Expand description
Selection vectors.
spec/07-execution.md section 7.1: a filter produces a u32 selection vector rather than
compacting. Compaction happens when a measured selectivity threshold is crossed and the
downstream operator is one that benefits, and the threshold is per operator and measured rather
than one global constant somebody picked.
The reason not to compact by default is that a filter over five columns which compacts has copied five columns to save the next operator a redirection. On a query that filters and then projects two of those columns, three of the copies were free work.
Structsยง
- Selection
- Which positions of a vector are still in play, as indices into it.