Is this compound index unnecessary?
It might be worth posting the table definition from your other question for clarity.

The composite index is doing a few things for you:


As you know, enforcing uniqueness on (sensor_id, timestamp); I'm unsure whether this is an important data integrity constraint.
Allowing queries that filter on both columns to look up matching rows by using a single index. MySQL can answer some queries (equality conditions on multiple columns are the ones I know about) by merging two indexes, but this tends to be significantly slower compared to using a single composite index.
The index can also be used to search for values in a left-based subset of the composite index, but not a right-based subset. So in this case it could help a query that filters on sensor_id values or sensor_id and timestamp values, but not timestamp values alone.


There are a number of caveats to this, so it's good idea to look at the EXPLAIN output for your queries and verify what indexes they're using. Keep also in mind that indexes can support  the read part of UPDATE and DELETE queries, as well as JOINs, GROUP BY, ORDER BY, and other operations I'm neglecting.

An example of a scenario where the composite index is unnecessary would be if you don't care about the uniqueness constraint and all your queries filter on timestamp or sensor_id, but not both.

The single-column index on sensor_id is actually redundant since the composite index on (sensor_id, timestamp) can be used by the same queries, but still you might find that some queries perform faster when doing scans on the single-column index compared to using a composite index with a wider key. The difference might not be enough to matter, though, and some testing will probably be required to find out. 

In addition to looking at the EXPLAIN output for your queries, tools such as pt-index-usage from the Percona toolkit or the table INFORMATION_SCHEMA.INDEX_STATISTICS if you're running Percona Server or MariaDB can help you assess what indexes are actually being used.

References:

The Optimization That (Often) Isn’t: Index Merge Intersection

Practical MySQL indexing guidelines