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Indexed select statements fetch primary key information from their internal materialized views and then use it to query the base table. Unfortunately, the current mechanism for retrieving base table rows makes it easy to overwhelm the replicas with unbounded concurrency - the number of concurrent ops is increased exponentially until a short read is encountered, but it's not enough to cap the concurrency - if data is fetched row-by-row, then short reads usually don't occur and as a result it's easy to see concurrency of 1M or higher. In order to avoid overloading the replicas, the concurrency of indexed queries is now capped at 4096 and additionally throttled if enough results are already fetched. For paged queries it means that the query returns as soon as 1MB of data is ready, and for unpaged ones the concurrency will no longer be doubled as soon as the previous iteration fetched 1MB of results. The fixed 4096 value can be subject to debate, its reasoning is as follows: for 2KiB rows, so moderately large but not huge, they result in fetching 10MB of data, which is the granularity used by replicas. For 200B rows, which is rather small, the result would still be around 1MB. At the same time, 4096 separate tasks also means 4096 allocations, so increasing the number also strains the allocator. Fixes #8799 Tests: unit(release), manual: observing metrics of modified index_paging_test Closes #8814 * github.com:scylladb/scylla: cql3: limit the transitional result size for indexed queries cql3: return indexed pages after 1MB worth of data cql3: limit the concurrency of indexed statements