Examples
This section contains examples for using HyperLogLog with Amazon Redshift.
Topics
Example: Return cardinality in a subquery
The following example returns the cardinality for each sketch in a subquery for a table named Sales.
CREATE TABLE Sales (customer VARCHAR, country VARCHAR, amount BIGINT); INSERT INTO Sales VALUES ('David Joe', 'Greece', 14.5), ('David Joe', 'Greece', 19.95), ('John Doe', 'USA', 29.95), ('John Doe', 'USA', 19.95), ('George Spanos', 'Greece', 9.95), ('George Spanos', 'Greece', 2.95);
The following query generates an HLL sketch for the customers of each country and extracts the cardinality. This shows unique customers from each country.
SELECT hll_cardinality(sketch), country FROM (SELECT hll_create_sketch(customer) AS sketch, country FROM Sales GROUP BY country) AS hll_subquery; hll_cardinality | country ----------------+--------- 1 | USA 2 | Greece ...
Example: Return an HLLSKETCH type from combined sketches in a subquery
The following example returns a single HLLSKETCH type that represents the combination of individual sketches from a subquery. The sketches are combined by using the HLL_COMBINE aggregate function.
SELECT hll_combine(sketch) FROM (SELECT hll_create_sketch(customer) AS sketch FROM Sales GROUP BY country) AS hll_subquery hll_combine -------------------------------------------------------------------------------------------- {"version":1,"logm":15,"sparse":{"indices":[29808639,35021072,47612452],"values":[1,1,1]}} (1 row)
Example: Return a HyperLogLog sketch from combining multiple sketches
For the following example, suppose that the table page-users
stores
preaggregated sketches for each page that users visited on a given website. Each row
in this table contains a HyperLogLog sketch that represents all user IDs that show
the visited pages.
page_users -- +----------------+-------------+--------------+ -- | _PARTITIONTIME | page | sketch | -- +----------------+-------------+--------------+ -- | 2019-07-28 | homepage | CHAQkAQYA... | -- | 2019-07-28 | Product A | CHAQxPnYB... | -- +----------------+-------------+--------------+
The following example unions the preaggregated multiple sketches and generates a single sketch. This sketch encapsulates the collective cardinality that each sketch encapsulates.
SELECT hll_combine(sketch) as sketch FROM page_users
The output looks similar to the following.
-- +-----------------------------------------+ -- | sketch | -- +-----------------------------------------+ -- | CHAQ3sGoCxgCIAuCB4iAIBgTIBgqgIAgAwY.... | -- +-----------------------------------------+
When a new sketch is created, you can use the HLL_CARDINALITY function to get the collective distinct values, as shown following.
SELECT hll_cardinality(sketch) FROM ( SELECT hll_combine(sketch) as sketch FROM page_users ) AS hll_subquery
The output looks similar to the following.
-- +-------+ -- | count | -- +-------+ -- | 54356 | -- +-------+
Example: Generate HyperLogLog sketches over S3 data using external tables
The following examples cache HyperLogLog sketches to avoid directly accessing Amazon S3 for cardinality estimation.
You can preaggregate and cache HyperLogLog sketches in external tables defined to hold Amazon S3 data. By doing this, you can extract cardinality estimates without accessing the underlying base data.
For example, suppose that you have unloaded a set of tab-delimited text files into
Amazon S3. You run the following query to define an external table named
sales
in the Amazon Redshift external schema named spectrum
.
The Amazon S3 bucket for this example is in the US East (N. Virginia) Amazon Web Services Region.
create external table spectrum.sales( salesid integer, listid integer, sellerid smallint, buyerid smallint, eventid integer, dateid integer, qtysold integer, pricepaid decimal(8,2), commission decimal(8,2), saletime timestamp) row format delimited fields terminated by '\t' stored as textfile location 's3://redshift-downloads/tickit/spectrum/sales/';
Suppose that you want to compute the distinct buyers who purchased an item on
arbitrary dates. To do so, the following example generates sketches for the buyer IDs
for each day of the year and stores the result in the Amazon Redshift table
hll_sales
.
CREATE TABLE hll_sales AS SELECT saletime, hll_create_sketch(buyerid) AS sketch FROM spectrum.sales GROUP BY saletime; SELECT TOP 5 * FROM hll_sales;
The output looks similar to the following.
-- hll_sales -- | saletime | sketch | -- +-----------------+---------------------------------------------------------------------+ -- | 7/22/2008 8:30 | {"version":1,"logm":15,"sparse":{"indices":[9281416],"values":[1]}} -- | 2/19/2008 0:38 | {"version":1,"logm":15,"sparse":{"indices":[48735497],"values":[3]}} -- | 11/5/2008 4:49 | {"version":1,"logm":15,"sparse":{"indices":[27858661],"values":[1]}} -- | 10/27/2008 4:08 | {"version":1,"logm":15,"sparse":{"indices":[65295430],"values":[2]}} -- | 2/16/2008 9:37 | {"version":1,"logm":15,"sparse":{"indices":[56869618],"values":[2]}} -- +---------------- +---------------------------------------------------------------------+
The following query shows the estimated number of distinct buyers that purchased an item during the Friday after Thanksgiving in 2008.
SELECT hll_cardinality(hll_combine(sketch)) as distinct_buyers FROM hll_sales WHERE trunc(saletime) = '2008-11-28';
The output looks similar to the following.
distinct_buyers --------------- 386
Suppose that you want the number of distinct users who bought an item on a certain
range of dates. An example might be from the Friday after Thanksgiving to the following
Monday. To get this, the following query uses the hll_combine
aggregate
function. This function enables you to avoid double-counting buyers who purchased an
item on more than one day of the selected range.
SELECT hll_cardinality(hll_combine(sketch)) as distinct_buyers FROM hll_sales WHERE saletime BETWEEN '2008-11-28' AND '2008-12-01';
The output looks similar to the following.
distinct_buyers --------------- 1166
To keep the hll_sales
table up-to-date, run the following query at the
end of each day. Doing this generates an HyperLogLog sketch based on the IDs of
buyers that purchased an item today and adds it to the hll_sales
table.
INSERT INTO hll_sales SELECT saletime, hll_create_sketch(buyerid) FROM spectrum.sales WHERE TRUNC(saletime) = to_char(GETDATE(), 'YYYY-MM-DD') GROUP BY saletime;