Examples - Amazon Redshift

Examples

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(customers) 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: Cache HyperLogLog sketches for cardinality estimation

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.

create external table spectrum.sales( sellerid integer, buyerid integer, dateid smallint, qtysold smallint, product_type integer, pricepaid decimal(8,2), commission decimal(8,2), saletime date) row format delimited fields terminated by '\t'stored as textfile location 's3://awssampledbuswest2/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;

The output looks similar to the following.

-- hll_sales -- | saletime | sketch | -- +----------------+---------------+ -- | 2018-11-23 | "CHAQkAQYA..." -- | 2018-11-24 | "TNLMLMLKK..." -- | 2018-11-25 | "KMNKLLOKM..." -- | 2018-11-26 | "MMKNKLLMO..." -- | 2018-11-27 | "MMLSKNLPM..." -- +----------------+---------------+

The following query extracts the estimated number of distinct buyers that purchased an item during the Friday after Thanksgiving.

SELECT hll_cardinality(sketch) as distinct_buyers FROM hll_sales WHERE saletime = '2018-11-23';

The output looks similar to the following.

distinct_buyers --------------- 1771

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 '2018-11-23' AND '2018-11-26';

The output looks similar to the following.

distinct_buyers --------------- 232152

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 saletime = to_char(now(), 'YYYY-MM-DD');