How can I know which shard contains a row for a table? #

You can use the primary Vindex column to query the Vindex and discover the shard ID. Once you have determined the shard ID you can use manual shard targeting to send that specific shard a query. Note that if the query contains the primary Vindex column, or an appropriate secondary Vindex column, you do not need to do this, and vtgate can route the query automatically.

Can I use Vitess to do cross-shard JOINs or Transactions? #

A horizontal sharding solution for MySQL like Vitess does allow you to do both cross-shard joins and transactions, but just because you can doesn’t mean you should.

A sharded architecture will perform best if you design it well and play to its strength, e.g. favoring single-shard targeted writes within any individual transaction. Enabling two-phase commit in Vitess to support cross-shard writes is possible, but will come at a significant performance cost.

Whether that tradeoff is worth it differs from application to application and, generally speaking, adjusting the schema/workload is considered the better approach.