When a search on some indices takes a long time, it may cause problems to other indices that are being searched as part of the same search request and being written to as well, because their search context needs to stay open for a long time. This is especially a problem when searching against throttled and non-throttled indices as part of the same request. The problem can be generalized though: this may happen whenever read-only indices are searched together with indices that are being written to. Search contexts staying open for a long time is only an issue for indices that are being written to, in practice.
This commit splits the search in two sub-searches: one for read-only indices, and one for ordinary indices. This way the two don't interfere with each other. The split is done only when size is greater than 0, no scroll is provided and query_then_fetch is used as search type. Otherwise, the search executes like before. Note that the returned num_reduce_phases reflect the number of reduction phases that were run. If the search is split in two, there are three reductions: one non-final for each search, and a final one that merges the results of the previous two.
Closes#40900
afeda1a7b9