Approximate Count Distinct Functions

The aggregate function COUNT(DISTINCT) computes the exact number of distinct values in a data set. COUNT(DISTINCT) performs well when it executes with the GROUPBY PIPELINED algorithm.

An aggregate COUNT operation performs well on a data set when the following conditions are true:

• One of the target table's projections has an ORDER BY clause that facilitates sorted aggregation.
• The number of distinct values is fairly small.
• Hashed aggregation is required to execute the query.

Alternatively, consider using the APPROXIMATE_COUNT_DISTINCT function instead of COUNT(DISTINCT) when the following conditions are true:

• You have a large data set and you do not require an exact count of distinct values.
• The performance of COUNT(DISTINCT) on a given data set is insufficient.
• You calculate several distinct counts in the same query.
• The plan for COUNT(DISTINCT) uses hashed aggregation.

The expected value that APPROXIMATE_COUNT_DISTINCT returns is equal to COUNT(DISTINCT), with an error that is lognormally distributed with standard deviation s. You can control the standard deviation by setting the function's optional error tolerance argument—by default, 1.25 percent.

Other APPROXIMATE_COUNT_DISTINCT Functions

Vertica supports two other functions that you can use together, instead of APPROXIMATE_COUNT_DISTINCT: APPROXIMATE_COUNT_DISTINCT_SYNOPSIS and APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS. Use these functions when the following conditions are true:

• You have a large data set and you don't require an exact count of distinct values.
• The performance of COUNT(DISTINCT) on a given data set is insufficient.
• You want to pre-compute the distinct counts and later combine them in different ways.

Use the two functions together as follows:

1. Pass APPROXIMATE_COUNT_DISTINCT_SYNOPSIS the data set and a normally distributed confidence interval. The function returns a subset of the data, as a binary synopsis object.
2. Pass the synopsis to the APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS function, which then performs an approximate count distinct on the synopsis.

You also use APPROXIMATE_COUNT_DISTINCT_SYNOPSIS_MERGE, which merges multiple synopses into one synopsis. With this function, you can continually update a "master" synopsis by merging in one or more synopses that cover more recent, shorter periods of time.

Example

The following example shows how to use APPROXIMATE_COUNT_DISTINCT functions to keep an approximate running count of users who click on a given web page within a given time span.

1. Create the pviews table to store data about website visits—time of visit, web page visited, and visitor:

   => CREATE TABLE pviews(
   visit_time TIMESTAMP NOT NULL,
   page_id INTEGER NOT NULL,
   user_id INTEGER NOT NULL)
   ORDER BY page_id, visit_time
   SEGMENTED BY HASH(user_id) ALL NODES KSAFE
   PARTITION BY visit_time::DATE GROUP BY CALENDAR_HIERARCHY_DAY(visit_time::DATE, 2, 2);

   pviews is segmented by hashing user_id data, so all visits by a given user are stored on the same segment, on the same node. This prevents inefficient cross-node transfer of data, when later we do a COUNT (DISTINCT user_id).

   The table also uses hierarchical partitioning on time of visit to optimize the ROS storage. Doing so improves performance when filtering data by time.

