IndexWriter buffers up all deletes (by Term and Query) and only
applies them if 1) commit or NRT getReader() is called, or 2) a merge
is about to kickoff.
We do this because, for a large index, it's very costly to open a
SegmentReader for every segment in the index. So we defer as long as
we can. We do it just before merge so that the merge can eliminate
the deleted docs.
But, most merges are small, yet in a big index we apply deletes to all
of the segments, which is really very wasteful.
Instead, we should only apply the buffered deletes to the segments
that are about to be merged, and keep the buffer around for the
I think it's not so hard to do; we'd have to have generations of
pending deletions, because the newly merged segment doesn't need the
same buffered deletions applied again. So every time a merge kicks
off, we pinch off the current set of buffered deletions, open a new
set (the next generation), and record which segment was created as of
This should be a very sizable gain for large indices that mix
deletes, though, less so in flex since opening the terms index is much