docs: postgres example (#958)

Author: badmonster0

docs/docs/examples/examples/postgres_source.md

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+---
+title:  Transform Data From Structured Source in PostgreSQL
+description: Transform data from PostgreSQL table as source, transform with both AI models and non-AI data mappings, and write them into PostgreSQL/PgVector for semantic + structured search.
+sidebar_class_name: hidden
+slug: /examples/postgres_source
+canonicalUrl: '/examples/postgres_source'
+sidebar_custom_props:
+  image: /img/examples/postgres_source/cover.png
+  tags: [data-mapping, vector-index, postgres]
+tags: [data-mapping, vector-index, postgres]
+---
+import { GitHubButton, YouTubeButton, DocumentationButton } from '../../../src/components/GitHubButton';
+
+<GitHubButton url="https://github.com/cocoindex-io/cocoindex/tree/main/examples/postgres_source" margin="0 0 24px 0" /
+>
+![PostgreSQL Product Indexing Flow](/img/examples/postgres_source/cover.png)
+
+[CocoIndex](https://github.com/cocoindex-io/cocoindex) is one framework for building **incremental** data flows across **structured and unstructured** sources. This tutorial shows how to take data from PostgreSQL table as source, transform with both AI and non-AI data mappings, and write them into a new PostgreSQL table with PgVector for semantic + structured search.
+
+## PostgreSQL Product Indexing Flow
+![PostgreSQL Product Indexing Flow](/img/examples/postgres_source/flow.png)
+
+- Reading data from a PostgreSQL table `source_products`.
+- Computing additional fields (`total_value`, `full_description`).
+- Generating embeddings for semantic search.
+- Storing the results in another PostgreSQL table with a vector index using pgvector
+
+
+### Connect to source
+
+`flow_builder.add_source` reads rows from `source_products`.
+
+```python
+@cocoindex.flow_def(name="PostgresProductIndexing")
+def postgres_product_indexing_flow(flow_builder: cocoindex.FlowBuilder, data_scope: cocoindex.DataScope) -> None:
+  
+    data_scope["products"] = flow_builder.add_source(
+        cocoindex.sources.Postgres(
+            table_name="source_products",
+            # Optional. Use the default CocoIndex database if not specified.
+            database=cocoindex.add_transient_auth_entry(
+                cocoindex.DatabaseConnectionSpec(
+                    url=os.environ["SOURCE_DATABASE_URL"],
+                )
+            ),
+            # Optional.
+            ordinal_column="modified_time",
+            notification=cocoindex.sources.PostgresNotification(),
+        ),
+    )
+```
+This step adds source data from PostgreSQL table `source_products` to the flow as a `KTable`.
+
+![Add PostgreSQL Source](/img/examples/postgres_source/source.png)
+
+- Incremental Sync: When new or updated rows are found, only those rows are run through the pipeline, so downstream indexes and search results reflect the latest data while unchanged rows are untouched.
+- `ordinal_column` is recommended for change detection so the pipeline processes what's changed.
+- `notification`: when present, enable change capture based on Postgres LISTEN/NOTIFY. 
+
+Check [Postgres source](https://cocoindex.io/docs/ops/sources#postgres) for more details. 
+
+If you use the Postgres database hosted by Supabase, please click Connect on your project dashboard and find the URL there. Check [DatabaseConnectionSpec](https://cocoindex.io/docs/core/settings#databaseconnectionspec)
+for more details.
+
+## Simple Data Mapping / Transformation
+
+Create a simple transformation to calculate the total price. 
+
+```python
+@cocoindex.op.function()
+def calculate_total_value(price: float, amount: int) -> float:
+    """Compute total value for each product."""
+    return price * amount
+```
+
+Plug into the flow:
+
+```python
+with data_scope["products"].row() as product:
+     # Compute total value
+    product["total_value"] = flow_builder.transform(
+        calculate_total_value,
+        product["price"],
+        product["amount"],
+    )
+```
+
+![Calculate Total Value](/img/examples/postgres_source/price.png)
+
+### Data Transformation & AI Transformation
+
+Create a custom function creates a `full_description` field by combining the product’s category, name, and description.
+
+```python
+@cocoindex.op.function()
+def make_full_description(category: str, name: str, description: str) -> str:
+    """Create a detailed product description for embedding."
+    return f"Category: {category}\nName: {name}\n\n{description}"
+
+```
+
+Embeddings often perform better with more context. By combining fields into a single text string, we ensure that the semantic meaning of the product is captured fully.
+
+Now plug into the flow:
+
+```python
+with data_scope["products"].row() as product:
+    #.. other transformations
+
+    # Compute full description
+    product["full_description"] = flow_builder.transform(
+        make_full_description,
+        product["product_category"],
+        product["product_name"],
+        product["description"],
+    )
+    
+    # Generate embeddings
+    product["embedding"] = product["full_description"].transform(
+        cocoindex.functions.SentenceTransformerEmbed(
+            model="sentence-transformers/all-MiniLM-L6-v2"
+        )
+    )
+    
+    # Collect data 
+    indexed_product.collect(
+        product_category=product["product_category"],
+        product_name=product["product_name"],
+        description=product["description"],
+        price=product["price"],
+        amount=product["amount"],
