Use EXISTS strategy for better performance

[bjester]

Summary

Description of the change(s) you made

• On @vkWeb's postgresql 12 upgrade PR, I was able to reproduce the issue we've been experiencing with Github action python tests stalling
• I was able to identify that postgres was busy running a query related to storage calculations
• The query needed optimization, and this does enough to prevent the issue for now

Manual verification steps performed

Run the full test suite

Screenshots (if applicable)

Does this introduce any tech-debt items?

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Reviewer guidance

How can a reviewer test these changes?

Are there any risky areas that deserve extra testing?

References

Comments

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Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
• If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md