---
title: Segment Count
---

By default, `CREATE INDEX`/`REINDEX` will create as many segments as there are CPUs on the host machine. This can be changed using the
`target_segment_count` index option.

```sql
CREATE INDEX search_idx ON mock_items USING bm25 (id, description, rating) WITH (key_field = 'id', target_segment_count = 32, ...);
```

This property is attached to the index so that during `REINDEX`, the same value will be used.

It can be changed with ALTER INDEX, like so:

```sql
ALTER INDEX search_idx SET (target_segment_count = 8);
```

However, a `REINDEX` is required to rebalance the index to that segment count.

For optimal performance, the segment count should equal the number of parallel workers that a query can receive, which is controlled by
[`max_parallel_workers_per_gather`](/documentation/configuration/parallel#parallel-workers). If `max_parallel_workers_per_gather` is greater than the number of CPUs on the host machine, then increasing the target segment count to match `max_parallel_workers_per_gather` can improve query
performance.

<Note>
`target_segment_count` is merely a suggestion.

While `pg_search` will endeavor to ensure the created index will have exactly this many segments, it is possible for it
to have less or more. Mostly this depends on the distribution of work across parallel builder processes, memory
constraints, and heap size.

</Note>