• Review existing indexes on the tally_snapshots table.
• Create a new partial index that can help with read and doesn't affect write process or update existing indexes.
• Check if runInTransaction=false helps since CREATE INDEX CONCURRENTLY cannot run inside a transaction and having this avoids locking issues on the 198M row table. https://docs.liquibase.com/reference-guide/changelog-attributes/runintransaction 
• Ensure the index aligns with current query patterns and filtering conditions. Not sure if adding snapshot_date in index will be helpful.
• Include migration and rollback support for index changes.
• Since is_primary defaults to false and index is WHERE true, the index starts empty (should be fast even with 198M)
• Determine a way to add the index concurrently (to avoid locking the table) and without creating an outage (which a Liquibase run would do).  See the comments for more information on how to do this.

Example:

 <changeSet id="" author="" runInTransaction="false">
    <sql>
      CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_tally_snapshots_primary
      ON tally_snapshots (org_id, product_id, granularity)
      WHERE is_primary = true;
    </sql>
    <rollback>
      DROP INDEX IF EXISTS idx_tally_snapshots_primary;
    </rollback>
  </changeSet> 

We can also try the same locally, by directly doing below for testing purpose

 CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_tally_snapshots_primary
  ON tally_snapshots (org_id, product_id, granularity)
  WHERE is_primary_record = true;

AC:

• The index will be added to the table in production manually through an out of band procedure to avoid Liquibase related issues. After completion in production we can create a PR to apply the same index to the lower environments.
• No functional impact to existing queries or data.
• No execution time degradation when running queries involving tally_snapshots (can be poked by automated component tests and compared against historical execution time of the same tests from report portal)
• Download prod data locally for tally_snapshots and tally_measurements and make sure the index gets used and time it takes to apply index. QE and DEV need to do the same.