Apache DataSketches (https://datasketches.apache.org/) is integrated into Hive via HIVE-22939.
This enables various kind of sketch operations thru regular sql statement.

Sketch functions

Naming convention

All sketch functions are registered using the following naming convention:

ds_{sketchType}_{functionName}

For example we have a function called: ds_hll_estimate which could be used to estimate the distinct values from an hll sketch.

sketchType

For detailed info about the sketches themself please refer to the datasketches site!

• frequency
  • hll
  • cpc
  • theta
• frequent items
  • freq
• histograms
  • kll

functionName

List declared sketch functions

Given that we have ~60 functions registered I would recommend to also consider listing/getting info about a single udf.

You could list all functions prefixed by ds_ using:

show functions like 'ds_%';

And you can access the description of a function like:

desc function ds_freq_sketch;

Integration with materialized views

Sketch aggregation(s) are exposed to Calcite by some extensions - which could enable both the usage of an MV in a smaller dimension query; or could help in incremental updates.

BI mode

Usage of sketches can give a performance boost in case we could afford to loose some accuracy. Which could come very handy in case of charts or live dashboards.
The BI mode is about making rewrites automatically to sketch functions if possible.

The BI mode can be enabled using:

set hive.optimize.bi.enabled=true;

Rewrite COUNT(DISTINCT(X))

This feature can be toggled using the hive.optimize.bi.rewrite.countdistinct.enabled conf key

The used distinct sketch family can be configured using: hive.optimize.bi.rewrite.countdistinct.sketch (currently only hll is available).

This feature could rewrite

select category, count(distinct id) from sketch_input group by category

to use a distinct count sketch to answer the query by rewriting it to

select category, round(ds_hll_estimate(ds_hll_sketch(id))) from sketch_input

Rewrite percentile_disc(p) withing group(order by x)

This feature can be toggled using the hive.optimize.bi.rewrite.percentile_disc.enabled conf key

The used histogram sketch family can be configured using: hive.optimize.bi.rewrite.percentile_disc.sketch (currently only kll is available).

This feature could rewrite

select percentile_disc(0.3) within group(order by id) from sketch_input

to use a histogram sketch to answer the query by rewriting to

select ds_kll_quantile(ds_kll_sketch(id), 0.3) from sketch_input

Rewrite cume_dist() over (order by id)

This feature can be toggled using the hive.optimize.bi.rewrite.cume_dist.enabled conf key

The used histogram sketch family can be configured using: hive.optimize.bi.rewrite.cume_dist.sketch (currently only kll is available).

select id,cume_dist() over (order by id) from sketch_input

to use a histogram sketch to answer the query by rewriting to

SELECT id, CAST(DS_KLL_RANK(t2.sketch, idVal) AS DOUBLE) 
FROM (SELECT id, CAST(COALESCE(CAST(id AS FLOAT), 340282346638528860000000000000000000000) AS FLOAT) AS idVal FROM sketch_input) AS t,
(SELECT DS_KLL_SKETCH(CAST(`id` AS FLOAT)) AS `sketch` FROM sketch_input) AS t2

Rewrite NTILE

This feature can be toggled using the hive.optimize.bi.rewrite.ntile.enabled conf key

The used histogram sketch family can be configured using: hive.optimize.bi.rewrite.ntile.sketch (currently only kll is available).

This feature can rewrite

select id,
       ntile(4) over (order by id
from sketch_input
order by id

To use a histogram sketch to calculate the NTILE's value:

select id,
        case when ceil(ds_kll_cdf(ds, CAST(id AS FLOAT) )[0]*4) < 1 then 1 else ceil(ds_kll_cdf(ds, CAST(id AS FLOAT) )[0]*4) end
from sketch_input
join ( select ds_kll_sketch(cast(id as float)) as ds from sketch_input ) q
order by id

select id,
                rank() over (order by id),
                case when ds_kll_n(ds) < (ceil(ds_kll_rank(ds, CAST(id AS FLOAT) )*ds_kll_n(ds))+1) then ds_kll_n(ds) else (ceil(ds_kll_rank(ds, CAST(id AS FLOAT) )*ds_kll_n(ds))+1) end

Rewrite RANK

This feature can be toggled using the hive.optimize.bi.rewrite.rank.enabled conf key

The used histogram sketch family can be configured using: hive.optimize.bi.rewrite.rank.sketch (currently only kll is available).

select id,
       rank() over (order by id)
from sketch_input
order by id

is rewritten to

select id,
       case when ds_kll_n(ds) < (ceil(ds_kll_rank(ds, CAST(id AS FLOAT) )*ds_kll_n(ds))+1) then ds_kll_n(ds) else (ceil(ds_kll_rank(ds, CAST(id AS FLOAT) )*ds_kll_n(ds))+1) end
from sketch_input
join ( select ds_kll_sketch(cast(id as float)) as ds from sketch_input ) q
order by id

Examples

Simple distinct counting examples using HLL

• Prepare sample table

  create table sketch_input (id int, category char(1))
  STORED AS ORC
  TBLPROPERTIES ('transactional'='true');
  
  insert into table sketch_input values
    (1,'a'),(1, 'a'), (2, 'a'), (3, 'a'), (4, 'a'), (5, 'a'), (6, 'a'), (7, 'a'), (8, 'a'), (9, 'a'), (10, 'a'),
    (6,'b'),(6, 'b'), (7, 'b'), (8, 'b'), (9, 'b'), (10, 'b'), (11, 'b'), (12, 'b'), (13, 'b'), (14, 'b'), (15, 'b')
  ; 

• Use HLL to compute distinct values using an intermediate table

  -- build sketches per category
  create temporary table sketch_intermediate (category char(1), sketch binary);
  insert into sketch_intermediate select category, ds_hll_sketch(id) from sketch_input group by category;
  
  -- get unique count estimates per category
  select category, ds_hll_estimate(sketch) from sketch_intermediate;
  
  -- union sketches across categories and get overall unique count estimate
  select ds_hll_estimate(ds_hll_union(sketch)) from sketch_intermediate;

• Use HLL to compute distinct values without intermediate table

  select category, ds_hll_estimate(ds_hll_sketch(id)) from sketch_input group by category;
  select ds_hll_estimate(ds_hll_sketch(id)) from sketch_input;

• Use HLL to compute distinct values transparently thru BI mode

  set hive.optimize.bi.enabled=true;
  select category,count(distinct id) from sketch_input group by category;
  select count(distinct id) from sketch_input;

• Use HLL to compute distinct values transparently thru BI mode - while utilizing a Materialized View to store the intermediate sketches.

  -- create an MV to store precomputed HLL values
  create  materialized view mv_1 as
    select category, ds_hll_sketch(id) from sketch_input group by category;
  
  set hive.optimize.bi.enabled=true;
  select category,count(distinct id) from sketch_input group by category;
  select count(distinct id) from sketch_input;