[FEATURE REQUEST]: Introduce Partial Indexes to index subset of data based on a filter condition

[samdvr]

Introduce Partial Indexes to index subset of data based on a filter condition

As a user I want to create index on a filtered dataframe with a condition
This filter can be used on read query as a more selective index for access patterns.

example:

hs.createIndex(df,IndexConfig("index1", indexedColumns = Seq("id"), condition: "id > 10")

[clee704]

@rapoth I think it would be an interesting feature to implement. WDYT?

[rapoth]

@clee704 I agree this is an interesting feature. I was thinking we could do this as part of #186 since the latter is much more general. What do you think?

[clee704]

I think a partial index or filtered index is a different thing, because materialization is not required for them. But as we only have covering indexes for now, there is little difference indeed.