Examples: Block Missing “Critical Filter” (Table/Column Heuristic)

Goal: block PRs that may have removed or weakened a “critical filter” like row_status = 'A'.

Important limitation: Lexega’s diff matching currently operates on structured signals (tables, columns, change types). It does not match the exact predicate text row_status = 'A' inside WHERE.

So the end-to-end workflow is a heuristic:

1. Detect WhereRemoved / WhereConditionChanged
2. Scope to a specific table
3. Scope to a specific column name (e.g., row_status)
4. Emit a custom policy signal
5. Block that custom rule in prod (with exceptions)

Files to commit

.lexega/
  custom_rules.yaml
  policy.yaml
  exceptions.yaml        # optional

1) Custom rules: emit a signal on dangerous WHERE changes

# yaml-language-server: $schema=/schemas/v1/custom_rules.schema.json
metadata:
  name: Critical Filters
  version: 1.0.0

rules:
  - id: CRITICAL-FILTER-001
    name: Potential critical filter change on row_status
    risk_level: High
    message: "Potential critical-filter change affecting {affected_object}. Verify row_status filtering remains correct."
    diff_triggers:
      - change_type: WhereRemoved
        match:
          tables:
            any_of: ["ANALYTICS.ORDERS"]
          columns:
            any_of: ["*row_status*"]
      - change_type: WhereConditionChanged
        match:
          tables:
            any_of: ["ANALYTICS.ORDERS"]
          columns:
            any_of: ["*row_status*"]

2) Policy: block that custom rule in prod

# yaml-language-server: $schema=/schemas/v1/policy.schema.json
schema_version: 1
policy_id: team-policy
policy_version: 1.0.0

default_action: allow

policies:
  - rule_id: CRITICAL-FILTER-001
    description: Block critical-filter changes in prod unless exception
    envs: [prod]
    action: block
    requires_exception: true

3) Run review (PR range) and gate CI

lexega-sql review main..HEAD models/ -r \
  --custom-rules .lexega/custom_rules.yaml \
  --policy .lexega/policy.yaml \
  --env prod \
  --decision-out decision.json \
  --format markdown > review.md

ALLOWED=$(jq -r '.outcome.allowed' decision.json)
test "$ALLOWED" = "true"

4) Exception (optional)

If the filter change is intentional and reviewed, grant an exception:

# yaml-language-server: $schema=/schemas/v1/exceptions.schema.json
schema_version: 1

exceptions:
  - exception_id: EX-2025-CRITFILTER-001
    policy_id: team-policy
    rule_id: CRITICAL-FILTER-001
    approved_by: data-governance@company.com
    approved_at: "2025-12-31T00:00:00Z"
    reason: "Reviewed critical-filter change; safe"
    ticket: DATA-5678
    scope:
      scoped:
        path_prefixes:
          - "models/"
        expires_at: "2026-03-01T00:00:00Z"

Run with exceptions:

lexega-sql review main..HEAD models/ -r \
  --custom-rules .lexega/custom_rules.yaml \
  --policy .lexega/policy.yaml \
  --exceptions .lexega/exceptions.yaml \
  --env prod \
  --decision-out decision.json

Key Concepts

Separation of concerns:

• Custom rules define WHAT to detect (detection logic)
• Policies define WHAT TO DO when a rule fires (enforcement)
• Exceptions grant one-off permissions for specific rules

This separation keeps policies simple and detection testable.