ページ分割

PostgreSQL 集計統計とウィンドウ関数

Michel Semaan

Data Scientist

ページ分割とは？

ページ分割: データを（ほぼ）等しい塊に分ける
用途
- 多くのAPIは送信データ量を減らすため「ページ」で返す
- パフォーマンス評価のため四分位や三分位（上位・中位・下位33%）に分ける

NTILE の登場

NTILE(n) はデータを約 n 等分のページに分ける

ページ分割 - 元テーブル

クエリ

SELECT
  DISTINCT Discipline
FROM Summer_Medals;

データを約15ページに等分
67 / 15 ≃ 4 のため、各ページは4〜5行になる

結果

| Discipline          |
|---------------------|
| Wrestling Freestyle |
| Archery             |
| Baseball            |
| Lacrosse            |
| Judo                |
| Athletics           |
| ...                 |

(67 rows)

ページ分割

クエリ

WITH Disciplines AS (
  SELECT
    DISTINCT Discipline
  FROM Summer_Medals)

SELECT
  Discipline, NTILE(15) OVER () AS Page
From Disciplines
ORDER BY Page ASC;

結果

| Discipline          | Page |
|---------------------|------|
| Wrestling Freestyle | 1    |
| Archery             | 1    |
| Baseball            | 1    |
| Lacrosse            | 1    |
| Judo                | 1    |
| Athletics           | 2    |
| ...                 | ...  |

上位・中位・下位の三分位

クエリ

WITH Country_Medals AS (
  SELECT
    Country, COUNT(*) AS Medals
  FROM Summer_Medals
  GROUP BY Country),

SELECT
  Country, Medals,
  NTILE(3) OVER (ORDER BY Medals DESC) AS Third
FROM Country_Medals;

結果

| Country | Medals | Third |
|---------|--------|-------|
| USA     | 4585   | 1     |
| URS     | 2049   | 1     |
| GBR     | 1720   | 1     |
| ...     | ...    | ...   |
| CZE     | 56     | 2     |
| LTU     | 55     | 2     |
| ...     | ...    | ...   |
| DOM     | 6      | 3     |
| BWI     | 5      | 3     |
| ...     | ...    | ...   |

三分位の平均

クエリ

WITH Country_Medals AS (...),

  Thirds AS (
  SELECT
    Country, Medals,
    NTILE(3) OVER (ORDER BY Medals DESC) AS Third
  FROM Country_Medals)

SELECT
  Third,
  ROUND(AVG(Medals), 2) AS Avg_Medals
FROM Thirds
GROUP BY Third
ORDER BY Third ASC;

結果

| Third | Avg_Medals |
|-------|------------|
| 1     | 598.74     |
| 2     | 22.98      |
| 3     | 2.08       |

Passons à la pratique !

PostgreSQL 集計統計とウィンドウ関数