7.21. Quantile Digest Functions
Presto implements the approx_percentile
function with the quantile digest
data structure. The underlying data structure, qdigest,
is exposed as a data type in Presto, and can be created, queried and stored
separately from approx_percentile
.
Data Structures
A quantile digest is a data sketch which stores approximate percentile
information. The presto type for this data structure is called qdigest
,
and it takes a parameter which must be one of bigint
, double
or
real
which represent the set of numbers that may be ingested by the
qdigest
. They may be merged without losing precision, and for storage
and retrieval they may be cast to/from VARBINARY
.
Functions

merge
(qdigest) → qdigest Merges all input
qdigest
s into a singleqdigest
.

value_at_quantile
(qdigest(T), quantile) → T Returns the approximate percentile values from the quantile digest given the number
quantile
between 0 and 1.

values_at_quantiles
(qdigest(T), quantiles) → T Returns the approximate percentile values as an array given the input quantile digest and array of values between 0 and 1 which represent the quantiles to return.

qdigest_agg
(x) → qdigest<[same as x]> Returns the
qdigest
which is composed of all input values ofx
.

qdigest_agg
(x, w) → qdigest<[same as x]> Returns the
qdigest
which is composed of all input values ofx
using the peritem weightw
.

qdigest_agg
(x, w, accuracy) → qdigest<[same as x]> Returns the
qdigest
which is composed of all input values ofx
using the peritem weightw
and maximum error ofaccuracy
.accuracy
must be a value greater than zero and less than one, and it must be constant for all input rows.