Add Pixeltable Document Store integration

integrations/pixeltable.md

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+---
+layout: integration
+name: Pixeltable
+description: Document Store and Retriever backed by Pixeltable — persistent, versioned, multimodal data infrastructure for AI applications.
+authors:
+    - name: Pixeltable
+      socials:
+        github: pixeltable
+        twitter: pixeltable
+        linkedin: https://www.linkedin.com/company/pixeltable/
+pypi: https://pypi.org/project/haystack-pixeltable/
+repo: https://github.com/pixeltable/haystack-pixeltable
+type: Document Store
+report_issue: https://github.com/pixeltable/haystack-pixeltable/issues
+version: Haystack 2.0
+toc: true
+---
+
+[![PyPI - Version](https://img.shields.io/pypi/v/haystack-pixeltable.svg)](https://pypi.org/project/haystack-pixeltable/)
+[![PyPI - Python Version](https://img.shields.io/pypi/pyversions/haystack-pixeltable.svg)](https://pypi.org/project/haystack-pixeltable/)
+[![CI](https://github.com/pixeltable/haystack-pixeltable/actions/workflows/ci.yml/badge.svg)](https://github.com/pixeltable/haystack-pixeltable/actions/workflows/ci.yml)
+
+---
+
+### Table of Contents
+
+- [Overview](#overview)
+- [Installation](#installation)
+- [Usage](#usage)
+  - [Document Store](#document-store)
+  - [Retriever](#retriever)
+  - [In a Haystack Pipeline](#in-a-haystack-pipeline)
+  - [Filtering](#filtering)
+  - [Pixeltable Escape Hatch](#pixeltable-escape-hatch)
+- [License](#license)
+
+## Overview
+
+[Pixeltable](https://pixeltable.com/) is open-source Python data infrastructure for multimodal AI. It provides persistent, versioned tables that store text, images, video, audio, and documents alongside embeddings and metadata — with automatic incremental computation via computed columns.
+
+This integration provides two components:
+
+- **`PixeltableDocumentStore`** — a Haystack `DocumentStore` backed by a Pixeltable table with a built-in embedding index.
+- **`PixeltableRetriever`** — a Haystack `Retriever` component that performs vector similarity search.
+
+### Why Pixeltable as a Document Store?
+
+| Feature | Pixeltable | Typical vector DBs |
+|---------|-----------|-------------------|
+| Persistent storage | Built-in (embedded PostgreSQL) | Varies |
+| Computed columns | Native — auto-run transforms on insert | No |
+| Version history | Built-in — time-travel queries | No |
+| Multimodal types | Image, Video, Audio, Document | Text only |
+| Metadata filtering | JSON + SQL predicates | Limited |
+| Embedding auto-compute | Via computed columns | Manual |
+
+The `.table` escape hatch lets you break out of the Haystack interface to use the full Pixeltable API — add computed columns, run LLM inference on insert, query version history, and work with multimodal data.
+
+## Installation
+
+```bash
+pip install haystack-pixeltable
+```
+
+## Usage
+
+### Document Store
+
+```python
+from haystack import Document
+from haystack_pixeltable import PixeltableDocumentStore
+
+store = PixeltableDocumentStore(
+    table_name="myproject.docs",
+    embedding_dimension=1536,
+)
+
+store.write_documents([
+    Document(
+        content="Pixeltable is multimodal data infrastructure.",
+        embedding=[0.1] * 1536,
+        meta={"category": "infra"},
+    ),
+    Document(
+        content="Haystack is a framework for building RAG pipelines.",
+        embedding=[0.2] * 1536,
+        meta={"category": "frameworks"},
+    ),
+])
+
+print(store.count_documents())  # 2
+```
+
+### Retriever
+
+```python
+from haystack_pixeltable import PixeltableDocumentStore, PixeltableRetriever
+
+store = PixeltableDocumentStore(
+    table_name="myproject.docs",
+    embedding_dimension=1536,
+)
+retriever = PixeltableRetriever(document_store=store, top_k=5)
+
+result = retriever.run(query_embedding=[0.1] * 1536)
+for doc in result["documents"]:
+    print(f"{doc.content} (score: {doc.score:.3f})")
+```
+
+### In a Haystack Pipeline
+
+```python
+from haystack import Pipeline
+from haystack.components.embedders import (
+    SentenceTransformersTextEmbedder,
+    SentenceTransformersDocumentEmbedder,
+)
+from haystack.components.writers import DocumentWriter
+from haystack_pixeltable import PixeltableDocumentStore, PixeltableRetriever
+
+store = PixeltableDocumentStore(
+    table_name="rag.knowledge",
+    embedding_dimension=384,
+)
+
+# Indexing pipeline
+indexing = Pipeline()
+indexing.add_component("embedder", SentenceTransformersDocumentEmbedder())
+indexing.add_component("writer", DocumentWriter(document_store=store))
+indexing.connect("embedder", "writer")
+
+# Query pipeline
+query = Pipeline()
+query.add_component("embedder", SentenceTransformersTextEmbedder())
+query.add_component("retriever", PixeltableRetriever(document_store=store, top_k=5))
+query.connect("embedder.embedding", "retriever.query_embedding")
+
+result = query.run({"embedder": {"text": "What is multimodal AI?"}})
+```
+
+### Filtering
+
+The Document Store supports the [Haystack filter specification](https://docs.haystack.deepset.ai/docs/metadata-filtering):
+
+```python
+# Simple comparison
+store.filter_documents(
+    filters={"field": "meta.category", "operator": "==", "value": "science"}
+)
+
+# Compound AND
+store.filter_documents(filters={
+    "operator": "AND",
+    "conditions": [
+        {"field": "meta.category", "operator": "==", "value": "science"},
+        {"field": "meta.score", "operator": ">", "value": 0.5},
+    ],
+})
+
+# Compound OR
+store.filter_documents(filters={
+    "operator": "OR",
+    "conditions": [
+        {"field": "meta.source", "operator": "==", "value": "arxiv"},
+        {"field": "meta.source", "operator": "==", "value": "pubmed"},
+    ],
+})
+```
+
+### Pixeltable Escape Hatch
+
+Access the underlying Pixeltable table for operations beyond the Haystack interface:
+
+```python
+store = PixeltableDocumentStore(
+    table_name="myproject.docs", embedding_dimension=1536
+)
+t = store.table
+
+# Add a computed column that auto-summarizes every document on insert
+import pixeltable.functions.openai as openai
+
+t.add_computed_column(
+    summary=openai.chat_completions(
+        messages=[{"role": "user", "content": t.content}],
+        model="gpt-4o-mini",
+    )
+)
+
+# Query with Pixeltable's full API
+results = t.select(t.content, t.summary).collect()
+```
+
+## License
+
+[Apache 2.0](https://github.com/pixeltable/haystack-pixeltable/blob/main/LICENSE)