spqrguard

spqrguard is a PostgreSQL extension that attaches to the executor and blocks writes to SPQR managed relations when they are meant to be read-only. It reads SPQR metadata to identify distributed and reference tables, then raises an error before any modification is executed.

How it works

• On load (via shared_preload_libraries), the extension installs an ExecutorRun hook.
• Each statement’s plan is scanned; if the target relation is recorded in spqr_metadata.spqr_distributed_relations or spqr_metadata.spqr_reference_relations, spqrguard consults its configuration to decide whether writes should be denied.
• Global toggles can be stored in spqr_metadata.spqr_global_settings; per-session overrides are available via GUCs.

Build & Install

make                # compile
make install        # install extension into the target PostgreSQL instance

Reload PostgreSQL after adding spqrguard to shared_preload_libraries.

Configuration

• spqrguard.prevent_distributed_table_modify (bool, superuser) — block writes to SPQR distributed relations.
• spqrguard.prevent_reference_table_modify (bool, superuser) — block writes to SPQR reference relations.
• Global defaults can be stored in spqr_metadata.spqr_global_settings with integer keys 42 (distributed) and 69 (reference); string values like on/yes/true/ok enable the block.

Quick usage example

CREATE EXTENSION spqrguard;
SELECT spqr_metadata.mark_distributed_relation('orders');
SET spqrguard.prevent_distributed_table_modify TO true;
INSERT INTO orders VALUES (1);  -- ERROR: unable to modify distributed relation within read-only transaction