2. Load data into pviews:

   => INSERT INTO pviews VALUES
        ('2022-02-01 10:00:02',1002,1),
        ('2022-02-01 10:00:03',1002,2),
        ('2022-02-01 10:00:04',1002,1),
        ('2022-02-01 10:00:05',1002,3),
        ('2022-02-01 10:00:01',1000,1),
        ('2022-02-01 10:00:06',1002,1),
        ('2022-02-01 10:00:07',1002,3),
        ('2022-02-01 10:00:08',1002,1),
        ('2022-02-01 10:00:09',1002,3),
        ('2022-02-01 10:00:12',1002,2),
        ('2022-02-02 10:00:01',1000,1),
        ('2022-02-02 10:00:02',1002,4),
        ('2022-02-02 10:00:03',1002,2),
        ('2022-02-02 10:00:04',1002,1),
        ('2022-02-02 10:00:05',1002,3),
        ('2022-02-02 10:00:06',1002,4),
        ('2022-02-02 10:00:07',1002,3),
        ('2022-02-02 10:00:08',1002,4),
        ('2022-02-02 10:00:09',1002,3),
        ('2022-02-02 10:00:12',1002,2),
        ('2022-03-02 10:00:01',1000,1),
        ('2022-03-02 10:00:02',1002,1),
        ('2022-03-02 10:00:03',1002,2),
        ('2022-03-02 10:00:04',1002,1),
        ('2022-03-02 10:00:05',1002,3),
        ('2022-03-02 10:00:06',1002,4),
        ('2022-03-02 10:00:07',1002,3),
        ('2022-03-02 10:00:08',1002,6),
        ('2022-03-02 10:00:09',1002,5),
        ('2022-03-02 10:00:12',1002,2),
        ('2022-03-02 11:00:01',1000,5),
        ('2022-03-02 11:00:02',1002,6),
        ('2022-03-02 11:00:03',1002,7),
        ('2022-03-02 11:00:04',1002,4),
        ('2022-03-02 11:00:05',1002,1),
        ('2022-03-02 11:00:06',1002,6),
        ('2022-03-02 11:00:07',1002,8),
        ('2022-03-02 11:00:08',1002,6),
        ('2022-03-02 11:00:09',1002,7),
        ('2022-03-02 11:00:12',1002,1),
        ('2022-03-03 10:00:01',1000,1),
        ('2022-03-03 10:00:02',1002,2),
        ('2022-03-03 10:00:03',1002,4),
        ('2022-03-03 10:00:04',1002,1),
        ('2022-03-03 10:00:05',1002,2),
        ('2022-03-03 10:00:06',1002,6),
        ('2022-03-03 10:00:07',1002,9),
        ('2022-03-03 10:00:08',1002,10),
        ('2022-03-03 10:00:09',1002,7),
        ('2022-03-03 10:00:12',1002,1);
    OUTPUT
   --------
        50
   (1 row)
   
   => COMMIT;
   COMMIT

3. Create the pview_summary table by querying pviews with CREATE TABLE...AS SELECT. Each row of this table summarizes data selected from pviews for a given date:

   • partial_visit_count stores the number of rows (website visits) in pviews with that date.
   • daily_users_acdp uses APPROXIMATE_COUNT_DISTINCT_SYNOPSIS to construct a synopsis that approximates the number of distinct users (user_id) who visited that website on that date.

   => CREATE TABLE pview_summary AS SELECT
         visit_time::DATE "date",
         COUNT(*) partial_visit_count,
         APPROXIMATE_COUNT_DISTINCT_SYNOPSIS(user_id) AS daily_users_acdp
      FROM pviews GROUP BY 1;
   CREATE TABLE
   => ALTER TABLE pview_summary ALTER COLUMN "date" SET NOT NULL;

4. Update the pview_summary table so it is partitioned like pviews. The REORGANIZE keyword forces immediate repartitioning of the table data:

   => ALTER TABLE pview_summary
        PARTITION BY "date"
        GROUP BY CALENDAR_HIERARCHY_DAY("date", 2, 2) REORGANIZE;
   vsql:/home/ale/acd_ex4.sql:93: NOTICE 8364:  The new partitioning scheme will produce partitions in 2 physical storage containers per projection
   vsql:/home/ale/acd_ex4.sql:93: NOTICE 4785:  Started background repartition table task
   ALTER TABLE

5. Use CREATE TABLE..LIKE to create two ETL tables, pviews_etl and pview_summary_etl with the same DDL as pviews and pview_summary, respectively. These tables serve to process incoming data:

   => CREATE TABLE pviews_etl LIKE pviews INCLUDING PROJECTIONS;
   CREATE TABLE
   => CREATE TABLE pview_summary_etl LIKE pview_summary INCLUDING PROJECTIONS;
   CREATE TABLE

6. Load new data into pviews_etl:

   => INSERT INTO pviews_etl VALUES
        ('2022-03-03 11:00:01',1000,8),
        ('2022-03-03 11:00:02',1002,9),
        ('2022-03-03 11:00:03',1002,1),
        ('2022-03-03 11:00:04',1002,11),
        ('2022-03-03 11:00:05',1002,10),
        ('2022-03-03 11:00:06',1002,12),
        ('2022-03-03 11:00:07',1002,3),
        ('2022-03-03 11:00:08',1002,10),
        ('2022-03-03 11:00:09',1002,1),
        ('2022-03-03 11:00:12',1002,1);
    OUTPUT
   --------
        10
   (1 row)
   
