PostgreSQL Guide

PGClient is the direct PostgreSQL client for LinkedQL. It gives you full SQL access, transactions, and realtime capabilities over a native PostgreSQL connection.

Use PGClient when your application talks directly to PostgreSQL.

PGClient uses the node-postgres connector under the hood and accepts all the existing constructor options.

---

Setup

import { PGClient } from '@linked-db/linked-ql/postgres';

const db = new PGClient({
  host: 'localhost',
  port: 5432,
  user: 'postgres',
  password: 'password',
  database: 'mydb',
  poolMode: false,
});

await db.connect();

const result = await db.query('SELECT 1::text AS result');
console.log(result.rows);
// [{ result: '1' }]

await db.disconnect();

---

Connection Mode

By default, PGClient runs on a single PostgreSQL connection.

You can opt into connection pooling by enabling poolMode:

const db = new PGClient({
  host: 'localhost',
  port: 5432,
  user: 'postgres',
  password: 'password',
  database: 'mydb',
  poolMode: true,
});

In poolMode, PGClient uses a connection pool to handle concurrent queries more efficiently.

After initializing the instance via db.connect(), subsequent db.connect() calls simply return a checked-out client.

This lets you explicitly check out a connection for session-sensitive work:

const client = await db.connect();
// ... run session-bound queries
client.release();

This is useful for transactions or workflows that require a stable connection.

---

Realtime Setup

LinkedQL uses PostgreSQL's logical replication to power live queries and db.wal.subscribe(). This requires logical replication to be enabled on the PostgreSQL instance.

At minimum, set in your database config file:

wal_level = logical

Depending on your setup, you may also need:

max_replication_slots = 10
max_wal_senders = 10

Refer to the official PostgreSQL documentation for enabling logical replication.

Restart PostgreSQL after changing these settings.

Once enabled, LinkedQL handles the rest automatically:

• creates a replication slot if it does not already exist
• creates a publication if it does not already exist
• subscribes to and decodes the WAL stream

The replication slot name and publication name that LinkedQL uses by default are:

Setting	Default
Replication slot	linkedql_default_slot (ephemeral by default)
Publication	linkedql_default_publication

Note that by default, when LinkedQL creates a publication, it creates it for all tables.

You can override these defaults if needed via constructor options:

Option	Type	Default	Meaning
walSlotName	string	'linkedql_default_slot'	logical replication slot name
walSlotPersistence	0 | 1	0	whether the slot should be ephemeral or persistent
pgPublications	string | string[]	'linkedql_default_publication'	publication(s) used for change streaming

Example:

const db = new PGClient({
  walSlotName: 'my_slot',
  walSlotPersistence: 1,
  pgPublications: 'my_publication',
});

This is useful when:

• you manage replication slots manually
• you need persistent slots
• you want to integrate with an existing replication setup

TIP

LinkedQL consumes PostgreSQL's WAL using the pgoutput plugin, the same mechanism PostgreSQL uses for native logical replication.

---

Additional Reading

If you want to learn about...	Go to...
the common application-facing methods	API
how this fits into larger app architectures	Integration Patterns