To recap, here are the best practices for defining indexes in VoltDB:
Avoid indexes that have a column list that is simply a prefix of another index's column list. The index with the longer column list will usually serve the same queries as the shorter one. If the primary key of table X is (A, B, C), then an index on (A, B) is of little use. An index on (B, C) may be of use in this scenario or it may be more effective to define the primary key as (B, C, A) — if B is likely to be filtered in queries where A is not equality-filtered.
Avoid "low-cardinality" indexes — An index defined solely on a column that only has a few distinct values is usually not very effective. Because of its large number of duplicate values, it does little to narrow the set of rows to be sequentially filtered by other filters in the query. Such an index can sometimes cause the planner to fail to select a more effective index for a query or even a more efficient sequential scan. One way to increase index effectiveness of low cardinality indexes is to add other filtered columns to the index, keeping in mind that the effectiveness of an index for a query "tops out" at the first column that has an inequality filter — or before the second column that has an IN filter.
When deciding how to order columns within an index (or primary key or unique constraint) definition, columns that are more likely to be filtered with an exact equality (such as A = ?), should be listed before columns that tend to be range-filtered (B <= ?). Queries that are run the most often or that benefit the most from indexing (perhaps because they lack filters that can be covered by other indexes) should weigh more heavily in this decision.
In some cases, with a roughly equal mix between queries using forms like "WHERE A = ? AND B <= ?" and other queries using forms like "WHERE A > ? AND B = ?", it may be worthwhile to define indexes on both permutations — on X(A, B ...) and on X(B, A ...). Otherwise, when two or more columns in an index tend to both get equality filtered in combination, it is generally better to list a column first if it also tends to be filtered (without the other) in other queries. A possible exception to this rule is when the column has low cardinality (to avoid the ineffective use of the index).
Placing the low-cardinality column later in the index's list prevents the index from being applied as a low-cardinality indexed filter and favors the selection of a more effective index or sequential scan.
Any non-unique filter that is listed in the schema report as having no procedures using it is a candidate for elimination. But first, it may make sense to look for queries that would be expected to use the index and determine what they are using instead for scans on the table. It may be that the index chosen by the planner is not actually as effective and that index may be the better candidate for elimination. Also, note that indexes that are only used to support recalculation of min and max values in materialized views may be erroneously reported as unused.
Index optimization is best accomplished iteratively, eliminating or tuning an index on a table and seeing its effect on statements before making other changes to other competing indexes.