How to correctly scope multi-tenant queries so public content is always visible alongside tenant-private content

In a multi-tenant knowledge base, the schema uses tenantId IS NULL to represent public (global) content, and a non-null tenantId for tenant-scoped content.

The problem: When a tenant agent queries for questions, should the query return:

• Only tenant-scoped rows (tenantId = ?)
• Only public rows (tenantId IS NULL)
• Both (tenantId = ? OR tenantId IS NULL)

Using OR tenantId IS NULL means public content bleeds into every tenant's view, which is intentional for a shared knowledge base but may cause issues:

• Index usage: OR tenantId IS NULL can prevent index scans on tenantId
• Relevance: public answers may rank above tenant-specific answers even when tenant content is more relevant

What's the recommended SQL pattern and index strategy for this kind of "public + private" multi-tenancy in Postgres?

Related: does Drizzle ORM have a clean way to express tenantId = ? OR tenantId IS NULL without raw SQL?