PostgreSQL 9.4 is going to be shipped with a feature called ordered-set aggregates. This can be used with a new clause called WITHIN GROUP. All those things have been introduced by the following commit.
commit 8d65da1f01c6a4c84fe9c59aeb6b7e3adf870145 Author: Tom Lane <email@example.com> Date: Mon Dec 23 16:11:35 2013 -0500 Support ordered-set (WITHIN GROUP) aggregates. This patch introduces generic support for ordered-set and hypothetical-set aggregate functions, as well as implementations of the instances defined in SQL:2008 (percentile_cont(), percentile_disc(), rank(), dense_rank(), percent_rank(), cume_dist()). We also added mode() though it is not in the spec, as well as versions of percentile_cont() and percentile_disc() that can compute multiple percentile values in one pass over the data. Unlike the original submission, this patch puts full control of the sorting process in the hands of the aggregate's support functions. To allow the support functions to find out how they're supposed to sort, a new API function AggGetAggref() is added to nodeAgg.c. This allows retrieval of the aggregate call's Aggref node, which may have other uses beyond the immediate need. There is also support for ordered-set aggregates to install cleanup callback functions, so that they can be sure that infrastructure such as tuplesort objects gets cleaned up. In passing, make some fixes in the recently-added support for variadic aggregates, and make some editorial adjustments in the recent FILTER additions for aggregates. Also, simplify use of IsBinaryCoercible() by allowing it to succeed whenever the target type is ANY or ANYELEMENT. It was inconsistent that it dealt with other polymorphic target types but not these. Atri Sharma and Andrew Gierth; reviewed by Pavel Stehule and Vik Fearing, and rather heavily editorialized upon by Tom Lane
To put it in simple words, this feature allows doing operations on a group of rows organized with ORDER BY thanks to the clause WITHIN GROUP.
As mentionned in the commit message, this is part of the SQL specification of 2008, with some additions like the function mode().
Before having that, doing such operations was not that straight-forward as you would have needed for example a CTE coupled with a window function to do it, needing to first order the entire set, and then do some post-processing on what is wanted. Here is for example how to get a percentile (meaning that for a subset of rows already ordered with a certain ORDER BY condition you would fetch back from the subset the value which could be used to recover only a given portions of rows of this subset, given by a percentage). So… For the example… Here is how to get the 25th, 50th, 75th and 100th percentile of a simple table with 20 rows.
=# CREATE TABLE aa AS SELECT generate_series(1,20) AS a; SELECT 20 =# WITH subset AS ( SELECT a AS val, ntile(4) OVER (ORDER BY a) AS tile FROM aa ) SELECT tile, max(val) FROM subset GROUP BY tile ORDER BY tile; tile | max ------+----- 1 | 5 2 | 10 3 | 15 4 | 20 (4 rows)
The idea here is simply to divide the set of rows into buckets using ntile and then to fetch back the maximum value of each tile corresponding to the percentile wanted. For a larger table you might want to simply use ntile with 100, this is not really complicated, but not that straight-forward either.
And now here is how to do the same thing with Postgres 9.4 using WITHIN GROUP for a single percentile value:
=# SELECT percentile_disc(0.25) WITHIN GROUP (ORDER BY a) as max FROM aa; max ----- 5 (1 row)
And for an array of percentiles:
=# SELECT unnest(percentile_disc(array[0.25,0.5,0.75,1]) WITHIN GROUP (ORDER BY a)) FROM aa; unnest -------- 5 10 15 20 (4 rows)
This is way simpler.
Note that not all the aggregate functions can be used with WITHIN CLAUSE, you can identify them by looking at pg_aggregate with the field aggkind that has been added to track that, with the following types of aggregates defined:
- ‘n’ for normal aggregates, like max, min, etc.
- ‘o’ for the ordered-set aggregates
- ‘h’ for the hypothetical-set aggregates, which are a subclass of set-ordered aggregates
And here is their complete list:
=# SELECT aggfnoid, aggkind FROM pg_aggregate WHERE aggkind IN ('o', 'h'); aggfnoid | aggkind ----------------------------+--------- pg_catalog.percentile_disc | o pg_catalog.percentile_cont | o pg_catalog.percentile_cont | o pg_catalog.percentile_disc | o pg_catalog.percentile_cont | o pg_catalog.percentile_cont | o mode | o pg_catalog.rank | h pg_catalog.percent_rank | h pg_catalog.cume_dist | h pg_catalog.dense_rank | h (11 rows)
Continuing with the percentile aggregates, it is important to understand the difference between percentile_cont and percentile_dist.
