Optimise productVariant.stocks query.

[zedzior]

Port to: #15766

Impact

• New migrations
• New/Updated API fields or mutations
• Deprecated API fields or mutations
• Removed API types, fields, or mutations

Docs

• Link to documentation:

Pull Request Checklist

• Privileged queries and mutations are either absent or guarded by proper permission checks
• Database queries are optimized and the number of queries is constant
• Database migrations are either absent or optimized for zero downtime
• The changes are covered by test cases
• All new fields/inputs/mutations have proper labels added (ADDED_IN_X, PREVIEW_FEATURE, etc.)
• All migrations have proper dependencies
• All indexes are added concurrently in migrations
• All RunSql and RunPython migrations have revert option defined

[db-queries]

Here is the report for 145c869 (saleor:merx-294-optimize-variant-stocks-query-3.16)
Missing base report (12c1ef0). The results couldn't be compared.

No differences were found.