August 2024
Intermediate to advanced
392 pages
10h 4m
Japanese
Content preview from 達人に学ぶDB設計徹底指南書 第2版
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SELECT 文を実行して、のです（このオ
リジナルのテーブルを「」と呼ぶこともあります）。
つまり、ビューへアクセスするときは、2 段階のSQL が発行されているようなイ
メージを持ってもらえば良いでしょう（図 8-1）。
少し細かい補足をすると、現実にはDBMS内部でこのようなSQL文が処理され
るときは、本当に 2 段階に分けて実行されるとは限りません。むしろ DBMS は、可
能な限りとのSELECT 文を合体（マージ）して、効率の良い形で実行しようと
します。ただ、いずれにせよその場合も、純粋にのSELECT 文が実行されるの
で は な く 、「 ＋」によって生まれる新たなSELECT 文（）を実行することに
なり、よりは複雑で高コストな処理になります。
このように、ビューへのSQL文によるアクセスにおいては、常に背後に存在す
る基底テーブルを意識しておく必要があります。もともとビューは基底テーブルを
隠蔽する技術でもあるのですが、本当に基底テーブルを意識しなくていいのはエン
ドユーザーだけであって、エンジニアは常に意識しなければなりません。エンジニ
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8
9
8
7
6
5
4
3
2
1
アは、決して物理層から自由になれないのです。
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