed from cache
+    - Typically shows **estimated plans only**
+
+---
+
+### 2. Query Store
+
+- Stores historical query performance data
+- Requirements:
+    - Must be enabled on the database
+
+#### Benefits
+
+- Allows:
+    - Retrieval of past execution plans
+    - Query performance analysis over time
+
+---
+
+## Why Execution Plans Matter
+
+- Provide visibility into:
+    - How SQL Server retrieves data
+- Essential for:
+    - **Performance tuning**
+    - Identifying inefficient operations
+    - Diagnosing slow queries
+
+---
+
+## Summary
+
+- Execution plans reveal **how SQL Server executes queries**.
+- The optimizer selects a **“good enough” plan**, not always optimal.
+- Main viewing options:
+    - Text plan
+    - Graphical plan (most useful)
+    - XML plan
+- Types of plans:
+    - **Estimated** → Pre-execution
+    - **Actual** → Includes runtime data
+- Advanced tools:
+    - **Live Query Statistics** → real-time insights
+    - **Plan Cache & Query Store** → historical analysis