7d0e3637ed
Prior to this change projection pushdown was implemented as a filter,
which meant a query using it would take the following steps:
* Query arrives
* Find necessary partition data
* Copy all the partition data into a RecordBatch
* Filter that RecordBatch to apply the projection
* Return results to caller
This is far from ideal, as the underlying partition data is copied in
its entirety and then the unneeded columns discarded - a pure waste!
After this PR, the projection is pushed down to the point of RecordBatch
generation:
* Query arrives
* Find necessary partition data
* Copy only the projected columns to a RecordBatch
* Return results to the caller
This minimises the amount of data copying, which for large amounts of
data should lead to a meaningful performance improvement when querying
for a subset of columns. It also uses a slightly more efficient
projection implementation by using a single pass over the columns (still
O(n) but less constant overhead).
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