add/remove some indices; more query optimization

[codecov[bot]]

Codecov Report

Merging #1147 (400ad05) into master (1ff1833) will decrease coverage by 0.08%. The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #1147      +/-   ##
==========================================
- Coverage   67.67%   67.59%   -0.08%     
==========================================
  Files         120      120              
  Lines        4727     4734       +7     
  Branches     1033     1039       +6     
==========================================
+ Hits         3199     3200       +1     
- Misses       1418     1423       +5     
- Partials      110      111       +1     

Flag	Coverage Δ
backend	67.59% <28.57%> (-0.08%)	:arrow_down:

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Impacted Files	Coverage Δ
...nd/bin/kafkaConsumer/common/userPoints/saveToDB.ts	88.03% <ø> (ø)
backend/graphql/ExerciseCompletion.ts	74.41% <0.00%> (-3.64%)	:arrow_down:
backend/graphql/User/model.ts	40.35% <0.00%> (-1.47%)	:arrow_down:
backend/graphql/UserCourseSummary.ts	62.79% <100.00%> (+0.88%)	:arrow_up:

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