COUNT (DISTINCT) and other DISTINCT aggregates
Computing a DISTINCT aggregate generally requires more work than other aggregates. Also, a query that uses a single DISTINCT aggregate consumes fewer resources than a query with multiple DISTINCT aggregates.
Tip
Vertica executes queries with multiple distinct aggregates more efficiently when all distinct aggregate columns have a similar number of distinct values.Examples
The following query returns the number of distinct values in a column:
=> SELECT COUNT (DISTINCT date_key) FROM date_dimension;
COUNT
-------
1826
(1 row)
This example returns the number of distinct return values from an expression:
=> SELECT COUNT (DISTINCT date_key + product_key) FROM inventory_fact;
COUNT
-------
21560
(1 row)
You can create an equivalent query using the LIMIT keyword to restrict the number of rows returned:
=> SELECT COUNT(date_key + product_key) FROM inventory_fact GROUP BY date_key LIMIT 10;
COUNT
-------
173
31
321
113
286
84
244
238
145
202
(10 rows)
The following query uses GROUP BY to count distinct values within groups:
=> SELECT product_key, COUNT (DISTINCT date_key) FROM inventory_fact
GROUP BY product_key LIMIT 10;
product_key | count
-------------+-------
1 | 12
2 | 18
3 | 13
4 | 17
5 | 11
6 | 14
7 | 13
8 | 17
9 | 15
10 | 12
(10 rows)
The following query returns the number of distinct products and the total inventory within each date key:
=> SELECT date_key, COUNT (DISTINCT product_key), SUM(qty_in_stock) FROM inventory_fact
GROUP BY date_key LIMIT 10;
date_key | count | sum
----------+-------+--------
1 | 173 | 88953
2 | 31 | 16315
3 | 318 | 156003
4 | 113 | 53341
5 | 285 | 148380
6 | 84 | 42421
7 | 241 | 119315
8 | 238 | 122380
9 | 142 | 70151
10 | 202 | 95274
(10 rows)
This query selects each distinct product_key value and then counts the number of distinct date_key values for all records with the specific product_key value. It also counts the number of distinct warehouse_key values in all records with the specific product_key value:
=> SELECT product_key, COUNT (DISTINCT date_key), COUNT (DISTINCT warehouse_key) FROM inventory_fact
GROUP BY product_key LIMIT 15;
product_key | count | count
-------------+-------+-------
1 | 12 | 12
2 | 18 | 18
3 | 13 | 12
4 | 17 | 18
5 | 11 | 9
6 | 14 | 13
7 | 13 | 13
8 | 17 | 15
9 | 15 | 14
10 | 12 | 12
11 | 11 | 11
12 | 13 | 12
13 | 9 | 7
14 | 13 | 13
15 | 18 | 17
(15 rows)
This query selects each distinct product_key value, counts the number of distinct date_key and warehouse_key values for all records with the specific product_key value, and then sums all qty_in_stock values in records with the specific product_key value. It then returns the number of product_version values in records with the specific product_key value:
=> SELECT product_key, COUNT (DISTINCT date_key),
COUNT (DISTINCT warehouse_key),
SUM (qty_in_stock),
COUNT (product_version)
FROM inventory_fact GROUP BY product_key LIMIT 15;
product_key | count | count | sum | count
-------------+-------+-------+-------+-------
1 | 12 | 12 | 5530 | 12
2 | 18 | 18 | 9605 | 18
3 | 13 | 12 | 8404 | 13
4 | 17 | 18 | 10006 | 18
5 | 11 | 9 | 4794 | 11
6 | 14 | 13 | 7359 | 14
7 | 13 | 13 | 7828 | 13
8 | 17 | 15 | 9074 | 17
9 | 15 | 14 | 7032 | 15
10 | 12 | 12 | 5359 | 12
11 | 11 | 11 | 6049 | 11
12 | 13 | 12 | 6075 | 13
13 | 9 | 7 | 3470 | 9
14 | 13 | 13 | 5125 | 13
15 | 18 | 17 | 9277 | 18
(15 rows)