feat: implement dynamic PostgreSQL source with composite key support

- Add PostgreSQL table source with dynamic schema generation
- Support both single and composite primary keys with proper KTable structure
- Implement type-safe column access using name lookup instead of indices
- Add comprehensive PostgreSQL to CocoIndex type mapping
- Support KeyValue::Struct for composite keys with individual field access
- Update example with environment variable validation for key configuration

Author: badmonster0

examples/postgres_embedding/.env

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+# Database Configuration
+# CocoIndex Database (for storing embeddings)
+COCOINDEX_DATABASE_URL=postgres://cocoindex:cocoindex@localhost/cocoindex
+
+# Source Database (for reading data - can be different from CocoIndex DB)
+SOURCE_DATABASE_URL=postgres://cocoindex:cocoindex@localhost/source_data
+
+   # ========================================
+   # Configuration for test_multiple table
+   # ========================================
+   TABLE_NAME=test_multiple
+   KEY_COLUMNS_FOR_MULTIPLE_KEYS=product_category,product_name
+   INDEXING_COLUMN=description
+   ORDINAL_COLUMN=modified_time
+
+

examples/postgres_embedding/.env.example

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+# Database Configuration
+# CocoIndex Database (for storing embeddings)
+COCOINDEX_DATABASE_URL=postgres://cocoindex:cocoindex@localhost/cocoindex
+
+# Database URLs
+SOURCE_DATABASE_URL=postgres://cocoindex:cocoindex@localhost/source_data
+
+# ========================================
+# Configuration for test_simple table
+# ========================================
+TABLE_NAME=test_simple
+KEY_COLUMN_FOR_SINGLE_KEY=id
+INDEXING_COLUMN=message
+ORDINAL_COLUMN=created_at
+
+# ========================================
+# Configuration for test_multiple table
+# ========================================
+TABLE_NAME=test_multiple
+KEY_COLUMNS_FOR_MULTIPLE_KEYS=product_category,product_name
+INDEXING_COLUMN=description
+ORDINAL_COLUMN=modified_time

