This simplifies some of the iterators by loading chunks from the
ChunkReader earlier, filtering of chunks vs filtering or series is
split into separate iterators for easier testing
Introduce a seperate mutex for the head blocks to avoid a race where
a post-compaction reload may run between switching the DB's base mutex
to create a new head block in an appender.
This adds write path support for segmented chunk data files.
Files of 512MB are pre-allocated and written to. If the file size
is exceeded, the next file is started. On completion, files
are truncated to their final size.
This adds a 4 sample buffer to every head chunk. The XOR
compression scheme may edit bytes in place. The minimum size
of a sample is 2 bits. So keeping the last 4 samples in an in-memory
buffer makes it safe to query the preceeding ones while samples
are added
This adds a position mapper that takes series from a head block
in the order they were appended and creates a mapping representing
them in order of their label sets.
Write-repair of the postings list would cause very expensive writing.
Hence, we keep them as they are and only apply the postition mapping
at the very end, after a postings list has been sufficienctly reduced
through intersections etc.
This changes the IndexReader API to expose plain labels
and chunk meta information instead of a Series interface.
Dropping of irrelevant chunks is moved into the querier.
A LabelIndices method is added to query for existing label
value indices.
This adds interval metadata to indexed chunks. The queried interval
is used to filter chunks when queried from the index to save
unnecessary accesses of the chunks file.
This is especially relevant for series that come and go often and larger
files.
This adds a proper duration based lookback buffer for series iterators
to allow advancing sequentially while remaining able to calculate time
aggregating functions such as `rate` backwards.
It uses an array ring buffer to minimize heap allocations for
potentially hundreds of thousands of series for a single query.
