---
title: Segment Size
---

During every `INSERT`/`UPDATE`/`COPY` statement, the BM25 index will look for opportunities to merge [segments](/documentation/concepts/index#segment) together.
This is done to optimize search performance by reducing the number of segments that need to be scanned.

The segment merge policy is a configurable policy that attempts to merge segments into layers of different sizes.

## Segment Layer Size

By default, the BM25 index will attempt to merge segments into layers of size `100KB`, `1MB`, and `100MB`. This means
that, if there are ten segments each `10KB` each in size, they will be merged into a single segment that is `100KB`. However,
if there are only eight segments that are `10KB` each, they will not be merged together because they are not enough to meet the size
of the next layer.

The segment layer size can be changed at index creation time or afterward:

```sql
CREATE INDEX search_idx ON mock_items USING bm25 (id, description, rating) WITH (key_field = 'id', layer_sizes = '1kb, 10kb, 100MB');
ALTER INDEX search_idx SET (layer_sizes = '100kb, 1mb, 100mb');
```

To pick the right layer sizes, we recommend the following approach:

1. Monitor the size of your index's [segments](/documentation/indexing/inspect_index#index-segments) over time to determine the size of the smallest segment that's created
   by typical writes to the index. The smallest layer should be 10-100x the size of this smallest segment - a higher layer size generally improves write throughput
   and the expense of search performance.
2. Do not create too many layers. Every additional layer introduces the write amplication of merging up to the next layer.
3. The larger the layer size, the more time it takes to merge into a segment of that size, and the lower the write throughput.

To view the current layer sizes, use `paradedb.layer_sizes`. This function returns the size of each layer in bytes.

```sql
SELECT paradedb.layer_sizes('search_idx');
```