# "The heap is empty" error on KNN search
**URL:** https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550
**Category:** Elasticsearch
**Tags:** vector-search
**Created:** [December 19, 2022, 10:58am UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550 "2022-12-19T10:58:18Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@abay.kenzhekhan](https://discuss.elastic.co/u/abay.kenzhekhan)
#### Post date: [December 19, 2022, 10:58am UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/1 "2022-12-19T10:58:18Z")
I am doing following KNN search request
```auto
{
"filter": {
"bool": {
"must_not": {
"term": {
"some_field": 14642652
}
},
"must": {
"term": {
"some_field2": true
}
}
}
},
"knn": {
"field": "photo_vector",
"k": 5,
"num_candidates": 5,
"query_vector": [
...
]
}
}
```
ES responses with 200 code, but I get following error on shards field:
```auto
{
"took": 522,
"timed_out": false,
"_shards": {
"total": 12,
"successful": 2,
"skipped": 0,
"failed": 10,
"failures": [
{
"shard": 0,
"index": "photos-v4",
"node": "ZF73KImLQ2qVcRhedvBKcA",
"reason": {
"type": "illegal_state_exception",
"reason": "The heap is empty"
}
}
]
},
"hits": {...}
}
```
If I remove condition ""some\_field2": true", everything is ok.
What can be the reason for this error?
---
### Author:  [@BenTrent](https://discuss.elastic.co/u/BenTrent)
#### Post date: [December 19, 2022, 2:16pm UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/2 "2022-12-19T14:16:16Z")
@abay.kenzhekhan
My gut tells me that this is because the query filter in KNN is very restrictive, so for certain document segments, no documents are found. But the `The heap is empty` error is thrown by an internal structure that is used many different places.
To be sure, can you re-run your failing query with `error_trace=true` so we get the full Java stacktrace to help debug the issue?
---
### Author:  [@abay.kenzhekhan](https://discuss.elastic.co/u/abay.kenzhekhan)
#### Post date: [December 20, 2022, 4:42am UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/3 "2022-12-20T04:42:17Z")
Unfortunately, it returns the same response with no additional info. What else can you suggest to do? Thanks 🙂
---
### Author:  [@BenTrent](https://discuss.elastic.co/u/BenTrent)
#### Post date: [December 20, 2022, 5:48pm UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/4 "2022-12-20T17:48:29Z")
Ah, please set `allow_partial_search_results=false` and `error_trace=true` That may work.
We have tests with restrictive filters (with empty results), so something strange indeed is going on.
Also, your Elasticsearch version would help as well 🙂
---
### Author:  [@abay.kenzhekhan](https://discuss.elastic.co/u/abay.kenzhekhan)
#### Post date: [December 21, 2022, 3:27am UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/5 "2022-12-21T03:27:56Z")
Elastic version: 8.3.1
Here is response:
```auto
{
"error": {
"root_cause": [
{
"type": "illegal_argument_exception",
"reason": "request [/photos-alias/_knn_search] contains unrecognized parameter: [allow_partial_search_results]",
"stack_trace": "org.elasticsearch.ElasticsearchException$1: request [/photos-alias/_knn_search] contains unrecognized parameter: [allow_partial_search_results]\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.ElasticsearchException.guessRootCauses(ElasticsearchException.java:638)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.ElasticsearchException.generateFailureXContent(ElasticsearchException.java:566)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.BytesRestResponse.build(BytesRestResponse.java:145)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.BytesRestResponse.(BytesRestResponse.java:101)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.BytesRestResponse.(BytesRestResponse.java:81)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.RestController.dispatchRequest(RestController.java:391)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.RestController.tryAllHandlers(RestController.java:468)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.RestController.dispatchRequest(RestController.java:304)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.http.AbstractHttpServerTransport.dispatchRequest(AbstractHttpServerTransport.java:371)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.http.AbstractHttpServerTransport.handleIncomingRequest(AbstractHttpServerTransport.java:450)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.http.AbstractHttpServerTransport.incomingRequest(AbstractHttpServerTransport.java:345)\n\tat org.elasticsearch.transport.netty4@8.3.1/org.elasticsearch.http.netty4.Netty4HttpPipeliningHandler.handlePipelinedRequest(Netty4HttpPipeliningHandler.java:98)\n\tat