examples/postgres_embedding/README.md

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+# PostgreSQL Source Embedding Example 🗄️
+
+[![GitHub](https://img.shields.io/github/stars/cocoindex-io/cocoindex?color=5B5BD6)](https://github.com/cocoindex-io/cocoindex)
+
+This example demonstrates the **PostgreSQL table source** feature in CocoIndex. It reads data from existing PostgreSQL tables, generates embeddings, and stores them in a separate CocoIndex database with pgvector for semantic search.
+
+We appreciate a star ⭐ at [CocoIndex Github](https://github.com/cocoindex-io/cocoindex) if this is helpful.
+
+## What This Example Does
+
+### 📊 Data Flow
+```
+Source PostgreSQL Table (messages)
+    ↓ [Postgres Source]
+Text Processing & Embedding Generation
+    ↓ [SentenceTransformer]
+CocoIndex Database (message_embeddings) with pgvector
+    ↓ [Semantic Search]
+Query Results
+```
+
+### 🔧 Key Features
+- **PostgreSQL Source**: Read from existing database tables
+- **Separate Databases**: Source data and embeddings stored in different databases
+- **Automatic Schema**: CocoIndex creates target tables automatically
+- **pgvector Integration**: Store embeddings for semantic search
+
+## Prerequisites
+
+Before running the example, you need to:
+
+1. **PostgreSQL with pgvector**: Follow the [CocoIndex PostgreSQL setup guide](https://cocoindex.io/docs/getting_started/quickstart) to install and configure PostgreSQL with pgvector extension.
+
+2. **Two databases**: You'll need two separate databases (names can be anything you choose):
+   - One database for your source table data
+   - One database for storing embeddings
+
+3. **Environment file**: Create a `.env` file with your database configuration:
+   ```bash
+   cp .env.example .env
+   $EDITOR .env
+   ```
+
+## Installation
+
+Install dependencies:
+
+```bash
+pip install -e .
+```
+
+## Quick Start
+
+### Environment Variables Explained
+
+The example uses these environment variables to configure the PostgreSQL source:
+
+- **`SOURCE_DATABASE_URL`**: Connection string to your source database containing the table you want to index
+- **`COCOINDEX_DATABASE_URL`**: Connection string to the database where CocoIndex will store embeddings
+- **`TABLE_NAME`**: Name of the table in your source database to read from
+- **`INDEXING_COLUMN`**: The text column to generate embeddings for (this example focuses on one column, but you can index multiple columns)
+- **`KEY_COLUMN_FOR_SINGLE_KEY`**: Primary key column name (for tables with single primary key)
+- **`KEY_COLUMNS_FOR_MULTIPLE_KEYS`**: Comma-separated primary key columns (for tables with composite primary key)
+- **`INCLUDED_COLUMNS`**: Optional - specify which columns to include (defaults to all)
+- **`ORDINAL_COLUMN`**: Optional - use for incremental updates
+
+### Option A: Test with Sample Data (Recommended for first-time users)
+
+1. **Setup test database with sample data**:
+   ```bash
+   python setup_test_database.py
+   ```
+   This will create both `test_simple` (single primary key) and `test_multiple` (composite primary key) tables with sample data.
+
+2. **Copy the generated environment configuration** to your `.env` file (the script will show you exactly what to copy).
+
+3. **Run the example**:
+   ```bash
+   python main.py
+   ```
+
+4. **Test semantic search** by entering queries in the interactive prompt
+
+### Option B: Use Your Existing Database
+
+1. **Update your `.env` file** with your database URLs and table configuration:
+   ```env
+   # CocoIndex Database (for storing embeddings)
+   COCOINDEX_DATABASE_URL=postgresql://username:password@localhost:5432/cocoindex
+
+   # Source Database (for reading data)
+   SOURCE_DATABASE_URL=postgresql://username:password@localhost:5432/your_source_db
+
+   # Table Configuration
+   TABLE_NAME=your_table_name
+   KEY_COLUMN_FOR_SINGLE_KEY=id  # or KEY_COLUMNS_FOR_MULTIPLE_KEYS=col1,col2
+   INDEXING_COLUMN=your_text_column
+   ORDINAL_COLUMN=your_timestamp_column  # optional
+   ```
+
+2. **Run the example**:
+   ```bash
+   python main.py
+   ```
+
+## How It Works
+
+The example demonstrates a simple flow:
+
+1. **Read from Source**: Uses `cocoindex.sources.PostgresDb` to read from your existing table
+2. **Generate Embeddings**: Processes text and creates embeddings using SentenceTransformers
+3. **Store Embeddings**: Exports to the CocoIndex database with automatic table creation
+4. **Search**: Provides interactive semantic search over the stored embeddings
+
+**Note**: This example indexes one text column for simplicity, but you can modify the flow to index multiple columns or add more complex transformations.
+
+### Key Benefits
+
+- **Separate Databases**: Keep your source data separate from embeddings
+- **Automatic Setup**: CocoIndex creates target tables automatically
+- **Real-time Updates**: Live updates as source data changes
+- **Interactive Search**: Built-in search interface for testing
+
+## Database Configuration
+
+The example uses two separate databases:
+
+1. **Source Database**: Contains your existing data table
+2. **CocoIndex Database**: Stores generated embeddings with pgvector support
+
+This separation allows you to:
+- Keep your production data unchanged
+- Scale embeddings independently
+- Use different database configurations for each purpose
+
+## Advanced Usage
+
+### Primary Key Configuration
+
+**Single Primary Key**:
+```env
+KEY_COLUMN_FOR_SINGLE_KEY=id
+```
+
+**Composite Primary Key**:
+```env
+KEY_COLUMNS_FOR_MULTIPLE_KEYS=product_category,product_name
+```
+
+
+
+## CocoInsight
+CocoInsight is in Early Access now (Free) 😊 You found us! A quick 3 minute video tutorial about CocoInsight: [Watch on YouTube](https://youtu.be/ZnmyoHslBSc?si=pPLXWALztkA710r9).
+
+Run CocoInsight to understand your RAG data pipeline:
+
+```sh
+cocoindex server -ci main.py
+```
+
+You can also add a `-L` flag to make the server keep updating the index to reflect source changes at the same time:
+
+```sh
+cocoindex server -ci -L main.py
+```
+
+Then open the CocoInsight UI at [https://cocoindex.io/cocoinsight](https://cocoindex.io/cocoinsight).