Added Citus example [skip ci]

Author: ankane

README.md

@@ -19,6 +19,7 @@ Or check out some examples:
 - [Sparse search](examples/sparse/example.php) with Text Embeddings Inference
 - [Morgan fingerprints](examples/rdkit/example.php) with RDKit
 - [Recommendations](examples/disco/example.php) with Disco
+- [Horizontal scaling](examples/citus/example.php) with Citus
 - [Bulk loading](examples/loading/example.php) with `COPY`
 
 ### Laravel

examples/citus/composer.json

@@ -0,0 +1,11 @@
+{
+    "repositories": [
+        {
+            "type": "path",
+            "url": "../.."
+        }
+    ],
+    "require": {
+        "pgvector/pgvector": "dev-master"
+    }
+}

examples/citus/example.php

@@ -0,0 +1,71 @@
+<?php
+
+require_once __DIR__ . '/vendor/autoload.php';
+
+use Pgvector\Vector;
+
+ini_set('memory_limit', '512M');
+
+// generate random data
+$rows = 100000;
+$dimensions = 128;
+$embeddings = [];
+$categories = [];
+for ($i = 0; $i < $rows; $i++) {
+    $embedding = [];
+    for ($j = 0; $j < $dimensions; $j++) {
+        $embedding[] = rand() / getrandmax();
+    }
+    $embeddings[] = $embedding;
+    $categories[] = rand(1, 100);
+}
+
+// enable extensions
+$db = pg_connect('postgres://localhost/pgvector_citus');
+pg_query($db, 'CREATE EXTENSION IF NOT EXISTS citus');
+pg_query($db, 'CREATE EXTENSION IF NOT EXISTS vector');
+
+// GUC variables set on the session do not propagate to Citus workers
+// https://github.com/citusdata/citus/issues/462
+// you can either:
+// 1. set them on the system, user, or database and reconnect
+// 2. set them for a transaction with SET LOCAL
+pg_query($db, "ALTER DATABASE pgvector_citus SET maintenance_work_mem = '512MB'");
+pg_query($db, 'ALTER DATABASE pgvector_citus SET hnsw.ef_search = 20');
+pg_close($db);
+
+// reconnect for updated GUC variables to take effect
+$db = pg_connect('postgres://localhost/pgvector_citus');
+
+echo "Creating distributed table\n";
+pg_query($db, 'DROP TABLE IF EXISTS items');
+pg_query($db, 'CREATE TABLE items (id bigserial, embedding vector(128), category_id bigint, PRIMARY KEY (id, category_id))');
+pg_query($db, 'SET citus.shard_count = 4');
+pg_query($db, "SELECT create_distributed_table('items', 'category_id')");
+
+echo "Loading data in parallel\n";
+
+pg_query($db, 'COPY items (embedding, category_id) FROM STDIN');
+foreach ($embeddings as $i => $e) {
+    $row = [new Vector($e), $categories[$i]];
+    $line = join("\t", array_map(fn ($v) => pg_escape_string($db, $v), $row)) . "\n";
+    pg_put_line($db, $line);
+}
+pg_put_line($db, "\\.\n");
+pg_end_copy($db);
+
+echo "Creating index in parallel\n";
+pg_query($db, 'CREATE INDEX ON items USING hnsw (embedding vector_l2_ops)');
+
+echo "Running distributed queries\n";
+for ($i = 0; $i < 10; $i++) {
+    $result = pg_query_params($db, 'SELECT id FROM items ORDER BY embedding <-> $1 LIMIT 10', [new Vector($embeddings[rand(0, $rows - 1)])]);
+    $ids = [];
+    while ($row = pg_fetch_array($result)) {
+        $ids[] = $row['id'];
+    }
+    echo join(', ', $ids) . "\n";
+    pg_free_result($result);
+}
+
+pg_close($db);