org.elasticsearch.transport.netty4@8.3.1/org.elasticsearch.http.netty4.Netty4HttpPipeliningHandler.channelRead(Netty4HttpPipeliningHandler.java:88)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:327)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:299)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.handler@4.1.76.Final/io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:722)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:623)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:586)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:496)\n\tat io.netty.common@4.1.76.Final/io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986)\n\tat io.netty.common@4.1.76.Final/io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)\n\tat java.base/java.lang.Thread.run(Thread.java:833)\nCaused by: java.lang.IllegalArgumentException: request [/photos-alias/_knn_search] contains unrecognized parameter: [allow_partial_search_results]\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.BaseRestHandler.handleRequest(BaseRestHandler.java:94)\n\tat org.elasticsearch.security@8.3.1/org.elasticsearch.xpack.security.rest.SecurityRestFilter.handleRequest(SecurityRestFilter.java:119)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.RestController.dispatchRequest(RestController.java:389)\n\t... 48 more\n"
}
],
"type": "illegal_argument_exception",
"reason": "request [/photos-alias/_knn_search] contains unrecognized parameter: [allow_partial_search_results]",
"stack_trace": "java.lang.IllegalArgumentException: request [/photos-alias/_knn_search] contains unrecognized parameter: [allow_partial_search_results]\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.BaseRestHandler.handleRequest(BaseRestHandler.java:94)\n\tat org.elasticsearch.security@8.3.1/org.elasticsearch.xpack.security.rest.SecurityRestFilter.handleRequest(SecurityRestFilter.java:119)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.RestController.dispatchRequest(RestController.java:389)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.RestController.tryAllHandlers(RestController.java:468)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.rest.RestController.dispatchRequest(RestController.java:304)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.http.AbstractHttpServerTransport.dispatchRequest(AbstractHttpServerTransport.java:371)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.http.AbstractHttpServerTransport.handleIncomingRequest(AbstractHttpServerTransport.java:450)\n\tat org.elasticsearch.server@8.3.1/org.elasticsearch.http.AbstractHttpServerTransport.incomingRequest(AbstractHttpServerTransport.java:345)\n\tat org.elasticsearch.transport.netty4@8.3.1/org.elasticsearch.http.netty4.Netty4HttpPipeliningHandler.handlePipelinedRequest(Netty4HttpPipeliningHandler.java:98)\n\tat org.elasticsearch.transport.netty4@8.3.1/org.elasticsearch.http.netty4.Netty4HttpPipeliningHandler.channelRead(Netty4HttpPipeliningHandler.java:88)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:327)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:299)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.handler@4.1.76.Final/io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.codec@4.1.76.Final/io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:722)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:623)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:586)\n\tat io.netty.transport@4.1.76.Final/io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:496)\n\tat io.netty.common@4.1.76.Final/io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986)\n\tat io.netty.common@4.1.76.Final/io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)\n\tat java.base/java.lang.Thread.run(Thread.java:833)\n"
},
"status": 400
}
```
It seems elastic doesn't understand `allow_partial_search_results`. Also what I found is that there is no error if instead of term condition with bool, I use some integer condition, everything works. It seems there is some problem with shards and bool condition.
---
### Author:  [@abay.kenzhekhan](https://discuss.elastic.co/u/abay.kenzhekhan)
#### Post date: [December 21, 2022, 4:23am UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/6 "2022-12-21T04:23:44Z")
I have some another question. We have to approach to filter KNN result:
1. Using filter. For instance, `filter:{"term": {"n": 1}}`
2. Using query. For instance, `query:{"term": {"n": 1}}`
The difference between them is that `query` with `knn` works like a `OR`. But we can anyway use it like a simple `filter` using `min_score`. The question is which on above of approaches is more suitable for filtering and have best speed? It seems `filter` is applied only after `knn` search gets completed, while `query` applied during (but why it works like a `OR`?)
---
### Author:  [@BenTrent](https://discuss.elastic.co/u/BenTrent)
#### Post date: [December 21, 2022, 3:40pm UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/7 "2022-12-21T15:40:37Z")
If you only want to filter documents with KNN, you should supply that filter within the KNN object.
`POST /_search?allow_partial_search_results=false&error_trace=true` should definitely work. I have just verified that these query parameters work just fine.
