From 0f170546274dfdc5fb1c489f4a803376de13542e Mon Sep 17 00:00:00 2001
From: Allen Chiang <allenjianggo@gmail.com>
Date: Mon, 4 Nov 2024 13:12:56 +0800
Subject: [PATCH] feat: add foreign key note

---
 database/06-check.md       |  6 ++++++
 database/07-foreign-key.md | 43 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)
 create mode 100644 database/07-foreign-key.md

diff --git a/database/06-check.md b/database/06-check.md
index ff9c77c..aff5439 100644
--- a/database/06-check.md
+++ b/database/06-check.md
@@ -1,3 +1,9 @@
+---
+layout: default
+parent: Database
+nav_order: 6
+---
+
 # Check
 
 在 PostgreSQL 中，可以使用 `CHECK` 來避免某些錯誤的資料被寫入。
diff --git a/database/07-foreign-key.md b/database/07-foreign-key.md
new file mode 100644
index 0000000..1b61002
--- /dev/null
+++ b/database/07-foreign-key.md
@@ -0,0 +1,43 @@
+---
+layout: default
+parent: Database
+nav_order: 7
+---
+
+# MySQL、SQLite 與 PostgreSQL 在加上 Foreign Key 上的差異。
+
+前幾天在寫 Laravel ORM 時，發現有一句關係查詢語句在 SQLite 中非常慢。
+
+```sql
+SELECT *,
+       (SELECT Count(*)
+        FROM "comments" AS "laravel_reserved_1"
+        WHERE "comments"."id" = "laravel_reserved_1"."parent_id") AS
+           "children_count"
+FROM "comments"
+WHERE "post_id" = 100
+  AND "parent_id" IS NULL
+ORDER BY "children_count" DESC
+```
+
+使用 `EXPLAIN` 發現查詢做了全表掃描，這時我才發現 `parent_id` 竟然沒有被加上索引 (Index)。
+在 `parent_id` 加上索引之後，查詢速度就變快非常多，基本上不到一秒就能返回結果。
+
+找資料才發現，**只有 MySQL 預設會幫 Foreign Key 加上索引，但是 SQLite 與 PostgreSQL 並不會這麼做**。
+
+所以如果你在 Laravel Database Migration 中使用這個 `foreignId` 方法來建立 Foreign Key。
+
+```php
+$table->foreignId('parent_id')
+    ->nullable()
+    ->constrained('comments')
+    ->onDelete('cascade');
+```
+
+那麼就只有 MySQL 會加上索引，可以參考[MySQL 文件](https://dev.mysql.com/doc/refman/9.1/en/constraint-foreign-key.html)的說明。
+
+> MySQL requires that foreign key columns be indexed; if you create a table with a foreign key constraint but no index on a given column, an index is created.
+
+相反的，PostgreSQL 與 SQLite 並不會這個做，可以參考[PostgreSQL 文件](https://www.postgresql.org/docs/current/ddl-constraints.html)的說明。
+
+> Because this is not always needed, and there are many choices available on how to index, the declaration of a foreign key constraint does not automatically create an index on the referencing columns.