Live Queries

The "live" extension enables you to subscribe to a query and receive updated results when the underlying tables change.

To use the extension, it needs to be added to the PGlite instance when creating it:

import { PGlite } from '@electric-sql/pglite'
import { live } from '@electric-sql/pglite/live'

const pg = await PGlite.create({
  extensions: {
    live,
  },
})

There are three methods on the live namespace:

• live.query() for basic live queries. With less machinery in PGlite, it's quicker for small results sets and narrow rows.
• live.incrementalQuery() for incremental queries. It materialises the full result set on each update from only the changes emitted by the live.changes API. Perfect for feeding into React, and with good performance for large result sets and wide rows.
• live.changes() a lower level API that emits the changes (insert/update/delete) that can then be mapped to mutations in a UI or other datastore.

live.query

live.query<T>()

This is very similar to a standard query, but takes an additional callback that receives the results whenever they change:

const ret = pg.live.query('SELECT * FROM test ORDER BY rand;', [], (res) => {
  // res is the same as a standard query result object
})

The returned value from the call is an object with this interface:

interface LiveQueryReturn<T> {
  initialResults: Results<T>
  unsubscribe: () => Promise<void>
  refresh: () => Promise<void>
}

• initialResults is the initial results set (also sent to the callback)
• unsubscribe allows you to unsubscribe from the live query
• refresh allows you to force a refresh of the query with the updated results sent to the callback

Internally it watches the tables that the query depends on, and reruns the query whenever they are changed.

live.incrementalQuery

live.incrementalQuery<T>()

Similar to above, but maintains a temporary table of the previous state inside of Postgres. When the tables it depends on change, the query is re-run and diffed with the last state. Only the changes from the last version of the query are copied from WASM into JS.

It requires an additional key argument - the name of a column (often a primary key) on which to key the diff.

const ret = pg.live.incrementalQuery(
  'SELECT * FROM test ORDER BY rand;',
  [],
  'id',
  (res) => {
    // res is the same as a standard query result object
  },
)

The returned value is of the same type as the query method above.

live.changes

live.changes()

A lower-level API which is the backend for the incrementalQuery, it emits the changes that have occurred. It requires a key on which to compare row differences:

const ret = pg.live.changes(
  'SELECT * FROM test ORDER BY rand;',
  [],
  'id',
  (res) => {
    // res is a change result object
  },
)

The returned value from the call is defined by this interface:

interface LiveChangesReturn<T = { [key: string]: any }> {
  fields: { name: string; dataTypeID: number }[]
  initialChanges: Array<Change<T>>
  unsubscribe: () => Promise<void>
  refresh: () => Promise<void>
}

The results passed to the callback are an array of Change objects:

type ChangeInsert<T> = {
  __changed_columns__: string[]
  __op__: 'INSERT'
  __after__: number
} & T

type ChangeDelete<T> = {
  __changed_columns__: string[]
  __op__: 'DELETE'
  __after__: undefined
} & T

type ChangeUpdate<T> = {
  __changed_columns__: string[]
  __op__: 'UPDATE'
  __after__: number
} & T

type Change<T> = ChangeInsert<T> | ChangeDelete<T> | ChangeUpdate<T>

Each Change includes the new values along with:

• __changed_columns__ the column names that were changed.
• __op__ the operation that is required to update the state (INSERT, UPDATE, DELETE).
• __after__ the key of the row after which this row should be positioned; it will be included in __changed_columns__ if it has been changed. This allows for very efficient moves within an ordered set of results.

This API can be used to implement very efficient in-place DOM updates.