Performing any long running task or CPU intensive work on transport_worker threads is problematic. This is typically very bad for cluster stability.

This issue has been filed to examine the usage of transport_worked threads by ESQL, with the intention of "offloading" as much as possible, including deserialising large(ish) amounts of data.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/exchange/ExchangeService.java
}
        return sinkHandler;
    }
    /**
     * Returns an exchange sink handler for the given id.
     */
    public ExchangeSinkHandler getSinkHandler(String exchangeId) {
        ExchangeSinkHandler sinkHandler = sinks.get(exchangeId);
        if (sinkHandler == null) {
            throw new ResourceNotFoundException("sink exchanger for id [{}] doesn't exist", exchangeId);
        }
        return sinkHandler;
    }
    /**
     * Removes the exchange sink handler associated with the given exchange id.
     */
    public void finishSinkHandler(String exchangeId, Exception failure) {
        final ExchangeSinkHandler sinkHandler = sinks.remove(exchangeId);
        if (sinkHandler != null) {
            if (failure != null) {
                sinkHandler.onFailure(failure);
            }
            assert sinkHandler.isFinished() : "Exchange sink " + exchangeId + " wasn't finished yet";
        }
    }
    /**
     * Creates an {@link ExchangeSourceHandler} for the specified exchange id.
     *
     * @throws IllegalStateException if a source handler for the given id already exists
     */