This will do it BUT there is a problem:
| rest /servicesNS/-/-/saved/searches
| table search title
| rex max_match=0 field=search "(?<=^|\s|\[)index\s*=\s*(?<index>(?:\"[^\"]+\")|\S+)"
| rex field=index mode=sed "s/\"//g"
| fillnull value="N/A" index
| stats values(title) BY index
The problem is that it does not handle any cases where index is not specified literally inside the search string. These cases include: |savedsearch, |loadjob, eventtypes, macros tags, the use of Indexes searched by default (which may vary depending on user and role) and probably other things that I am missing. Also note that values (and limit) are limited to a maximum of 1000 values.
