This change adds a zap PostingsIterator.nextBytes() method, which is
similar to Next(), but instead of returning a Posting instance,
nextBytes() returns the encoded freq/norm and location byte slices.
The zap merge code then provides those byte slices directly to the
intCoder's via a new method, intCoder.AddBytes(), thereby avoiding
having to encode many uvarint's.
Due to the usage rules of iterators, mem.PostingsIterator.Next() can
reuse its returned Postings instance.
Also, there's a micro optimization in persistDocValues() for one fewer
access to the docTermMap in the inner-loop.
This change allows the introducer to become the only goroutine to
modify the root, which in turn allows the introducer to greatly reduce
its root lock holding surface area.
There's now multiple competing merge activities (file-merging and
in-memory merging during persistence), so the simple math to
precalculate capacity for the slice of segments in introduceMerge() no
longer works for all cases and might have negative capacity.
This change removes that (sometimes wrong) precalculation, and instead
depends on append() to grow the slice correctly.
The optimizations / changes include...
- reuse of a memory buf when serializing varint's.
- reuse of a govarint.U64Base128Encoder instance, as it's a thin,
wrapper around an underlying chunkBuf, so Reset()'s on the
chunkBuf is enough for encoder reuse.
- chunkedIntcoder.Write() method was changed to invoke w.Write() less
often by forming a larger, reused buf. Profiling and analysis
showed w.Write() was getting called a lot, often with tiny 1 or 2
byte inputs. The theory is w.Write() and its underlying memmove()
can be more efficient when provided with larger bufs.
- some repeated code removal, by reusing the Close() method.
As part of this, zap.MergeToWriter() now returns more information --
enough so that callers can now create their own SegmentBase instances.
Also, the fieldsMap maintained and returned by zap.MergeToWriter() is
now a mapping from fieldName ==> fieldID+1 (instead of the previous
mapping from fieldName ==> fieldID). This makes it similar to how
fieldsMap are handled in other parts of zap to avoid "zero value"
issues.
