ClickHouseでのPostgreSQLクエリの書き換え
これはPostgreSQLからClickHouseへの移行ガイドの第4部です。このコンテンツは導入として考えられ、ユーザーがClickHouseのベストプラクティスに従った初期の機能的なシステムを展開するのを助けることを目的としています。このガイドは複雑なトピックを避け、完全に最適化されたスキーマを生み出すわけではなく、ユーザーがプロダクションシステムを構築し、学習を基にするための堅実な基盤を提供します。
以下は、PostgreSQLとClickHouseを比較するクエリの例を提供します。このリストは、ClickHouseの機能を活用してクエリを大幅に簡略化する方法を示すことを目的としています。これらのクエリはほとんどの場合、ClickHouseでより迅速に実行されます。ここでの例は、PostgreSQLとClickHouseの同等のリソース（8コア、32GiB RAM）での完全なStack Overflow データセット（2024年4月まで）の使用に基づいています。
ここでのカウントは若干異なります。Postgresデータは外部キーの参照整合性を満たす行のみが含まれているためです。ClickHouseはそのような制約を課さず、匿名ユーザーを含むフルデータセットを持っています。
最もビューを獲得した質問が10件を超えるユーザー:
-- ClickHouse
SELECT OwnerDisplayName, sum(ViewCount) AS total_views
FROM stackoverflow.posts
WHERE (PostTypeId = 'Question') AND (OwnerDisplayName != '')
GROUP BY OwnerDisplayName
HAVING count() > 10
ORDER BY total_views DESC
LIMIT 5
┌─OwnerDisplayName────────┬─total_views─┐
│ Joan Venge │ 25520387 │
│ Ray Vega │ 21576470 │
│ anon │ 19814224 │
│ Tim │ 19028260 │
│ John │ 17638812 │
└─────────────────────────┴─────────────┘
5 rows in set. Elapsed: 0.360 sec. Processed 24.37 million rows, 140.45 MB (67.73 million rows/s., 390.38 MB/s.)
Peak memory usage: 510.71 MiB.
--Postgres
SELECT OwnerDisplayName, SUM(ViewCount) AS total_views
FROM public.posts
WHERE (PostTypeId = 1) AND (OwnerDisplayName != '')
GROUP BY OwnerDisplayName
HAVING COUNT(*) > 10
ORDER BY total_views DESC
LIMIT 10;
ownerdisplayname | total_views
-------------------------+-------------
Joan Venge | 25520387
Ray Vega | 21576470
Tim | 18283579
J. Pablo Fernández | 12446818
Matt | 12298764
Time: 107620.508 ms (01:47.621)
最もビューを獲得したタグ:
--ClickHouse
SELECT arrayJoin(arrayFilter(t -> (t != ''), splitByChar('|', Tags))) AS tags,
sum(ViewCount) AS views
FROM posts
GROUP BY tags
ORDER BY views DESC
LIMIT 5
┌─tags───────┬──────views─┐
│ javascript │ 8190916894 │
│ python │ 8175132834 │
│ java │ 7258379211 │
│ c# │ 5476932513 │
│ android │ 4258320338 │
└────────────┴────────────┘
5 rows in set. Elapsed: 0.908 sec. Processed 59.82 million rows, 1.45 GB (65.87 million rows/s., 1.59 GB/s.)
--Postgres
WITH tags_exploded AS (
SELECT
unnest(string_to_array(Tags, '|')) AS tag,
ViewCount
FROM public.posts
),
filtered_tags AS (
SELECT
tag,
ViewCount
FROM tags_exploded
WHERE tag <> ''
)
SELECT tag AS tags,
SUM(ViewCount) AS views
FROM filtered_tags
GROUP BY tag
ORDER BY views DESC
LIMIT 5;
tags | views
------------+------------
javascript | 7974880378
python | 7972340763
java | 7064073461
c# | 5308656277
android | 4186216900
(5 rows)
Time: 112508.083 ms (01:52.508)
集計関数
可能な場合、ユーザーはClickHouseの集計関数を活用するべきです。以下は、毎年最もビューを獲得した質問を計算するためのargMax関数の使用を示しています。
--ClickHouse
SELECT toYear(CreationDate) AS Year,
argMax(Title, ViewCount) AS MostViewedQuestionTitle,
max(ViewCount) AS MaxViewCount
FROM stackoverflow.posts
WHERE PostTypeId = 'Question'
GROUP BY Year
ORDER BY Year ASC
FORMAT Vertical
Row 1:
──────
Year: 2008
MostViewedQuestionTitle: How to find the index for a given item in a list?
MaxViewCount: 6316987
Row 2:
──────
Year: 2009
MostViewedQuestionTitle: How do I undo the most recent local commits in Git?
MaxViewCount: 13962748
...
Row 16:
───────
Year: 2023
MostViewedQuestionTitle: How do I solve "error: externally-managed-environment" every time I use pip 3?
