When a search is in progress, the options to cancel or pause the search appear.
Pause a Search
Search always retrieves and displays messages in reverse chronological order. Results are found walking backward in time from your most current data and progressing through older data. So, if you pause a search, you can check the timestamps for the messages in the Messages tab. If you reverse the sort order of the messages so that the oldest message is at the top, then all messages with more recent timestamps have been retrieved and processed.
You can resume a paused search; just click Resume under the Start button.
Once a search has returned 100,000 results the search is automatically paused. Click Resume to continue the search.
Cancel a Search
When you cancel a search, you are stopping all progress on the current search and removing all results. Your query remains in the search query field.