https://xinqilin.github.io/tags/n+1/ Recent content in N+1 on Bill.Lin's Notes Hugo zh-tw Mon, 15 Jan 2024 00:00:00 +0800 https://xinqilin.github.io/post/backend/jpa-n-plus-1/ Mon, 15 Jan 2024 00:00:00 +0800 https://xinqilin.github.io/post/backend/jpa-n-plus-1/ <h2 id="什麼是-n1-問題">什麼是 N+1 問題</h2> <p>載入 1 筆 <code>Order</code> 後，Hibernate 額外發出 N 條 SQL 去撈每筆 <code>Order</code> 的 <code>OrderItem</code>：</p> <div class="highlight"><div class="chroma"> <table class="lntable"><tr><td class="lntd"> <pre tabindex="0" class="chroma"><code><span class="lnt">1 </span><span class="lnt">2 </span><span class="lnt">3 </span><span class="lnt">4 </span><span class="lnt">5 </span></code></pre></td> <td class="lntd"> <pre tabindex="0" class="chroma"><code class="language-fallback" data-lang="fallback"><span class="line"><span class="cl">SELECT * FROM orders WHERE user_id = 1; -- 1 條 </span></span><span class="line"><span class="cl">SELECT * FROM order_items WHERE order_id = 10; -- N 條（每筆 Order 一條） </span></span><span class="line"><span class="cl">SELECT * FROM order_items WHERE order_id = 11; </span></span><span class="line"><span class="cl">SELECT * FROM order_items WHERE order_id = 12; </span></span><span class="line"><span class="cl">... </span></span></code></pre></td></tr></table> </div> </div><p>根源在於 <code>@OneToMany</code> 預設是 <code>LAZY</code> 載入——存取集合屬性時才觸發查詢，在迴圈中存取就變成 N 條。</p>