# FST index

The FST index supports regex queries on text. Decreases on-disk index by 4-6 times. 

* Only supports regex queries
* Only supported on stored or completed Pinot segments (no consuming segments).
* Only supported on dictionary-encoded columns.
* Works better for prefix queries 

For more information on the FST construction and code, see [Lucene documentation](https://lucene.apache.org/core/9_10_0/core/org/apache/lucene/util/fst/FST.html).

## Enable the FST index

To enable the FST index on a dictionary-encoded column, include the following configuration:

```javascript
"fieldConfigList":[
{
"name":"text_col_1",
"encodingType":"DICTIONARY",
"indexType":"FST"
}
]
```

The FST index generates one FST index file (`.lucene.fst)`. If the inverted index is enabled, this is further able to take advantage of that.

For more information about enabling the FST index, see ways to [enable indexes](/release-1.2.0/basics/indexing.md#enabling-indexes).

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# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.pinot.apache.org/release-1.2.0/basics/indexing/fst-index.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.