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Not true.

An index isn't free so you can impact the write performance, depending on the workload and usecases slower writes could have more of an impact than faster reads.

But even in a read heavy application it's possible that your databases query planner is "tricked" into using a specific index which can actually be slower than another (or even none) so you should always measure the impact that adding an index can have on your application.

Yes it could be slower, but it’s very unlikely to actually break the application.

Not true, as always it depends.

Tell me you haven't used spanner without telling me...