# Aggregations on multiple indices much slower than on single index
**URL:** https://discuss.elastic.co/t/aggregations-on-multiple-indices-much-slower-than-on-single-index/43228
**Category:** Elasticsearch
**Created:** [March 2, 2016, 10:32am UTC](https://discuss.elastic.co/t/aggregations-on-multiple-indices-much-slower-than-on-single-index/43228 "2016-03-02T10:32:37Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@rkrombho](https://discuss.elastic.co/u/rkrombho)
#### Post date: [March 2, 2016, 10:32am UTC](https://discuss.elastic.co/t/aggregations-on-multiple-indices-much-slower-than-on-single-index/43228/1 "2016-03-02T10:32:37Z")
Hi,
I noticed a huge performance decrease when it comes to aggregations on multiple indices compared to aggregations one index at a time.
I got the following two indices:
- indexA ~ 1,250,000,000 docs (8 shards)
- indexB ~ 89,000,000 docs (4 shards)
My aggregation (which one doesnt really matter here) runs ~ 2100ms on indexA and ~1400ms on indexB.
When I run the same aggregation on an Alias which points to both indices, it runs ~20000ms.
I notices the same type of decrease for any type of aggregation I run (simple terms, cardinality etc).
Is this expected? Maybe due to the coordination work that needs to be done to merge those results together?
I mostly just split my indiced because it makes changes over time easier and because I didnt expect such a hit in terms of performance when querying multiple indices.
Should I try to put all my data into one big Index in order to avoid this kind of issues?
Another stange thing is, that I can not see any issues with regards to hot threads.
There is no thread which e.g. runs the same thing for the complete 'interval'.
Cheers,
Robert
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### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [July 5, 2017, 11:12pm UTC](https://discuss.elastic.co/t/aggregations-on-multiple-indices-much-slower-than-on-single-index/43228/2 "2017-07-05T23:12:07Z")