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How the two isolation models work
Schema-per-tenant
A PostgreSQL schema is a namespace for tables and other objects. Different schemas can contain objects with the same name, so an application can organize each tenant’s objects separately. But PostgreSQL explicitly says schemas in one database are not rigidly separated: a user who has the required privileges can access objects in another schema. A schema is therefore an organizational and privilege boundary, not an automatic security barrier. See PostgreSQL’s schema documentation.
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Shared tables with RLS
In a pooled RLS design, tenants share tables, and rows are distinguished by a tenant key such as tenant_id. Once row security is enabled, normal access to rows must be allowed by an applicable policy. If no policy applies, PostgreSQL uses default deny. Policies can govern SELECT, INSERT, UPDATE, and DELETE. RLS adds a row-access layer alongside ordinary SQL privileges; it does not replace them. See PostgreSQL’s row security documentation.
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| Decision area | Schema-per-tenant | Shared tables with RLS |
|---|---|---|
| Data organization | Tenant objects live in separate namespaces; object names can be reused across schemas. | Tenant rows live in shared tables and are filtered through table policies. |
| What enforces access | Schema and object privileges, ownership, and safe object-name resolution. | RLS being enabled, applicable policies, reliable tenant context, and roles that do not bypass RLS. |
| Key security review | Review schema USAGE and CREATE grants, ownership, search_path, and writable schemas. |
Review policy coverage and logic, application role behavior, tenant-context handling, and constraint-related information leakage. |
| Operational design | Plan how tenant namespaces and grants are created, migrated, backed up, and audited. The cited PostgreSQL documentation does not quantify per-tenant migration costs. | Ensure every tenant-data table is covered, context is set consistently, and application access uses intended roles. |
| Performance evidence | The cited material provides no direct comparison or per-tenant-schema benchmark. | PostgreSQL calls policies based only on the current row’s values the simplest and best-performing case when possible. That is not a claim that RLS is faster than separate schemas. |
PostgreSQL’s documentation establishes these mechanics, but does not provide a head-to-head latency, throughput, storage, tenant-count, or migration-cost figure for the two designs. If performance is decisive, compare representative workloads in your own deployment rather than inferring a winner from the isolation model.
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What to check before choosing
For schema-per-tenant: grants and name resolution
- Grant only the schema and object privileges each role needs. Separate schema names do not prevent access when a role has privileges across schemas.
- Review
search_pathwherever queries use unqualified object names. PostgreSQL warns that a schema on the search path is trusted: a user with CREATE privilege there can create objects that affect query behavior. - Check the deployed PostgreSQL version and configuration. PostgreSQL 15 and later support a private-schema secure usage pattern by default, while upgraded databases and older configurations may need public-schema CREATE privileges revoked. Confirm the actual grants and path settings rather than assuming the default applies.
- Make tenant provisioning, grants, schema changes, backups, and audits repeatable. The mechanics are documented, but the workload-specific effort and migration cost must be established for your own system.
For RLS: role behavior and policy coverage
- Run application queries under a role that is subject to RLS. Superusers and roles with
BYPASSRLSalways bypass policies. Table owners normally bypass them too, unless the table usesFORCE ROW LEVEL SECURITY. - Check every table that contains tenant data. AWS Prescriptive Guidance recommends enabling RLS on all such tables in its pooled PostgreSQL model; this is AWS’s guidance for that architecture, not a universal PostgreSQL mandate. See AWS Prescriptive Guidance on RLS for pooled PostgreSQL.
- Define how each request establishes tenant context, and ensure the database comparison uses the intended tenant value. AWS’s example compares a tenant column with a runtime setting and favors runtime tenant context over creating a separate PostgreSQL user for every tenant.
- Test normal reads and writes under the same roles the application will use, including paths where a tenant key is inserted or changed.
Review RLS policies by command and effect
USING and WITH CHECK do different jobs
A policy’s USING expression determines which existing rows are visible or available to a command. WITH CHECK determines whether a proposed new or changed row is allowed. Review both sides of inserts and updates: filtering which rows an application can target is not the same as validating the tenant identity of the row it writes.
Policy combinations can broaden or narrow access
Permissive policies are the default and combine with OR. Restrictive policies combine with AND. Review the complete set of applicable policies for each command and role; reading one policy in isolation may not reveal the effective access rule.
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Constraints can reveal information outside RLS filtering
PostgreSQL’s referential-integrity checks bypass RLS so the database can preserve integrity. The documentation warns that constraint behavior can create covert information channels if policies and constraints are not designed carefully. Include constraint errors and cross-tenant references in security review and testing, especially where an error could reveal whether another tenant’s value exists. See PostgreSQL’s CREATE POLICY documentation.
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Favor schema-per-tenant when separate namespaces fit the way you organize tenant objects and you can reliably manage schema privileges, ownership, and safe name resolution. Favor shared tables with RLS when pooled data and row-level access policies fit the application, and you can set tenant context consistently while ensuring application roles remain subject to RLS.
For either design, write down the intended boundary and test it using the actual application role. For RLS, test every tenant-data table and relevant command with tenant context set to different tenants and unset or invalid. For schema separation, test that roles cannot reach another tenant’s objects and that writable schemas cannot alter name resolution. PostgreSQL’s own guidance is: “As with any security settings, it’s important to test and ensure that the system is behaving as expected.”
The available official guidance does not establish a universal scale cutoff, security ranking, or performance winner. Compare migration and provisioning workflows, cross-tenant query needs, privilege design, and the threat model; use a representative workload benchmark if runtime performance determines the choice.
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