+        total_value=product["total_value"],
+        embedding=product["embedding"],
+    )
+```
+
+This takes each product row, and does the following:
+
+1. builds a rich description. 
+
+    ![Make Full Description](/img/examples/postgres_source/description.png)
+
+2. turns it into an embedding
+
+    ![Embed Full Description](/img/examples/postgres_source/embed.png)
+
+3. collects the embedding along with structured fields (category, name, price, etc.).
+
+    ![Collect Embedding](/img/examples/postgres_source/collector.png)
+
+
+## Export
+
+```python
+indexed_product.export(
+    "output",
+    cocoindex.targets.Postgres(),
+    primary_key_fields=["product_category", "product_name"],
+    vector_indexes=[
+        cocoindex.VectorIndexDef(
+            field_name="embedding",
+            metric=cocoindex.VectorSimilarityMetric.COSINE_SIMILARITY,
+        )
+    ],
+)
+```
+
+All transformed rows are collected and exported to a new PostgreSQL table with a vector index, ready for semantic search.
+
+
+## Field lineage
+When the transform flow starts to getting complex, it's hard to understand how each field is derived.
+CocoIndex provides a way to visualize the lineage of each field, to make it easier to trace and troubleshoot field origins and downstream dependencies.
+
+For example, the following image shows the lineage of the `embedding` field, you can click from the final output backward all the way to the source fields, step by step.
+
+![Field Lineage](/img/examples/postgres_source/lineage.png)
+
+
+## Running the Pipeline
+
+1. Set up dependencies:
+    
+    ```bash
+    pip install -e .
+    ```
+    
+2. Create the source table with sample data:
+    
+    ```bash
+    psql "postgres://cocoindex:cocoindex@localhost/cocoindex" -f ./prepare_source_data.sql
+    ```
+    
+3. Setup tables and update the index:
+    
+    ```bash
+    cocoindex update --setup main.py
+    ```
+    
+4. Run CocoInsight:
+    
+    ```bash
+    cocoindex server -ci main.py
+    ```
+    You can walk through the project step by step in CocoInsight to see exactly how each field is constructed and what happens behind the scenes. It connects to your local CocoIndex server, with zero pipeline data retention.
+
+
+## Continuous Updating
+
+For continuous updating when the source changes, add `-L`:
+
+```bash
+cocoindex server -ci -L main.py
+```
+Check [live updates](https://cocoindex.io/docs/tutorials/live_updates) for more details.
+
+## Search and Query the Index
+
+### Query
+
+Runs a semantic similarity search over the indexed products table, returning the top matches for a given query.
+
+```python
+def search(pool: ConnectionPool, query: str, top_k: int = 5) -> list[dict[str, Any]]:
+    # Get the table name, for the export target in the text_embedding_flow above.
+    table_name = cocoindex.utils.get_target_default_name(
+        postgres_product_indexing_flow, "output"
+    )
+    # Evaluate the transform flow defined above with the input query, to get the embedding.
+    query_vector = text_to_embedding.eval(query)
+    # Run the query and get the results.
+    with pool.connection() as conn:
+        register_vector(conn)
+        with conn.cursor(row_factory=dict_row) as cur:
+            cur.execute(
+                f"""
+                SELECT
+                    product_category,
+                    product_name,
+                    description,
+                    amount,
+                    total_value,
+                    (embedding <=> %s) AS distance
+                FROM {table_name}
+                ORDER BY distance ASC
+                LIMIT %s
+            """,
+                (query_vector, top_k),
+            )
+            return cur.fetchall()
+```
+This function
+
+- Converts the query text into an embedding (`query_vector`).
+- Compares it with each product’s stored embedding (`embedding`) using vector distance.
+- Returns the closest matches, including both metadata and the similarity score (`distance`).
+
+### Create an command-line interactive loop
+
+```python
+def _main() -> None:
+    # Initialize the database connection pool.
+    pool = ConnectionPool(os.environ["COCOINDEX_DATABASE_URL"])
+    # Run queries in a loop to demonstrate the query capabilities.
+    while True:
+        query = input("Enter search query (or Enter to quit): ")
+        if query == "":
+            break
+        # Run the query function with the database connection pool and the query.
+        results = search(pool, query)
+        print("\nSearch results:")
+        for result in results:
+            score = 1.0 - result["distance"]
+            print(
+                f"[{score:.3f}] {result['product_category']} | {result['product_name']} | {result['amount']} | {result['total_value']}"
+            )
+            print(f"    {result['description']}")
+            print("---")
+        print()
+
+if __name__ == "__main__":
+    load_dotenv()
+    cocoindex.init()
+    _main()
+```
+
+### Run as a Service
+
+This [example](https://cocoindex.io/docs/examples/image_search#fast-api-application) runs as a service using Fast API.
+
+
+## Why One Framework for Structured + Unstructured?
+
+- One mental model: Treat files, APIs, and databases uniformly; AI steps are ordinary ops.
+- Incremental by default: Use an ordinal column to sync only changes; no fragile glue jobs.
+- Consistency: Embeddings are always derived from the exact transformed row state.
+- Operational simplicity: One deployment, one lineage view, fewer moving parts.
+