## Index and Limit

<div class="warning">`limit` cannot be used in conjunction with `index`.</div>

Initializing a `Table` with an `index` tells Perspective to treat a column as
the primary key, allowing in-place updates of rows. Only a single column (of any
type) can be used as an `index`. Indexed `Table` instances allow:

-   In-place _updates_ whenever a new row shares an `index` values with an
    existing row
-   _Partial updates_ when a data batch omits some column.
-   _Removes_ to delete a row by `index`.

To create an indexed `Table`, provide the `index` property with a string column
name to be used as an index:

<div class="javascript">

JavaScript:

```javascript
const indexed_table = await perspective.table(data, { index: "a" });
```

</div>
<div class="python">

Python

```python
indexed_table = perspective.Table(data, index="a");
```

</div>

Initializing a `Table` with a `limit` sets the total number of rows the `Table`
is allowed to have. When the `Table` is updated, and the resulting size of the
`Table` would exceed its `limit`, rows that exceed `limit` overwrite the oldest
rows in the `Table`. To create a `Table` with a `limit`, provide the `limit`
property with an integer indicating the maximum rows:

<div class="javascript">

JavaScript:

```javascript
const limit_table = await perspective.table(data, { limit: 1000 });
```

</div>
<div class="python">

Python:

```python
limit_table = perspective.Table(data, limit=1000);
```

</div>

## `page_to_disk`

By default a `Table` keeps its columns in memory. Initializing a `Table` with
`page_to_disk` backs its column data with on-disk storage instead, so the
`Table` can be larger than the memory available to the engine. It is otherwise
an ordinary `Table`: `index`, `update()`, views, aggregates, expressions and
Arrow round-trips all behave as they do in memory, and produce identical
results.

<div class="javascript">

JavaScript:

```javascript
const table = await perspective.table(arrow, { page_to_disk: true });
```

</div>
<div class="python">

Python:

```python
table = perspective.table(arrow, page_to_disk=True)
```

</div>

Where the data goes depends on the runtime:

| Runtime | Backing storage |
| --- | --- |
| Browser (WebAssembly, in a Web Worker) | The [Origin Private File System](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API/Origin_private_file_system) (OPFS) |
| Node.js (WebAssembly) | Files under the OS temp directory, via `node:fs` |
| Python and Rust (native) | Memory-mapped files under the OS temp directory |