Decision Tree

Need query optimization?   → Use EXPLAIN ANALYZE
Need cached aggregations?  → Use Materialized View
Need complex breakdown?    → Use CTEs
Need row ranking?          → Use Window Functions
Need partial coverage?     → Use Partial Index

Reporting & Optimization Guidelines

Reporting queries often scan large datasets. Inefficient queries can degrade the performance of the entire system. Follow these guidelines to ensure speed and stability.

1. Query Optimization

1.1 SELECT Efficiency

• No SELECT *: Fetching unnecessary columns increases I/O and network payload. Select only required fields.
• SARGable Queries: Ensure predicates (WHERE clauses) are “Search ARGument ABLE” to perform Index Seeks instead of Scans.
  • Bad: WHERE YEAR(created_at) = 2023 (Function on column prevents index use).
  • Good: WHERE created_at >= '2023-01-01' AND created_at < '2024-01-01'.

1.2 CTEs vs Temporary Tables

• Common Table Expressions (CTEs): Use WITH clauses for readability and to break down complex logic. In modern PostgreSQL (12+), CTEs are materialized when beneficial, making them performant.
• Temporary Tables: For extremely complex multi-step processing involving heavy intermediate indexing, use CREATE TEMP TABLE.

1.3 Subqueries

• Avoid Correlated Subqueries in the SELECT list that run once per row. Rewrite them as JOINs or LATERAL joins.
  • Bad: SELECT id, (SELECT count(*) FROM orders WHERE user_id = u.id) FROM users u
  • Good: SELECT u.id, count(o.id) FROM users u LEFT JOIN orders o ON u.id = o.user_id GROUP BY u.id

2. Aggregation Strategies

2.1 Materialized Views

• For dashboards requiring heavy aggregation (Count, Sum, Avg) over millions of rows, do not query the live transactional table every time.
• Use Materialized Views to cache the result:

  CREATE MATERIALIZED VIEW mv_daily_sales AS
  SELECT day, SUM(total) FROM sales GROUP BY day;
  

• Refresh explicitly: REFRESH MATERIALIZED VIEW CONCURRENTLY mv_daily_sales;.

2.2 Window Functions

• Use Window Functions (ROW_NUMBER(), RANK(), LEAD(), LAG()) for complex reporting intra-row logic instead of self-joins.

3. Safety Guardrails

3.1 Timeouts

• Set statement_timeout for reporting roles to prevent a runaway query from locking resources indefinitely.

3.2 Result Limits

• Always verify the estimated cardinality. If a report could return 1M+ rows, enforce LIMIT or pagination logic.

4. Query Profiling

4.1 EXPLAIN ANALYZE

Before deploying complex queries, use EXPLAIN ANALYZE to understand the execution plan.

• Look for Seq Scans on large tables (potential missing index).
• Check Actual vs Estimated Rows for significant discrepancies (stale statistics).
• Identify Nested Loops with high row counts that could be rewritten as Hash Joins.

EXPLAIN ANALYZE
SELECT u.id, COUNT(o.id)
FROM USER u
LEFT JOIN ORDER o ON u.id = o.user_id
WHERE u.is_active = TRUE
GROUP BY u.id;

4.2 Partial Indexes

Create indexes that only cover a subset of rows to save space and speed up specific queries.

• Example: Index only active users.

  CREATE INDEX idx_user_active ON USER(email) WHERE is_active = TRUE;
  

• Use for columns with low cardinality where only specific values are frequently queried.