MaxViewCount: 506822
Row 17:
───────
Year: 2024
MostViewedQuestionTitle: Warning "Third-party cookie will be blocked. Learn more in the Issues tab"
MaxViewCount: 66975
17 rows in set. Elapsed: 0.677 sec. Processed 24.37 million rows, 1.86 GB (36.01 million rows/s., 2.75 GB/s.)
Peak memory usage: 554.31 MiB.
これはPostgresクエリの同等のものよりも大幅に簡単（かつ速い）です:
--Postgres
WITH yearly_views AS (
SELECT
EXTRACT(YEAR FROM CreationDate) AS Year,
Title,
ViewCount,
ROW_NUMBER() OVER (PARTITION BY EXTRACT(YEAR FROM CreationDate) ORDER BY ViewCount DESC) AS rn
FROM public.posts
WHERE PostTypeId = 1
)
SELECT
Year,
Title AS MostViewedQuestionTitle,
ViewCount AS MaxViewCount
FROM yearly_views
WHERE rn = 1
ORDER BY Year;
year | mostviewedquestiontitle | maxviewcount
------+-----------------------------------------------------------------------------------------------------------------------+--------------
2008 | How to find the index for a given item in a list? | 6316987
2009 | How do I undo the most recent local commits in Git? | 13962748
...
2023 | How do I solve "error: externally-managed-environment" every time I use pip 3? | 506822
2024 | Warning "Third-party cookie will be blocked. Learn more in the Issues tab" | 66975
(17 rows)
Time: 125822.015 ms (02:05.822)
条件文と配列
条件文と配列関数を使用すると、クエリが大幅に簡略化されます。以下のクエリは、2022年から2023年にかけて10000件以上の発生があったタグの中から、最大の割合増加を計算します。以下のClickHouseクエリでは、条件文、配列関数、およびHAVINGおよびSELECT句でのエイリアスの再利用が可能であることがわかります。
--ClickHouse
SELECT arrayJoin(arrayFilter(t -> (t != ''), splitByChar('|', Tags))) AS tag,
countIf(toYear(CreationDate) = 2023) AS count_2023,
countIf(toYear(CreationDate) = 2022) AS count_2022,
((count_2023 - count_2022) / count_2022) * 100 AS percent_change
FROM stackoverflow.posts
WHERE toYear(CreationDate) IN (2022, 2023)
GROUP BY tag
HAVING (count_2022 > 10000) AND (count_2023 > 10000)
ORDER BY percent_change DESC
LIMIT 5
┌─tag─────────┬─count_2023─┬─count_2022─┬──────percent_change─┐
│ next.js │ 13788 │ 10520 │ 31.06463878326996 │
│ spring-boot │ 16573 │ 17721 │ -6.478189718413183 │
│ .net │ 11458 │ 12968 │ -11.644046884639112 │
│ azure │ 11996 │ 14049 │ -14.613139725247349 │
│ docker │ 13885 │ 16877 │ -17.72826924216389 │
└─────────────┴────────────┴────────────┴─────────────────────┘
5 rows in set. Elapsed: 0.247 sec. Processed 5.08 million rows, 155.73 MB (20.58 million rows/s., 630.61 MB/s.)
Peak memory usage: 403.04 MiB.
--Postgres
SELECT
tag,
SUM(CASE WHEN year = 2023 THEN count ELSE 0 END) AS count_2023,
SUM(CASE WHEN year = 2022 THEN count ELSE 0 END) AS count_2022,
((SUM(CASE WHEN year = 2023 THEN count ELSE 0 END) - SUM(CASE WHEN year = 2022 THEN count ELSE 0 END))
/ SUM(CASE WHEN year = 2022 THEN count ELSE 0 END)::float) * 100 AS percent_change
FROM (
SELECT
unnest(string_to_array(Tags, '|')) AS tag,
EXTRACT(YEAR FROM CreationDate) AS year,
COUNT(*) AS count
FROM public.posts
WHERE EXTRACT(YEAR FROM CreationDate) IN (2022, 2023)
AND Tags <> ''
GROUP BY tag, year
) AS yearly_counts
GROUP BY tag
HAVING SUM(CASE WHEN year = 2022 THEN count ELSE 0 END) > 10000
AND SUM(CASE WHEN year = 2023 THEN count ELSE 0 END) > 10000
ORDER BY percent_change DESC
LIMIT 5;
tag | count_2023 | count_2022 | percent_change
-------------+------------+------------+---------------------
next.js | 13712 | 10370 | 32.22757955641273
spring-boot | 16482 | 17474 | -5.677005837243905
.net | 11376 | 12750 | -10.776470588235295
azure | 11938 | 13966 | -14.520979521695546
docker | 13832 | 16701 | -17.178612059158134
(5 rows)
Time: 116750.131 ms (01:56.750)
これで、PostgresからClickHouseへの移行を検討しているユーザー向けの基本ガイドは終了です。Postgresから移行するユーザーには、ClickHouseでのモデルデータの設計ガイドを読み、ClickHouseの高度な機能について学ぶことをお勧めします。