[FEATURE] Optimize SQL table tx_solr_statistics for frequent search terms by adding an index

[mmoeller129]

Is your feature request related to a problem? Please describe.
Frequent search terms statistics sql is slow when table grows big.

Describe the solution you'd like
Add an index to the table like this:
CREATE INDEX idx_frequentsearches ON tx_solr_statistics (root_pid, language, num_found, keywords);

Describe alternatives you've considered
Normalize or using hashes manually - but they are probably too complex

Additional context
I had this issue in frontend when clearing "all caches" unusually slowed the page down. I am not sure why because i could only trace the call back from getFrequentSearchTermsFromStatisticsByFrequentSearchConfiguration in the StatisticsRepository to the FrequentlySearchedViewHelper...
With the index the time for the query execution went down from 93,663ms to 575ms with approx. 482 503 entries in tx_solr_statistics (according to xdebug output in admPanel)

Target versions
12.0.5 - probably previous versions also