Schema Evaluation

Schema Evaluation

Purpose

Evaluate and design data warehouse schemas for analytical workloads. Covers star schemas, snowflake schemas, data vault, and One Big Table (OBT) patterns. Assesses grain definition, normalization trade-offs, slowly changing dimension strategies, and data contracts between producers and consumers.

Inputs

• Business domain and key entities (e.g., e-commerce: orders, products, customers)
• Analytical queries the schema must support (e.g., "revenue by product category by month")
• Data volume estimates (row counts, growth rate)
• Source systems and their update patterns (CDC, full refresh, event stream)
• Existing schema (if evaluating rather than designing from scratch)

Process

Step 1: Define the Grain

Identify the grain of each fact table — what does one row represent? A single transaction? A daily snapshot? A session event? The grain determines everything downstream. Document the grain as a clear English sentence: "One row = one order line item" or "One row = one daily active user per product."

Step 2: Identify Facts and Dimensions

Separate measurable facts (revenue, quantity, duration, count) from descriptive dimensions (customer, product, date, geography). For each:

• Facts: Data type, aggregation method (SUM, AVG, COUNT DISTINCT), nullability
• Dimensions: Cardinality, hierarchy levels, whether it changes over time (SCD candidate)

Step 3: Choose a Modeling Approach

Evaluate which pattern fits the requirements:

• Star schema — Simple, fast queries, denormalized dimensions. Best for straightforward BI with a known query pattern.
• Snowflake schema — Normalized dimensions for storage efficiency and consistency. Best when dimensions are large or shared across many facts.
• Data vault — Hub, link, satellite pattern for auditability and flexibility. Best when source systems change frequently or full history is required.
• One Big Table (OBT) — Fully denormalized single table. Best for small teams, simple analytics, or when query simplicity outweighs storage concerns.

Document the chosen approach and the reasoning behind it.

Step 4: Design Slowly Changing Dimension Strategy

For each dimension that changes over time, specify the SCD type:

• Type 1 — Overwrite. No history. Simple, but you lose the old value.
• Type 2 — Add new row with effective dates. Full history, but increases row count and query complexity.
• Type 3 — Add column for previous value. Limited history (only one prior value), but simple to query.
• Type 6 — Hybrid (1+2+3). Current value column plus history rows. Best of both but most complex.

Document which SCD type applies to each changing attribute and why.

Step 5: Define Data Contracts

For each source-to-warehouse interface, specify the contract:

• Schema expectations (required fields, data types, allowed values)
• Freshness SLA (how soon after source update must the warehouse reflect it?)
• Quality thresholds (max null rate, uniqueness constraints, referential integrity)
• Breaking change policy (how are schema changes communicated and handled?)

Step 6: Validate Against Query Patterns

Test the proposed schema against the required analytical queries:

• Can each query be answered with at most 2-3 joins?
• Are the most common filters (date, category, status) on dimension keys?
• Is the grain appropriate — not too fine (wasteful) or too coarse (lossy)?
• Are aggregate tables or materialized views needed for high-frequency dashboards?

Step 7: Document the Schema

Produce a complete schema specification with DDL, relationships, and usage notes.

Output Format

# Schema Evaluation: [Domain/Project Name]

## Grain Definitions

| Fact Table | Grain (one row = ...) | Estimated Rows | Growth Rate |
|------------|----------------------|----------------|-------------|
| ...        | ...                  | ...            | ...         |

## Entity Relationship Summary

[ASCII diagram showing fact and dimension relationships]

## Modeling Approach

**Chosen:** [Star / Snowflake / Data Vault / OBT]
**Rationale:** [1-2 sentences]

## Fact Tables

### fct_[name]
| Column | Type | Description | Aggregation |
|--------|------|-------------|-------------|
| ...    | ...  | ...         | SUM/AVG/... |

## Dimension Tables

### dim_[name]
| Column | Type | Description | SCD Type |
|--------|------|-------------|----------|
| ...    | ...  | ...         | 1/2/3    |

## Slowly Changing Dimensions

| Dimension | Attribute | SCD Type | Rationale |
|-----------|-----------|----------|-----------|
| ...       | ...       | ...      | ...       |

## Data Contracts

| Source → Target | Freshness SLA | Quality Checks | Breaking Change Policy |
|-----------------|---------------|----------------|----------------------|
| ...             | ...           | ...            | ...                  |

## Query Validation

| Query Pattern | Tables Involved | Join Count | Performance Notes |
|---------------|----------------|------------|-------------------|
| ...           | ...            | ...        | ...               |

Quality Checks

• Every fact table has a clearly stated grain in plain English
• Facts and dimensions are properly separated — no business logic in fact tables
• SCD strategy is specified for every dimension attribute that changes
• Data contracts define freshness SLA, schema expectations, and quality thresholds
• The schema supports all required analytical queries with minimal joins
• Cardinality estimates are documented for key dimensions
• Surrogate keys vs natural keys decision is documented and consistent
• Indexing strategy is defined for common filter and join columns

Evolution Notes

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