Avoid walking the complete list of search contexts on shard creation #123855
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This I found in the many-shards benchmark during some manual testing. Creating indices slows down measurably when there's concurrent searches going on. Interestingly enough, the bulk of the cost is coming from this hook. This makes sense to some extend because the map can quickly grow to a massive size as it scales as O(shards_searched_on_average * concurrent_searches) and a CHM generally is anything but cheap to iterate over.
=> no need to do this iteration if we're creating a new shard.