The main thing here, is if you want the `term` query to provide scores to the document, or you only want documents scored according to KNN.
Here is what a filter looks like with using KNN (just now noticed you are using a filter in the top level query 🤦♂️ my bad...)
```auto
{
"knn": {
"filter": {
"bool": {
"must_not": {
"term": {
"some_field": 14642652
}
},
"must": {
"term": {
"some_field2": true
}
}
}
},
"field": "photo_vector",
"k": 5,
"num_candidates": 5,
"query_vector": [...]
}
}
```
^ This is the way to filter with KNN search. This way only vectors that match the filter are considered.
If you want to use "hybrid search" (meaning docs are scored according to term query & KNN and the disjoint `OR` is returned), then use a term query clause in addition to the knn clause.
Example from the docs
```auto
{
"query": {
"match": {
"title": {
"query": "mountain lake",
"boost": 0.9
}
}
},
"knn": {
"field": "image-vector",
"query_vector": [54, 10, -2],
"k": 5,
"num_candidates": 50,
"boost": 0.1
},
"size": 10
}
```
---
### Author:  [@abay.kenzhekhan](https://discuss.elastic.co/u/abay.kenzhekhan)
#### Post date: [December 22, 2022, 11:09am UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/8 "2022-12-22T11:09:34Z")
Thanks for the great answer. But it leads to a lot of questions:
1. `filter` might be inside `knn` and outside. Are there any difference? I hope they work the same way.
2. is `filter` in `knn` the same as `post_filter`? Does ES do KNN first and only after do what is inside `filter`?
3. `query` with `knn` might be used just like `filter`. Even if it works like `OR`, we can just use `min_score` or just take some top scored documents. It creates a question like which one is faster and performs better: `filter` or `query`?
---
### Author:  [@BenTrent](https://discuss.elastic.co/u/BenTrent)
#### Post date: [December 23, 2022, 7:48pm UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/9 "2022-12-23T19:48:33Z")
1. Yes, there is a difference, doing a filter outside KNN is an OR combination. Either docs match the filter, or KNN or both. Note, the filtered documents are unscored, but they may be scored by the KNN query.
2. It is not a post-filter, but it is an AND combination.
3. Yes, this is how you should do it.
Something like this would be an OR combination with a filter & KNN.
```auto
{
"query": {
"bool": {
"must_not": { "term": {"some_field": 14642652} },
"filter": {"term": {"some_field2": true}}
}
},
"knn": {
"field": "photo_vector",
"k": 5,
"num_candidates": 5,
"query_vector": [
...
]
}
}
```
To get the full stack trace to fully debug your original issue, I have confirmed the query parameters `allow_partial_search_results=false&error_trace=true ` are what you need. If you want to dig into that further.
---
### Author:  [@abay.kenzhekhan](https://discuss.elastic.co/u/abay.kenzhekhan)
#### Post date: [December 26, 2022, 3:37am UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/10 "2022-12-26T03:37:39Z")
1. There is a great [article](https://medium.com/gsi-technology/a-comparison-of-elasticsearch-k-nn-solutions-be8d668812e) telling that it is not possible to do filtering first and only after, on small subset of data, making the KNN search. According to this article, we can say that there is no benefit of adding some `filter`, because anyway, KNN does his job first (because of graphs) and only after (or during), docs will be filtered. The performance would be way better if filtering does his job first and only after, KNN search will be executed on smaller data. Are my observations are correct?
---
### Author:  [@BenTrent](https://discuss.elastic.co/u/BenTrent)
#### Post date: [December 26, 2022, 12:28pm UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/11 "2022-12-26T12:28:35Z")
We don't do pre/post filtering. We take advantage of the KNN index and the filter index living in the same document. Additionally, an optimization kicks in if the filter set is tiny, we skip running through the HNSW graph as it wouldn't provide benefit.
Doing an 'OR' combination does not reduce the work done by KNN at all. It is just saying, either it's a closest neighbor or it's part of this filter.
If you want to do a filter that says "I only want these documents and give me the ones that are closest neighbors" the example I gave you is what you need
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [January 23, 2023, 12:28pm UTC](https://discuss.elastic.co/t/the-heap-is-empty-error-on-knn-search/321550/12 "2023-01-23T12:28:42Z")
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