Gold Layer Documentation Standards

Overview

Gold layer tables require comprehensive, dual-purpose documentation that serves both business users and technical users (including LLMs like Genie). This skill standardizes naming conventions, description formats, and metadata requirements for production-grade Gold layer implementations.

When to Use This Skill

• Creating Gold layer table DDL with comments
• Writing column descriptions for dimensions and facts
• Documenting surrogate keys, business keys, and foreign keys
• Implementing SCD Type 2 dimensions
• Setting up table properties and governance metadata
• Preparing documentation for Genie Spaces and AI/BI tools

Critical Rules

1. YAML Schema Files Are Single Source of Truth

CRITICAL: Before writing any Gold layer SQL (TVFs, queries, MERGE statements), ALWAYS consult the YAML schema definitions in gold_layer_design/yaml/**/*.yaml.

Why: 100% of SQL compilation errors in production deployments were caused by not consulting YAML schemas first.

2. Dual-Purpose Documentation

Every description must serve both business and technical audiences without requiring an "LLM:" prefix.

Pattern:

[Natural description]. Business: [business context and use cases]. Technical: [implementation details and calculations].

See LLM Optimization Guide for complete patterns.

3. Surrogate Keys as Primary Keys

• Surrogate keys (store_key, product_key, date_key) are PRIMARY KEYS
• Business keys (store_number, upc_code, date) are denormalized for readability
• Facts reference surrogate PKs via foreign keys

Quick Reference

Column Naming Conventions

Column Type	Pattern	Example	Notes
Surrogate Key	{entity}_key	store_key, product_key	Always NOT NULL, PRIMARY KEY
Business Key	Natural name	store_number, upc_code	Denormalized in facts
Foreign Key	{entity}_key	store_key	References surrogate PK
Measure	Descriptive	net_revenue, units_sold	Include calculation formula
Percentage	{name}_pct	return_rate_pct	Document NULL handling
Boolean Flag	is_{state}	is_current, is_weekend	Document TRUE/FALSE meanings
Timestamp	{purpose}_timestamp	record_created_timestamp	Document update behavior

Table Comment Structure

Every Gold table comment must include:

1. Layer and purpose
2. Grain (what one row represents)
3. Business use cases
4. Technical implementation

See Documentation Templates for complete examples.

Required Table Properties

TBLPROPERTIES (
    # Performance
    'delta.enableChangeDataFeed' = 'true',
    'delta.enableRowTracking' = 'true',
    'delta.enableDeletionVectors' = 'true',
    'delta.autoOptimize.autoCompact' = 'true',
    'delta.autoOptimize.optimizeWrite' = 'true',
    
    # Layer and Quality
    'quality' = 'gold',
    'layer' = 'gold',
    'source_layer' = 'silver',
    'source_table' = '<silver_source_table>',
    
    # Domain and Classification
    'domain' = '<retail|sales|inventory|product>',
    'entity_type' = '<dimension|fact>',
    
    # Governance
    'contains_pii' = '<true|false>',
    'data_classification' = '<confidential|internal|public>',
    'business_owner' = '<Team Name>',
    'technical_owner' = 'Data Engineering',
    
    # Dimension-specific
    'scd_type' = '<1|2>',
    
    # Fact-specific
    'grain' = '<description of grain>',
    'gold_type' = '<aggregated|snapshot>'
)

Core Patterns

Column Description Format

Standard Template:

[One-sentence definition]. Business: [Business purpose, use cases, and context]. Technical: [Data type, format, calculation logic, source, constraints].

Example - Surrogate Key:

store_key STRING NOT NULL COMMENT 'Surrogate key uniquely identifying each version of a store record. Business: Used for joining fact tables to dimension. Technical: MD5 hash generated from store_id and processed_timestamp to ensure uniqueness across SCD Type 2 versions.'

Example - Measure:

net_revenue DECIMAL(18,2) COMMENT 'Net revenue after subtracting returns from gross revenue. Business: The actual revenue realized from sales, primary KPI for financial reporting. Technical: gross_revenue - return_amount, represents true daily sales value.'

See Documentation Templates for examples by field type.

Table Comment Patterns

Dimension Table:

COMMENT 'Gold layer conformed {entity} dimension with {SCD Type}. Business: {Business purpose, history tracking, use cases}. Technical: {SCD implementation details, key strategy, update behavior}.'

Fact Table:

COMMENT 'Gold layer {period} {subject} fact table with {aggregation level} at {grain}. Business: {Primary use cases, metrics included, reporting purpose}. Technical: Grain is {what one row represents}. {Performance optimizations, key relationships}.'

