How to get better relevance without compromising on performance, scalability and avoid the sharding effect of Elasticsearch

Let's suppose I have a big index, consists 500 million docs and by default, ES creates 5 primary shards for below reasons and I also go with the same setting.

1. Performance:- There will be less time to search in a shard with less no of documents(100 million in my use case) than in just 1 shard with a huge number of documents(500 million). Also, allows to distribute and parallelize operations across shards.

2. Horizontal scalability(HS) :- horizontally split/scale your content volume.

But when we search by default it just goes to 1 shard and gives the result. in this case, relevance isn't accurate(as idf be majorly impacted) and also it might even not give any result if my matched document is on another shard. and its called as The Sharding Effect.

Above issue is explained in details here and there are below 2 options to avoid this issue but I think both the solutions have some cons :-

1. Document routing: I this case all the documents will be on the same shards which lose the whole purpose of sharding.
2. dfs_query_then_fetch search type: there is performance cost associated with it.

I am interested to know below:

1. What ES does by default? or is there is any config by which it can be controlled?
2. Is there is other Out of the box solution which ES provides to avoid the sharding effect?

first of all this part of your question if not accurate :

But when we search by default it just goes to 1 shard and gives the result. in this case, relevance isn't accurate(as idf be majorly impacted) and also it might even not give any result if my matched document is on another shard. and its called as The Sharding Effect.

The bold part is false. The search request is sent to all shards ( of course, or no one would use elasticsearch !) but the score is computed on shard basis. So yes you can have an accuracy problem with multiple shards but only if you have very few documents. With 500 million the accuracy will not be a problem ( unless you u make a bad usage of document routing see here for more informations

So when you search for 10 results for a query, each shard return the 10 best matches for the query, then the results from the shards are aggregated by the coordination node to give the best 10 results for the whole index.

You can use 5 shards without fearing any relevancy problem. But don't try to avoid sharding effect! It is what makes elasticsearch so cool :D