> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cyborg.co/llms.txt
> Use this file to discover all available pages before exploring further.

# Train Index

<Note>This function is only present in the [core library](../../../guides/get-started/about#deployment-models) version of CyborgDB.
In other versions (microservice, serverless), it is automatically called once enough vector embeddings have been indexed.</Note>

Builds the index using the specified training configuration. Required before efficient querying.
Prior to calling this, all queries will be conducted using encrypted exhaustive search.
After, they will be conducted using encrypted ANN search.

```cpp theme={null}
void TrainIndex(const TrainingConfig& training_config = TrainingConfig());
```

### Parameters

| Parameter         | Type                                        | Description                                                          |
| ----------------- | ------------------------------------------- | -------------------------------------------------------------------- |
| `training_config` | [`TrainingConfig`](../types#trainingconfig) | *(Optional)* Training parameters (batch size, max iterations, etc.). |

<Tip>There must be at least `2 * n_lists` vector embeddings in the index prior to to calling this function.</Tip>

### Exceptions

Throws if there are not enough vector embeddings in the index for training (must be at least `2 * n_lists`).

<AccordionGroup>
  <Accordion title="std::runtime_exception">
    * Throws if there are not enough vector embeddings in the index for training (must be at least `2 * n_lists`).
    * Throws if the index could not be trained.
  </Accordion>
</AccordionGroup>

### Example Usage

```cpp theme={null}
cyborg::TrainingConfig config(128, 10, 1e-4, 1024);
index->TrainIndex(config);
```