Update readme

packages/pglite/README.md

@@ -359,6 +359,127 @@ PGlite supports the pl/pgsql procedural language extension, this is included and
 
 In future we plan to support additional extensions, see the [roadmap](#roadmap).
 
+## Live Queries
+
+The "live" extension enables you to subscribe to a query and receve updated results when the underlying tables change.
+
+To use the extension it needs adding to the PGlite instance when creating it:
+
+```ts
+import { PGlite } from "@electric-sql/pglite";
+import { live } from "@electric-sql/pglite/live";
+
+const pg = new PGlite({
+  extensions: {
+    live,
+  },
+});
+```
+
+There are three methods on the `live` namespace:
+- `live.query()` for basic live queries. With less machinery in PG 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 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<T>()
+
+This is very similar to a standard query, but takes an additional callback that receives the results whenever they change:
+
+```js
+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:
+
+```ts
+interface LiveQueryReturn<T> { 
+  initialResults: Results<T>; 
+  unsubscribe: () => Promise<void>; 
+  refresh: () => Promise<void>; 
+}
+```
+
+- `initialResults` is the initial results set (also sent to the callback
+- `unsubscribe` allow you to unsubscribe from the live query
+- `refresh` allows you to force a refresh of the query
+
+Internally it watches for the tables that the query depends on, and reruns the query whenever they are changed.
+
+### live.incrementalQuery<T>()
+
+Similar to above, but maintains a temporary table inside of Postgres of the previous state. 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 PK) to key the diff on.
+
+```ts
+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()
+
+A lower level API which is the backend for the `incrementalQuery`, it emits the change that have happened. It requires a `key` to key the diff on:
+
+```ts
+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:
+
+```ts
+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 array of `Change` objects:
+
+```ts
+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 columns names that were changes
+- `__op__` the operation that is required to update the state (`INSERT`, `UPDATE`, `DELETE`)
+- `__after__` the `key` of the row that this row should be after, it will be included in `__changed_columns__` if it has been changed.
+
+This API can be used to implement very efficient in-place DOM updates.
+
 ## ORM support.
 
 - Drizzle ORM supports PGlite, see [their docs here](https://orm.drizzle.team/docs/get-started-postgresql#pglite).