PostgreSQL row-level security (RLS) can restrict which tenant rows an application role may read or change in a shared-table database. It is a useful authorization control for the pool model, but it is not a complete tenant-security guarantee: database privileges, table ownership, policy composition, integrity checks, and application connection handling all affect the real boundary. RLS also does not replace transaction isolation, which governs concurrency rather than tenant authorization.
Choose the tenant data layout before choosing RLS
Multi-tenant systems commonly use three layouts. AWS describes them as pool, bridge, and silo. Its managed-PostgreSQL recommendations are workload guidance, not universal rules; validate the trade-offs against your hosting environment and workload. See AWS’s multi-tenant architecture guidance and its managed PostgreSQL decision matrix.
| Model | Data and resource separation | Where it can fit | Trade-offs to assess |
|---|---|---|---|
| Pool | Tenants share tables in a schema and share the database resources. | AWS guidance presents this as a fit for large numbers of smaller tenants. | Efficient sharing and a common data model come with shared resource contention and a need for strong row-level authorization. Cross-tenant reporting may be simpler structurally, but must be deliberately authorized. |
| Bridge | Tenants use tenant-specific schemas or databases on shared infrastructure. | Can suit systems needing more separation than a shared-table pool while retaining shared underlying infrastructure. | Tenant-specific provisioning, migrations, monitoring, backups, and connection selection can increase operational work. The actual separation depends on the chosen schema/database design and privileges. |
| Silo | Each tenant has dedicated infrastructure, such as a separate database instance or stack. | AWS guidance recommends considering it when stronger resource control or very large or performance-sensitive tenants are key needs. | Provides more control over per-tenant resources, but multiplies provisioning and ongoing operational responsibilities. Dedicated infrastructure does not by itself settle authorization or application-access questions. |
AWS’s managed PostgreSQL guide discusses RLS in the pool model and describes the shared-resource approach in more detail: guide introduction and pool model. The right choice depends on tenant count and shape, resource isolation needs, cross-tenant query requirements, and the operational cost of provisioning and maintaining tenant-specific environments.
What PostgreSQL RLS does—and does not do
RLS adds row authorization alongside ordinary SQL privileges. After row security is enabled on a table, applicable policies govern normal row selection and modification. PostgreSQL 18 documents the default as: “If no policy exists for the table, a default-deny policy is used, meaning that no rows are visible or can be modified.” Read the full PostgreSQL 18 row security documentation.
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Default deny is not the same as automatic tenant isolation. An application role still needs ordinary table privileges, and the policy set must correctly grant access only to the intended rows. Row policies also do not cover every table operation: TRUNCATE is outside row security. A deployment must restrict such operations through privileges and role design.
Roles that can bypass row policies
Table owners ordinarily bypass RLS. Superusers and roles with the BYPASSRLS attribute always bypass it. ALTER TABLE ... FORCE ROW LEVEL SECURITY subjects the table owner to row policies, but it does not constrain superusers or BYPASSRLS roles. The application’s everyday database role should therefore be considered separately from migration, administration, and other elevated roles.
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Read and write policies have different jobs
PostgreSQL policy expressions are command- and role-specific. Review all policies that apply to each operation; a single policy read in isolation may not describe the effective access rule. The key distinction is between USING and WITH CHECK, as detailed in PostgreSQL 18 CREATE POLICY.
| Clause | What it evaluates | Tenant-isolation role |
|---|---|---|
USING |
Existing rows that a command may see or target. | Restricts which tenant-owned rows can be selected, updated, or deleted where the policy applies. |
WITH CHECK |
Proposed row values for inserts or updates. | Restricts which tenant assignment or other values may be written, including attempts to insert a row for another tenant or change a row’s tenant ownership. |
For policy forms that support it, PostgreSQL can use the USING expression as the check when WITH CHECK is omitted. Explicitly defining and reviewing write rules is important when inserts or updates can set or change the tenant identifier.
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Understand how policies combine
Policies are permissive by default. When multiple policies apply, permissive policies combine with OR, while restrictive policies combine with AND. A restrictive policy narrows access; it does not grant access by itself, so at least one applicable permissive policy must allow the operation.
This means an additional permissive policy can broaden access in ways that are easy to miss if reviewers inspect only the tenant policy. Conversely, a restrictive condition can further narrow access but cannot serve as the only grant. Review the full set of policies by command and role, including policies added later by migrations or extensions.
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RLS caveats beyond the policy expression
- Referential integrity: PostgreSQL’s integrity checks are not governed by row security. Depending on constraints and the operations allowed, constraint outcomes can reveal information about otherwise hidden values. The CREATE POLICY documentation describes this caveat; design constraints and error handling with potential inference in mind.
- Referenced objects and functions: Policy expressions run with the querying user’s privileges. Tables or functions referenced in a policy must be accessible as required. PostgreSQL discusses security-definer functions as one possible way to access data unavailable to the caller, but such privileged helpers need careful design and review.
- Connection and transaction context: In a shared-table design, the database must evaluate policy conditions against the correct tenant context for each operation. The application’s connection-pool reuse and transaction/session handling are part of that boundary. Do not assume a tenant value set for one request cannot affect another; define and verify the lifecycle for your application and PostgreSQL version.
- Other access paths: RLS governs row access for applicable operations, not all actions on the database. Audit grants, ownership, elevated roles, maintenance paths, and operations such as
TRUNCATEas part of the security design.
Tenant isolation is not transaction isolation
These terms answer different questions. Tenant data isolation asks whether a role is authorized to access a particular tenant’s rows. Transaction isolation defines what concurrent transactions can observe and which concurrent outcomes are permitted.
PostgreSQL’s transaction isolation documentation states that Serializable isolation makes concurrent serializable transactions have an effect equivalent to running them one at a time in some order. That concurrency guarantee does not determine which tenant is entitled to a row. A system may need both correct row authorization and an appropriate transaction isolation level, but one cannot substitute for the other.
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How to evaluate a pool design
Before choosing a shared-table pool, assess the requirements that RLS cannot decide for you:
- How many tenants do you expect, and are they mostly small or do some have unusually large or performance-sensitive workloads?
- Must database resources or noisy-neighbor effects be controlled per tenant, or is shared capacity acceptable?
- Do legitimate reports or relationships need to span tenants, and which roles may run them?
- Can your team reliably operate tenant-specific schemas or databases, or would that provisioning and migration burden be better avoided?
- Which roles own tables, bypass RLS, or perform migrations, and what privileges do application connections actually use?
- How are policies reviewed across reads, inserts, updates, and deletes, including policy composition and tenant reassignment?
- How is tenant context selected and scoped as pooled connections are reused across application requests and transactions?
For a shared PostgreSQL database, RLS is a database-enforced layer in the authorization design, not the whole design. Choose the storage layout for the required separation and operating model, then verify effective policies, privileges, and application connection behavior together.
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