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an aggregation function for PostgreSQL
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README.md

Quantile aggregates

This extension provides three simple aggregate functions to compute quantiles (http://en.wikipedia.org/wiki/Quantile). There are two forms of aggregate functions available.

1) quantile(p_value numeric, p_quantile float)

Computes arbitrary quantile of the values - the quantile has to be between 0 and 1. For example this should return 500 just like the previous example

SELECT quantile(i, 0.5) FROM generate_series(1,1000) s(i);

but you can choose arbitrary quantile.

2) quantile(p_value numeric, p_quantiles float[])

If you need multiple quantiles at the same time (e.g. all four quartiles), you can use this function instead of the one described above. This version allows you to pass an array of quantiles and returns an array of values.

So if you need all three quartiles, you may do this

SELECT quantile(i, ARRAY[0.25, 0.5, 0.75])
 FROM generate_series(1,1000) s(i);

and it should return ARRAY[250, 500, 750]. Compared to calling the simple quantile function like this

SELECT quantile(i, 0.25), quantile(i, 0.5), quantile(i, 0.75)
 FROM generate_series(1,1000) s(i);

the advantage is that the values are collected just once (into a single array), not for each expression separately. If you're working with large data sets, this may save a lot of time and memory (so it's a significant advantage).

Just as in the first case, thre are three functions handling other basic numeric types, i.e. double, int (32-bit) and bigint (64-bit).

Installation

Installing this is very simple, especially if you're using pgxn client. All you need to do is this:

$ pgxn install quantile
$ pgxn load -d mydb quantile

and you're done. You may also install the extension manually:

$ make install
$ psql dbname -c "CREATE EXTENSION quantile"

And if you're on an older version (pre-9.1), you have to run the SQL script manually

$ psql dbname < `pg_config --sharedir`/contrib/quantile--1.1.sql

That's all.

License

This software is distributed under the terms of BSD 2-clause license. See LICENSE or http://www.opensource.org/licenses/bsd-license.php for more details.

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