# Understanding how Elasticsearch segments, merges, and compacts its vector indexes

**URL:** <https://discuss.elastic.co/t/understanding-how-elasticsearch-segments-merges-and-compacts-its-vector-indexes/383796>\
**Category:** Elasticsearch\
**Tags:** vector-search\
**Created:** [December 2, 2025, 12:30am UTC](https://discuss.elastic.co/t/understanding-how-elasticsearch-segments-merges-and-compacts-its-vector-indexes/383796 "2025-12-02T00:30:01Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![sfmqrb](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/sfmqrb/32/146005_2.png) [@sfmqrb](https://discuss.elastic.co/u/sfmqrb)\
**Post date:** [December 2, 2025, 12:30am UTC](https://discuss.elastic.co/t/understanding-how-elasticsearch-segments-merges-and-compacts-its-vector-indexes/383796/1 "2025-12-02T00:30:01Z")

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In Elasticsearch’s vector indexing pipeline, how exactly do segmenting and segment compaction work? Specifically, what compaction policy does Elasticsearch follow for segments and their vector indexes, and during compaction, are old segments reused and merged, or does Elasticsearch discard them and perform a complete rebuild of the vector index?