* feat(query): hyper log log counting in query engine
In addition to helping with normal queries, this can improve the 'SHOW CARDINALITY'
meta-queries:
time influx -database mydb -execute 'select count_hll(sum_hll(_seriesKey)) from big'
name: big
time count_hll
---- ---------
0 200767781
influx -database mydb -execute 0.06s user 0.12s system 0% cpu 8:49.99 total
This commit adds initial empty sketches back to the tsi1 index, as well
as ensuring that ephemeral sketches in the index `LogFile` are updated
accordingly.
The commit also adds a test that verifies that the merged sketches at
the store level produce the correct results under writes, deletions and
re-opening of the store.
This commit does not provide working sketches for post-compaction on the
tsi1 index.
The new algorithm uses only one formula and needs no additional bias corrections for the entire range of cardinalities,
therefore, it is more efficient and simpler to implement. Our simulations show that the accuracy provided by the new
algorithm is as good as or better than the accuracy provided by either of HyperLogLog or HyperLogLog++. The sparse
representation was kept in to provide better low cardinality accuracy. However the linear counting and range estimations
are replaced.
Sam Arnold
Ben Wells
Jacob Marble
Mark Rushakoff
Edd Robinson
Seif Lotfy
Ben Johnson
Stuart Carnie