Major - P3 T-digest's accuracy vs performance can be tuned using the compaction factor, size of the merge buffer and details of the scaling function. We are not planning to expose any of these parameters to the customers but instead will tune them to achieve a general "sweet" spot. However, we might in the end introduce dial knobs for this, if helpful for testing or no definitive sweet spot exists.
Micro-benchmarks from the initial impl of t-digest (the non-expr tests using 1e6 inputs and expr tests using 100 inputs, both with normal distribution)
------------------------------------------------------------------------------------------------------ Benchmark Time CPU Iterations ------------------------------------------------------------------------------------------------------ PercentileAlgoBenchmarkFixture/tdigest_k0_delta1000 627636909 ns 627617190 ns 1 PercentileAlgoBenchmarkFixture/tdigest_k1_delta1000 682104826 ns 682094139 ns 1 PercentileAlgoBenchmarkFixture/tdigest_k2_delta500 643034220 ns 643014710 ns 1 PercentileAlgoBenchmarkFixture/tdigest_k2_delta1000 646875381 ns 646855993 ns 1 PercentileAlgoBenchmarkFixture/tdigest_k2_delta5000 742438555 ns 742403578 ns 1 PercentileAlgoBenchmarkFixture/tdigest_k2_delta1000_sorted 167359114 ns 167354307 ns 4 PercentileAlgoBenchmarkFixture/tdigest_k2_delta1000_batched 638900042 ns 638880589 ns 1 PercentileAlgoBenchmarkFixture/tdigest_expr_99_100 1684 ns 1684 ns 414402 PercentileAlgoBenchmarkFixture/tdigest_expr_01_100 1507 ns 1507 ns 464590 PercentileAlgoBenchmarkFixture/tdigest_expr_01_1000 33418 ns 33416 ns 20924 PercentileAlgoBenchmarkFixture/sortAndRank_expr_100 632 ns 632 ns 1109422 PercentileAlgoBenchmarkFixture/sortAndRank_expr_1000 22419 ns 22418 ns 31093
Note: $group with $avg and null group key on a collection with 1e7 small documents, takes ~4500 msec in SBE and ~7200 msec in classic. So even major differences in runtimes of t-digest itself are unlikely to affect the query latency in a significant way. However, for the expressions it might make sense not to use t-digest at all.