Aggregate Functions

Aggregate functions operate on a set of values to compute a single result. Please refer to PostgreSQL for usage of standard SQL functions.

General

  • min

  • max

  • count

  • avg

  • sum

  • array_agg

Statistical

  • var / var_samp / var_pop

  • stddev / stddev_samp / stddev_pop

  • covar / covar_samp / covar_pop

  • corr

Approximate

approx_distinct

approx_distinct(x) -> uint64 returns the approximate number (HyperLogLog) of distinct input values

approx_median

approx_median(x) -> x returns the approximate median of input values. it is an alias of approx_percentile_cont(x, 0.5).

approx_percentile_cont

approx_percentile_cont(x, p) -> x return the approximate percentile (TDigest) of input values, where p is a float64 between 0 and 1 (inclusive).

It supports raw data as input and build Tdigest sketches during query time, and is approximately equal to approx_percentile_cont_with_weight(x, 1, p).

approx_percentile_cont_with_weight

approx_percentile_cont_with_weight(x, w, p) -> x returns the approximate percentile (TDigest) of input values with weight, where w is weight column expression and p is a float64 between 0 and 1 (inclusive).

It supports raw data as input or pre-aggregated TDigest sketches, then builds or merges Tdigest sketches during query time. TDigest sketches are a list of centroid (x, w), where x stands for mean and w stands for weight.

It is suitable for low latency OLAP system where a streaming compute engine (e.g. Spark Streaming/Flink) pre-aggregates data to a data store, then queries using Datafusion.