Combine multiple indexes when it's likely to improve performance

[rbasralian]

When a table has multiple independent data indexes, the query engine does not leverage them in situations where it may be beneficial to do so.

For example, if you have a PartitionAwareSourceTable (partitioned by Date), where each partition has hundreds of millions of rows and several thousand distinct Syms, and data indexes are available for the Sym column, then an operation like t.partitionBy("Date", "Sym") should probably 'combine' the index by Date and the index by Sym into an index by Date/Sym.