Which preconfigured index serves as the default for summary indexing?

The preconfigured index that serves as the default for summary indexing is the summary index. Summary indexing is a feature in Splunk that allows for the storage of pre-processed summary data to improve the performance of searches. By default, when you create a summary index, it stores summarized data in the summary index, which makes it efficient for generating reports and dashboards, especially when dealing with large datasets.

Using the summary index helps to reduce the volume of data that needs to be queried during searches, resulting in faster search performance and lower resource consumption. This index is specifically designed to work with summary data, and it is tailored to store the results of scheduled searches that aggregate data over time.

In contrast, other indices like _thefishbucket or defaultdb serve specific functions, such as tracking file changes or default indexing, but they are not designed for summary indexing purposes. Similarly, the main index is the default index for incoming data but is not dedicated to storing summarized outputs. The summary index's unique characteristics make it the correct answer in this scenario, as it directly relates to the concept of summary indexing within Splunk.