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Understanding how an SQL query executes in a database is essential for performance tuning and system design. Here’s a step-by-step breakdown of what happens under the hood when you run an SQL query like:

SELECT name FROM employees WHERE department = 'Sales';

🧭 0. Query Input (Your SQL)

You submit the SQL query via:

• SQL CLI / GUI (like pgAdmin, MySQL Workbench)
• Application backend
• Notebook (e.g., Databricks, Jupyter)

⚙️ Step-by-Step SQL Query Execution

🧩 Step 1: Parsing

• The SQL Parser checks for syntax errors.
• Converts SQL into an abstract syntax tree (AST).
• Example: Confirms SELECT, FROM, WHERE are valid clauses.

✅ If there’s a syntax error → You get an error immediately.

📊 Step 2: Query Rewrite (Optional)

• The query rewrite engine transforms the SQL to an equivalent but more efficient version.
• Includes:
  • View expansion (resolving a view into its base query)
  • Subquery flattening
  • Predicate pushdown
  • Common subexpression elimination

Example:

SELECT * FROM view_emp WHERE dept = 'HR';

→ gets rewritten to underlying table query.

🧠 Step 3: Query Optimization

• The query optimizer generates multiple query plans and picks the least-cost plan using:
  • Table statistics (row count, distinct values)
  • Index availability
  • Join order and type
  • Available keys, partitions

Example decision:

Should I scan the whole table or use an index on department?

✅ Outputs an execution plan (physical plan).

🛠️ Step 4: Query Plan Execution (Physical Operators)

The engine executes the physical plan with:

• Scans (full table or index)
• Filters (apply WHERE conditions)
• Joins, Aggregations
• Sorts, Grouping

All operators work in a pipeline, usually row-by-row or batch-by-batch.

💾 Step 5: Data Fetch / Buffer / Storage Layer

• Fetches actual data blocks/pages from:
  • Memory (buffer pool / cache)
  • Disk (if not cached)
• Applies filtering logic (e.g., department = 'Sales')

🔍 Uses indexes (if available) to avoid full scan.

📤 Step 6: Results Return

• The output rows are passed back:
  • Through client interface (ODBC, JDBC)
  • Shown in your query editor / app
• May be paginated or streamed depending on client

🗂️ Summary Flowchart

SQL Query 
   ↓
1. Parsing → Syntax Tree
   ↓
2. Rewrite (Views, Subqueries)
   ↓
3. Optimization (Plan + Cost Model)
   ↓
4. Execution Plan (Scan, Filter, Join, Aggregate)
   ↓
5. Fetch Data from Memory/Disk
   ↓
6. Return Result to User

🔍 Example: Optimized Plan for This Query

SELECT name FROM employees WHERE department = 'Sales';

• Use index on department (if exists)
• Perform index seek to locate matching rows
• Retrieve name column using a covering index (if name is in index)
• Avoid full scan

🧪 Tools to View Execution Plan

🧠 Bonus: What Affects Execution?

• Indexes and stats
• Query complexity (joins, subqueries)
• Table size and partitioning
• Caching
• Database engine (PostgreSQL vs MySQL vs Spark SQL)

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