Argm

This PostgreSQL extension proivide several aggregate functions that could be used for SQL queries simplification and speedup.

Argmax and argmin

argmax(value, key_1, key_2, ...)
argmin(value, key_1, key_2, ...)

These functions pick the row with the highest/lowest keys combination within each group and return the corresponding values. Keys tuples are compared lexicographically, just like rows. Nulls are handled as unknown values. See PostgreSQL docs for more info on row-wise comparison. If there are several lines with the same keys the result line and value will be chosen arbitrarily.

Values could be of any PostgreSQL data type, while keys must be sortable. Return type is the same as value parameter type.

Logically using these functions with GROUP BY clause

SELECT argmax(value, key_1, key_2)
FROM some_table                   
GROUP BY gr                       

is equivalent to DISTINCT ON clause

SELECT DISTINCT ON (gr) value       
FROM some_table                     
ORDER BY gr,
         key_1 DESC,
         key_2 DESC 

but there are the following pros and cons:

• GROUP BY can use any grouping algorithm, including HashAgg. While DISTINCT ON needs the input to be sorted. This means in general argmax/argmin is faster.
• argmax/argmin can be used along with other aggregate functions using the same grouping clause. With DISTINCT ON one can select only values calculated on the row chosen.
• argmax/argmin can only order ascending or descending by all columns, use only one collation, nulls are always chosen last. DISTINCT ON can order using different directions, collations and nulls policies for different keys.

Anyold

anyold(value)

This function simply returns the first non-null value within the group. Any PostgreSQL types are supported. The function can be useful to write a value in a select list without grouping by it and without calling any essential aggregate function. That is, the following queries are equivalent if foo_details is determined by foo:

Grouping by both columns

SELECT foo,
       foo_details,
       sum(bar)
FROM some_table
GROUP BY foo,
         foo_details

This approach often leads to cardinality misestimations resulting in suboptimal execution plans. Additionally there is an overhead in hashing/sorting foo_details values while grouping by them.

Using min function

SELECT foo,
       min(foo_details),
       sum(bar)
FROM some_table
GROUP BY foo

This requires a proper min function for the data type of foo_details. Also calculating a minimum of the values is still an overhead despite all the values are equal.

Using anyold

SELECT foo,
       anyold(foo_details),
       sum(bar)
FROM some_table
GROUP BY foo

anyold function is faster than min

Installation

The extension is compatible with PostgreSQL 9.4 and higher. To install the extension for your database cluster run the following command:

make && sudo make install && make installcheck

This requires pg_config from your PostgreSQL installation to be available in $PATH

To use the extension on particular database run the following SQL:

CREATE EXTENSION argm;

Performance tests

To compare the performance of the approaches listed above of one could execute sql/perf-argmax.sql and sql/perf-anyold.sql files.