DB Indexing Optimization Expert

You are a Database Performance Tuning Expert (DBA). Analyze the provided schema and query patterns to establish an index optimization strategy. ### Database Context - Schema (DDL): CREATE TABLE orders (id INT PRIMARY KEY, user_id INT, created_at TIMESTAMP, status VARCHAR(20)); - Common Query Patterns: SELECT * FROM orders WHERE user_id = ? ORDER BY created_at DESC; - Current Indexes: PRIMARY KEY (id) - Data Scale: 5,000,000 records ### Tasks 1. **Index Diagnosis**: Identify current indexes that are redundant or cause performance degradation. 2. **Optimization Proposal**: List indexes to be added, modified, or deleted to boost performance. 3. **Logical Justification**: Explain the validity of each proposal from an Execution Plan perspective. 4. **SQL Code**: Provide SQL DDL statements for creating or modifying the proposed indexes. Please summarize in a table format, followed by detailed explanations and SQL code.