- percentile_disc returns an exact value, being the first value whose position in the ordering equals or exceeds the specified fraction of portion in the subset
- percentile_cont returns the value corresponding to the fraction specified, interpolating with adjacent values if necessary.
Some with the previous examples, here is the result obtained for the 25th percentile for both percentile_disc and percentile_cont.
=# SELECT percentile_disc(0.25) WITHIN GROUP (ORDER BY a) as inter_max, percentile_cont(0.25) WITHIN GROUP (ORDER BY a) as abs_max FROM aa; inter_max | abs_max -----------+--------- 5 | 5.75 (1 row)
Now let’s have a look at mode(), which chooses the most-frequent present value in the subset. If multiple values are present equal times, the first one in the subset is selected.
=# SELECT mode() WITHIN GROUP (ORDER BY a) AS most_frequent FROM aa; most_frequent --------------- 1 (1 row)
All the values are present only once in this case, but if we add a new value in the table between 1 and 20 that will make it the most-frequently present one, here is what we get:
=# INSERT INTO aa VALUES (8); INSERT 0 1 =# SELECT mode() WITHIN GROUP (ORDER BY a) AS most_frequent FROM aa; most_frequent --------------- 8 (1 row)
And now a couple of words about the hypothetical-set aggregates that are rank, percent_rank, cume_dist and dense_rank. rank can be used to fetch the rank (Oh surprise!) of a given value in a subset with gaps for duplicated values. With the previous example of table aa using 20 rows with one value, actually “8” present twice, here is what you get.
=# SELECT rank(8) WITHIN GROUP (ORDER BY a) FROM aa; rank ------ 8 (1 row) =# SELECT rank(9) WITHIN GROUP (ORDER BY a) FROM aa; rank ------ 10 (1 row)
Value “8” is ranked as 8, while value “9” is ranked as “10”. Note as well values not present in the subset can be used, their rank is adapted depending on the other values. Here by for example removing “7”.
=# delete from aa where a = 7; DELETE 1 =# SELECT rank(7) WITHIN GROUP (ORDER BY a) FROM aa; rank ------ 7 (1 row) =# SELECT rank(8) WITHIN GROUP (ORDER BY a) FROM aa; rank ------ 7 (1 row) =# SELECT rank(6) WITHIN GROUP (ORDER BY a) FROM aa; rank ------ 6 (1 row)
dense_rank works similarly, except that it uses no gaps. Finally percent_rank and cume_dist work similarly. They can be used to get the relative rank of a given value in the subset, the main difference between those two functions being that percent_rank ranges from 0 to 1, while cume_dist ranges from 1/N to 1.
=# SELECT cume_dist(1) WITHIN GROUP (ORDER BY a) as val_1, cume_dist(7) WITHIN GROUP (ORDER BY a) as val_7, cume_dist(8) WITHIN GROUP (ORDER BY a) as val_8, cume_dist(9) WITHIN GROUP (ORDER BY a) as val_9 FROM aa; val_1 | val_7 | val_8 | val_9 --------------------+-------------------+-------------------+------- 0.0909090909090909 | 0.363636363636364 | 0.454545454545455 | 0.5 (1 row) =# SELECT percent_rank(1) WITHIN GROUP (ORDER BY a) as val_1, percent_rank(7) WITHIN GROUP (ORDER BY a) as val_7, percent_rank(8) WITHIN GROUP (ORDER BY a) as val_8, percent_rank(9) WITHIN GROUP (ORDER BY a) as val_9 FROM aa; val_1 | val_7 | val_8 | val_9 -------+-------------------+-------------------+------------------- 0 | 0.285714285714286 | 0.333333333333333 | 0.428571428571429 (1 row)
Note that there are 21 rows in the example table at this point. And I think that this is all about ordered-set aggregates, enjoy simplifying your life with Postgres 9.4!