   => COMMIT;
   COMMIT

7. Summarize the new data in pview_summary_etl:

   => INSERT INTO pview_summary_etl SELECT
         visit_time::DATE visit_date,
         COUNT(*) partial_visit_count,
         APPROXIMATE_COUNT_DISTINCT_SYNOPSIS(user_id) AS daily_users_acdp
       FROM pviews_etl GROUP BY visit_date;
    OUTPUT 
   --------
         1
   (1 row)
   

8. Append the pviews_etl data to pviews with COPY_PARTITIONS_TO_TABLE:

   => SELECT COPY_PARTITIONS_TO_TABLE('pviews_etl', '01-01-0000'::DATE, '01-01-9999'::DATE, 'pviews');
                 COPY_PARTITIONS_TO_TABLE
   ----------------------------------------------------
    1 distinct partition values copied at epoch 1403.
   
   (1 row)
   
   => SELECT COPY_PARTITIONS_TO_TABLE('pview_summary_etl', '01-01-0000'::DATE, '01-01-9999'::DATE, 'pview_summary');
                 COPY_PARTITIONS_TO_TABLE
   ----------------------------------------------------
    1 distinct partition values copied at epoch 1404.
   
   (1 row)
   

9. Create views and distinct (approximate) views by day for all data, including the partition that was just copied from pviews_etl :

   => SELECT
        "date" visit_date,
        SUM(partial_visit_count) visit_count,
        APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS(daily_users_acdp) AS daily_users_acd  
      FROM pview_summary GROUP BY visit_date ORDER BY visit_date;
    visit_date | visit_count | daily_users_acd
   ------------+-------------+-----------------
    2022-02-01 |          10 |               3
    2022-02-02 |          10 |               4
    2022-03-02 |          20 |               8
    2022-03-03 |          20 |              11
   (4 rows)
   

10. Create views and distinct (approximate) views by month:

    => SELECT
         DATE_TRUNC('MONTH', "date")::DATE "month",
         SUM(partial_visit_count) visit_count,
         APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS(daily_users_acdp) AS monthly_users_acd
       FROM pview_summary GROUP BY month ORDER BY month;
       month    | visit_count | monthly_users_acd
    ------------+-------------+-------------------
     2022-02-01 |          20 |                 4
     2022-03-01 |          40 |                12
    (2 rows)
    

11. Merge daily synopses into monthly synopses:

    => CREATE TABLE pview_monthly_summary AS SELECT
         DATE_TRUNC('MONTH', "date")::DATE "month",
         SUM(partial_visit_count) partial_visit_count,
         APPROXIMATE_COUNT_DISTINCT_SYNOPSIS_MERGE(daily_users_acdp) AS monthly_users_acdp
       FROM pview_summary GROUP BY month ORDER BY month;
    CREATE TABLE

12. Create views and distinct views by month, generated from the merged synopses:

    => SELECT
         month,
         SUM(partial_visit_count) monthly_visit_count,
         APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS(monthly_users_acdp) AS monthly_users_acd
       FROM pview_monthly_summary GROUP BY month ORDER BY month;
       month    | monthly_visit_count | monthly_users_acd 
    ------------+---------------------+-------------------
     2019-02-01 |                  20 |                 4
     2019-03-01 |                  40 |                12
    (2 rows)

13. You can use the monthly summary to produce a yearly summary. This approach is likely to be faster than using a daily summary if a lot of data needs to be processed:

    => SELECT
         DATE_TRUNC('YEAR', "month")::DATE "year",
         SUM(partial_visit_count) yearly_visit_count,
         APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS(monthly_users_acdp) AS yearly_users_acd
       FROM pview_monthly_summary GROUP BY year ORDER BY year;
        year    | yearly_visit_count | yearly_users_acd
    ------------+--------------------+------------------
     2022-01-01 |                 60 |               12
    (1 row)
    

14. Drop the ETL tables:

    => DROP TABLE IF EXISTS pviews_etl, pview_summary_etl;
    DROP TABLE

See Also

• APPROXIMATE_COUNT_DISTINCT_SYNOPSIS
• APPROXIMATE_COUNT_DISTINCT_SYNOPSIS
• APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS
• APPROXIMATE_COUNT_DISTINCT_SYNOPSIS_MERGE
• COUNT [Aggregate]