SCD Type 2 Pattern

For slowly changing dimensions, include:

• effective_from TIMESTAMP NOT NULL - Start timestamp
• effective_to TIMESTAMP - End timestamp (NULL = current)
• is_current BOOLEAN NOT NULL - Current version flag

Always filter with WHERE is_current = true when joining to facts.

Validation Checklist

Naming Conventions

• Surrogate keys use {entity}_key pattern
• Business keys use natural terminology
• Measures use descriptive names (no cryptic abbreviations)
• Percentages end with _pct
• Boolean flags start with is_ or action verb
• Timestamps end with _timestamp
• SCD fields use effective_from, effective_to, is_current

Column Documentation

• Every column has a comment
• Comments follow: [Definition]. Business: [context]. Technical: [details]. format
• Surrogate keys document hash generation method
• Business keys document source and immutability
• Foreign keys document referenced table and column
• Measures document calculation formula
• Flags document TRUE/FALSE meanings
• Timestamps document update behavior

Table Documentation

• Table comment includes layer and grain
• Table comment explains business use cases
• Table comment describes technical implementation
• TBLPROPERTIES includes all required fields
• grain property set for fact tables
• scd_type property set for dimensions
• CLUSTER BY AUTO specified

Primary and Foreign Keys

• Surrogate keys are NOT NULL
• PRIMARY KEYs defined on surrogate keys
• FOREIGN KEYs reference surrogate PKs
• Facts have composite PKs matching grain
• UNIQUE constraints on business keys (where applicable)

Common Mistakes to Avoid

❌ Mistake 1: Using "LLM:" prefix

# OLD PATTERN (Don't use)
store_key STRING NOT NULL COMMENT 'LLM: Surrogate key (unique per version)'

✅ Correct: Natural dual-purpose description

store_key STRING NOT NULL COMMENT 'Surrogate key uniquely identifying each version of a store record. Business: Used for joining fact tables to dimension. Technical: MD5 hash generated from store_id and processed_timestamp to ensure uniqueness across SCD Type 2 versions.'

❌ Mistake 2: Business key as PRIMARY KEY

# WRONG: Breaks dimensional modeling
CREATE TABLE dim_store (
    store_key STRING NOT NULL,
    store_number STRING NOT NULL PRIMARY KEY,  -- ❌ Wrong
    ...
)

✅ Correct: Surrogate key as PRIMARY KEY

CREATE TABLE dim_store (
    store_key STRING NOT NULL,
    store_number STRING NOT NULL,
    ...
    CONSTRAINT pk_dim_store PRIMARY KEY (store_key) NOT ENFORCED,  -- ✅ Correct
    CONSTRAINT uk_store_number UNIQUE (store_number) NOT ENFORCED  -- Business key is UNIQUE
)

❌ Mistake 3: Missing grain in TBLPROPERTIES

# Fact table without grain
TBLPROPERTIES (
    'layer' = 'gold',
    'entity_type' = 'fact'
    # Missing 'grain' property
)

✅ Correct: Grain documented

TBLPROPERTIES (
    'layer' = 'gold',
    'entity_type' = 'fact',
    'grain' = 'daily_store_product'  -- ✅ Clear grain definition
)

Reference Files

Documentation Templates

references/documentation-templates.md - Complete column and table comment templates with examples for all field types (surrogate keys, business keys, foreign keys, measures, percentages, boolean flags, timestamps, SCD Type 2 fields).

LLM Optimization

references/llm-optimization.md - Dual-purpose documentation patterns optimized for Genie and AI/BI tools, including natural language optimization techniques and testing strategies.

Assets

Documentation Template

assets/templates/gold-table-docs.yaml - YAML template for Gold layer table documentation with all required fields and examples.

References

Dimensional Modeling

• Kimball Dimensional Modeling
• Star Schema Design

Unity Catalog

• Unity Catalog Constraints
• Primary and Foreign Keys

Delta Lake

• Delta Table Properties
• Automatic Clustering

Inputs

• From 05-erd-diagrams: ERD diagrams with domain groupings, table/column names, and relationship notation
• From 01-grain-definition through 04-conformed-dimensions: Grain types, dimension patterns, fact table patterns, and conformed dimension list

Outputs

• YAML schema files with dual-purpose descriptions for every table and column
• Complete TBLPROPERTIES metadata (layer, domain, entity_type, grain, scd_type)
• Surrogate key patterns and SCD Type 2 field documentation
• Column-level lineage metadata embedded in each YAML

Design Notes to Carry Forward

After completing this skill, note:

• Which dimensions use SCD Type 2 (need effective_from, effective_to, is_current fields)
• YAML files are the single source of truth — implementation reads from them
• Any non-standard column descriptions that may need semantic layer attention

Next Step

Proceed to design-workers/07-design-validation/SKILL.md to cross-validate all design artifacts (YAML ↔ ERD ↔ Lineage) before handoff to implementation.