bigquery-optimization

BigQuery Optimization

When to Use

• Designing tables for an analytics use case on GCP
• Queries are slow or unexpectedly expensive
• Planning partitioning/clustering strategy
• Preparing for GCP Professional Data Engineer exam

Core Jobs

1. Schema Design

• Prefer denormalized schemas for analytics (avoid JOINs at scale)
• Use ARRAY and STRUCT for nested/repeated data instead of separate tables
• Choose correct column types: INT64 over STRING for IDs, TIMESTAMP over STRING for dates
• Avoid nullable columns on high-cardinality fields

2. Partitioning Strategy

• Time-unit partitioned (DAY, HOUR, MONTH, YEAR) — most common, use for time-series data
• Integer range partitioned — for numeric IDs with predictable ranges
• Ingestion-time partitioned — auto-partitions on load time (_PARTITIONTIME pseudo-column)
• Always add partition filter in queries (WHERE date = '2024-01-01') to reduce bytes billed

3. Clustering

• Apply after partitioning; up to 4 clustering columns
• Order columns by highest cardinality first (most selective → least selective)
• Best for filter columns: user_id, country, event_type
• Clustering improves query performance but does NOT reduce bytes billed (unlike partitioning)

4. Query Optimization

• Avoid SELECT * — select only needed columns (columnar storage)
• Filter early with WHERE on partitioned/clustered columns
• Use WITH CTEs for readability; avoid deep nesting
• Avoid self-joins; use window functions instead
• Use APPROX_COUNT_DISTINCT for large cardinality estimates

5. Cost Management

• On-demand pricing — pay per bytes scanned (good for irregular workloads)
• Capacity pricing (slots) — flat-rate, good for predictable high-volume workloads
• Monitor with INFORMATION_SCHEMA.JOBS_BY_PROJECT
• Use BI Engine for in-memory acceleration of dashboard queries
• Materialized views for repeated aggregations (auto-refresh on DML)

6. Access Control

• Use Authorized Views for row/column-level security without data duplication
• Apply dataset-level IAM before table-level (principle of least privilege)
• Column-level security via Policy Tags (BigQuery column security + Data Catalog)

Key Concepts

• Slot — unit of BigQuery compute (1 slot = 1 CPU thread for query processing)
• Shuffle — cross-node data transfer; minimize with partition pruning
• Materialized View — pre-computed results, auto-refreshed; use for repeated aggregations
• INFORMATION_SCHEMA — metadata views for monitoring jobs, tables, partitions

Checklist

• Table partitioned on a date/timestamp column?
• Clustering applied (up to 4 cols, high cardinality first)?
• All queries use partition filter in WHERE clause?
• SELECT * avoided — only needed columns selected?
• Authorized views used for sensitive data instead of data copies?
• Materialized views created for repeated aggregations?
• Cost monitoring via INFORMATION_SCHEMA.JOBS?

Output Format

• 🔴 Critical — full table scan with no partition filter (unbounded cost)
• 🟡 Warning — missing clustering, suboptimal JOIN order, no partition pruning
• 🟢 Suggestion — materialized view opportunity, slot reservation candidate

Exam Tips

• Partitioning reduces cost (fewer bytes scanned); clustering improves performance (not cost)
• Authorized views → share data without duplicating; row-level security without VPC-SC
• INFORMATION_SCHEMA.JOBS_BY_PROJECT → monitor slot usage and bytes billed
• Flat-rate (capacity) pricing → predictable cost for high-volume; on-demand → pay-per-query
• BI Engine → in-memory cache for Looker/Data Studio; does NOT replace Bigtable for OLTP
• Cross-region BigQuery JOIN → significant data transfer cost; prefer co-located datasets