Use a golf analogy when explaining to executives.
Use a car analogy for all others. — Confucius.

The purpose of window functions is to translate the business reporting requirements declaratively and effectively to SQL so query performance and developer/business-analyst efficiency improve dramatically.

I've seen real-world reports and dashboards go from hours to minutes and minutes to seconds after using window functions. Query size decreases from 40-pages to a few pages. Back in the '90s, Redbrick database really understood the business use case and created a new layer of functionality to do business reporting that included ranking, running totals, and calculating commissions and inventory based on subgroups, positions, etc. These have been in SQL standard in 2003. Every BI layer (like Tableau, Looker, Cognos) exploits this functionality.

You may also like: Fun With SQL: Window Functions in Postgres

Introduction to Window Functions

Insert the data into Couchbase.

INSERT INTO golf 
VALUES("KP1", {"player": "Marco", "round1":75, "round2":73}),
VALUES("KP2", {"player": "Johan", "round1":72, "round2":68}),
VALUES("KP3", {"player": "Chang", "round1":67, "round2":76}),
VALUES("KP4", {"player": "Isha", "round1":74, "round2":71}),
VALUES("KP5", {"player": "Sitaram", "round1":68, "round2":72}),
VALUES("KP6", {"player": "Bingjie", "round1":71, "round2":67});

WITHOUT window functions (current state — Couchbase 6.0)

To write the query without the use of window functions, you need a subquery to calculate the rank for each player. This subquery has to scan through all of the data, resulting in the worst algorithmic complexity of O(N^2), which dramatically increases the execution time and throughput.

WITH g1 as (select player, round1, round2 from golf)
SELECT    g3.player                                AS player,
          (g3.round1+g3.round2)                    AS T,
          ((g3.round1+g3.round2) - 144)            AS ToPar, 
          (select raw 1+COUNT(*) 
             from g1 as g2 
               where (g2.round1 + g2.round2) < 
                     (g3.round1 + g3.round2))[0]   AS sqlrankR2
FROM g1 as g3
ORDER BY sqlrankR2

Results: 
T ToPar player sqlrankR2
138 -6 "Bingjie"    1
140 -4 "Johan"      2
140 -4 "Sitaram"    2
143 -1 "Chang"      4
145 1 "Isha"       5
148 4 "Marco"      6

With window functions in Mad-Hatter (upcoming release)

This query returns the player, the total after two rounds (T), how much of the score is over/under par (ToPar), and then ranks them based on the scores of the first two rounds. This is the NEW functionality in Mad-Hatter. The time complexity of this is O(N), meaning execution time will only increase linearly.

SELECT    player                                AS player,
          (round1+round2)                       AS T,
          ((round1+round2) - 144)               AS ToPar,
          RANK() OVER(ORDER BY (round1+round2)) AS rankR2
FROM golf;


T ToPar player    rankR2
138 -6 "Bingjie"    1
140 -4 "Johan"      2
140 -4 "Sitaram"    2
143 -1 "Chang"      4
145 1 "Isha"      5
148 4 "Marco"      6

Observations

The query expresses the requirements simply and clearly.

1. Performance of this query in a real-world scenario is much better. We plan to measure.
2. When the ranking requirements depend on multiple documents, the query becomes quite complex to write, optimize, and run.
3. All of this affects the TCO overall.

Now, let's create an expanded dashboard.

Show add dense rank, row number, who's ahead, and the number of strokes behind the leader. All very common things in a reporting resituation. You're seeing the new window function whenever you see the OVER() clause. The query below has six window functions.

SELECT    player                                AS player,
          (round1+round2)                       AS T,
          ((round1+round2) - 144)               AS ToPar,
          RANK() OVER(ORDER BY (round1+round2)) AS rankR2,
          DENSE_RANK() OVER (ORDER BY (round1+round2)) AS rankR2Dense,
          ROW_NUMBER() OVER() rownum,
          ((round1+round2) - 
              FIRST_VALUE(round1+round2) 
                OVER(ORDER BY (round1+round2))) AS strokesbehind,
          RANK() OVER(ORDER BY (round1))        AS rankR1,
          LAG(player, 1, "None") OVER(ORDER BY round1+round2) 
                                                AS inFront
FROM golf
ORDER BY rankR2


T ToPar inFront   player    rankR1  rankR2 rankR2Dense rownum strokesbehind
138 -6 "None"    "Bingjie" 3 1 1 3 0
140 -4 "Johan"   "Sitaram" 2 2 2 2 2
140 -4 "Bingjie" "Johan" 4 2 2 4 2
143 -1 "Sitaram" "Chang" 1 4 3 1 5
145 1 "Chang"  "Isha"        5 5 4 5 7
148 4 "Isha"   "Marco" 6 6 5 6 10

As you saw earlier, doing this query with six window functions using the subquery method will be a larger effort, expensive, error-prone query.

In addition to making the built-in aggregates (COUNT, SUM, AVG, etc) as window functions, Sitaram has added the following window functions. The syntax and semantics of each of them are well-defined in the standard and well-described in the articles of the reference section below.

• RANK()
• DENSE_RANK()
• PERCENT_RANK()
• CUME_DIST()
• NTILE()
• RATIO_TO_REPORT()
• ROW_NUMBER()
• LAG()
• FIRST_VALUE()
• LAST_VALUE()
• NTH_VALUE()
• LEAD()

References