Otherwise GlobalThreadPool can be terminated (for example due to an
exception from the ParallelInputsHandler::onFinish/onFinishThread, from
ParallelAggregatingBlockInputStream::Handler::onFinish/onFinishThread,
since writeToTemporaryFile() can definitelly throw) and the server will
not accept new connections (or/and execute queries) anymore.
Here is possible stacktrace (it is a bit inaccurate, due to
optimizations I guess, and it had been obtained with the
DB::tryLogCurrentException() in the catch block of the
ThreadPoolImpl::worker()):
2020.02.16 22:30:40.415246 [ 45909 ] {} <Error> ThreadPool: Unhandled exception in the ThreadPool(10000,1000,10000) the loop will be shutted down: Code: 241, e.displayText() = DB::Exception: Memory limit (total) exceeded: would use 279.40 GiB (attempt to allocate chunk of 4205536 bytes), maximum: 279.40 GiB, Stack trace (when copying this message, always include the lines below):
1. Common/Exception.cpp:35: DB::Exception::Exception(...)
...
6. Common/Allocator.h:102: void DB::PODArrayBase<8ul, 4096ul, Allocator<false, false>, 15ul, 16ul>::reserve<>(unsigned long) (.part.0)
7. Interpreters/Aggregator.cpp:1040: void DB::Aggregator::writeToTemporaryFileImpl<...>(...)
8. Interpreters/Aggregator.cpp:719: DB::Aggregator::writeToTemporaryFile(...)
9. include/memory:4206: DB::Aggregator::writeToTemporaryFile(...)
10. DataStreams/ParallelInputsProcessor.h:223: DB::ParallelInputsProcessor<DB::ParallelAggregatingBlockInputStream::Handler>::thread(...)
Refs: https://github.com/ClickHouse/ClickHouse/issues/6833#issuecomment-579221732
(Reference to particular comment, since I'm not sure about the initial issue)
