After you find the index that Cloud Spanner used for your query, try running your query with a different index, or by scanning the base table instead of using an index.
To specify the index, add a FORCE_INDEX directive to the query.
If you find a faster version of the query, update your application to use the faster version.
For more info refer https://cloud.google.com/spanner/docs/troubleshooting-performance-regressions#choose-index