Speed up Emap by adding a 3-column index

[jeremyestein]

The query generated by uk.ac.ucl.rits.inform.datasinks.emapstar.repos.visit_observations.VisitObservationRepository#findByHospitalVisitIdAndVisitObservationTypeIdAndObservationDatetime is in the top ~5 of all queries by running time according to my 1-day sample.

We already had single-column indexes for each of the three columns in the query, but a single index of all three makes the query run about 30% faster. I didn't check which, if any, of the single column indexes were being used.

(Can old indexes be removed?)

[github-actions]

PR checklist

Default guide for a PR (if multiple PRs for the work, only keep one version of it and link to it on the other PRs)

• From the UCLH data science desktop, a validation run has been set off
• Check that content that reveals Epic IP (eg. Clarity/Caboodle queries) has not been checked into a public repo.
• load times
  in UCL teams has been populated with the run information
• During the run, glowroot has been checked for any queries which are taking a substantial proportion of the
  total processing time. This can be useful to identify indexes that are required.
• After the run, look for any unexpected errors in the etl_per_message_logging table, the error_search.sql file
  on the shared drive can be used for this \\sharefs6\UCLH6\EMAP\Shared\EmapSqlScripts\devops\error_search.sql.
  Create an issue if you find an unexpected exception and is not related to the changes you've made, otherwise
  fix them!
• After the run, populate the end time in
  load times
• Let Aasiyah know about the completed validation and give her information on the changes and where to start
  with the validation
• Check validation report and give any feedback to Aasiyah if there are any changes needed on her side,
  iterate on getting the validation to match at least 99% (validation and emap code).

[skeating]

I don't see why old indexes cannot be removed but probably want to check with somebody else with more experience of this