# Performance implications for elasticsearch when specifying fields that won't be in the indexed documents? **URL:** \ **Category:** Elasticsearch\ **Created:** [March 19, 2021, 8:18pm UTC](https://discuss.elastic.co/t/performance-implications-for-elasticsearch-when-specifying-fields-that-wont-be-in-the-indexed-documents/267858 "2021-03-19T20:18:03Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Supermathie](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/supermathie/32/44936_2.png) [@Supermathie](https://discuss.elastic.co/u/Supermathie)\ **Post date:** [March 19, 2021, 8:18pm UTC](https://discuss.elastic.co/t/performance-implications-for-elasticsearch-when-specifying-fields-that-wont-be-in-the-indexed-documents/267858/1 "2021-03-19T20:18:03Z")
I'm working on [switching to using the ECS toolchain](https://discuss.elastic.co/t/managed-index-templates-composition-and-ecs/266734/4) for our indices (mostly logs). The [usage notes for the generator](https://github.com/elastic/ecs/blob/master/USAGE.md#subset) say: > If your indices will never populate particular ECS fields, there's no need to include those field definitions in your index mappings. I'm considering implementing this, but would like to know: Are there performance/storage/etc. implications for elasticsearch when defining more fields than will be used in the documents in the index? ---
**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\ **Post date:** [April 16, 2021, 8:18pm UTC](https://discuss.elastic.co/t/performance-implications-for-elasticsearch-when-specifying-fields-that-wont-be-in-the-indexed-documents/267858/2 "2021-04-16T20:18:29Z")
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