APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS

Calculates the number of distinct non-NULL values from the synopsis objects created by APPROXIMATE_COUNT_DISTINCT_SYNOPSIS.

Behavior type

Immutable

Syntax

APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS ( synopsis-obj[, error-tolerance ] )

Parameters

synopsis-obj

A synopsis object created by APPROXIMATE_COUNT_DISTINCT_SYNOPSIS.

error-tolerance

Numeric value that represents the desired percentage of error tolerance, distributed around the value returned by this function. The smaller the error tolerance, the closer the approximation.

You can set error-tolerance to a minimum value of 0.88. Vertica imposes no maximum restriction, but any value greater than 5 is implemented with 5% error tolerance.

If you omit this argument, Vertica uses an error tolerance of 1.25(%).

For more details, see APPROXIMATE_COUNT_DISTINCT.

Restrictions

APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS and DISTINCT aggregates cannot be in the same query block.

Examples

The following examples review and compare different ways to obtain a count of unique values in a table column:

Return an exact count of unique values in column product_key, from table store.store_sales_fact:

=> \timing
Timing is on.
=> SELECT COUNT(DISTINCT product_key) from store.store_sales_fact;
 count
-------
 19982
(1 row)

Time: First fetch (1 row): 553.033 ms. All rows formatted: 553.075 ms

Return an approximate count of unique values in column product_key:

=> SELECT APPROXIMATE_COUNT_DISTINCT(product_key) as unique_product_keys
   FROM store.store_sales_fact;
 unique_product_keys
---------------------
               19921
(1 row)

Time: First fetch (1 row): 394.562 ms. All rows formatted: 394.600 ms

Create a synopsis object that represents a set of store.store_sales_fact data with unique product_key values, store the synopsis in the new table my_summary:


=> CREATE TABLE my_summary AS SELECT APPROXIMATE_COUNT_DISTINCT_SYNOPSIS (product_key) syn
   FROM store.store_sales_fact;
CREATE TABLE
Time: First fetch (0 rows): 582.662 ms. All rows formatted: 582.682 ms

Return a count from the saved synopsis:


=> SELECT APPROXIMATE_COUNT_DISTINCT_OF_SYNOPSIS(syn) FROM my_summary;
 ApproxCountDistinctOfSynopsis
-------------------------------
                         19921
(1 row)

Time: First fetch (1 row): 105.295 ms. All rows formatted: 105.335 ms