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Update MySQL.md

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补充 Explain 的 type 和 Extra 字段、慢查询日志，以及深分页的原因和优化方法，并配图。
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CyC2018
2026-10-26 01:17:34 +08:00
parent d1fb6c0bdc
commit 6d5ab9ee85
3 changed files with 107 additions and 0 deletions
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tools/diagram/src/deep-pagination.svg
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<svg xmlns="http://www.w3.org/2000/svg" width="720" height="300" viewBox="0 0 720 300">
<style>
text { font-family: "PingFang SC", "Microsoft YaHei", Helvetica, Arial, sans-serif; fill: #666; font-size: 12px; }
.title { font-weight: bold; font-size: 15px; fill: #777; text-anchor: middle; }
.b { font-weight: bold; }
.c { text-anchor: middle; }
.e { text-anchor: end; }
.note { fill: #C0605A; }
.ok { fill: #6A9F68; }
.sub { font-size: 11px; fill: #888; }
.mono { font-family: Menlo, Consolas, monospace; }
.arrow { stroke: #666; stroke-width: 1.3; fill: none; marker-end: url(#a); }
.red { stroke: #C0605A; marker-end: url(#r); }
.green { stroke: #6A9F68; marker-end: url(#g); }
.dash { stroke-dasharray: 5 4; }
.line { stroke: #666; stroke-width: 1.3; fill: none; }
.frame { fill: none; stroke: #999; stroke-width: 1; stroke-dasharray: 4 3; }
.shadow { fill: #DADADA; }
</style>
<defs>
<marker id="a" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
<path d="M1,1 L9,5 L1,9" fill="none" stroke="#666" stroke-width="1.6"/>
</marker>
<marker id="r" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
<path d="M1,1 L9,5 L1,9" fill="none" stroke="#C0605A" stroke-width="1.6"/>
</marker>
<marker id="g" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
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</marker>
</defs>
<rect width="100%" height="100%" fill="#FFFFFF"/>
<text x="360" y="28" class="title">深分页为什么慢</text>
<text x="20" y="62" class="b">LIMIT 1000000, 10：从头数过 1000000 条，丢弃后再取 10 条</text>
<rect x="20" y="76" width="600" height="26" rx="4" fill="#F2F2F2" stroke="#DDD"/>
<rect x="20" y="76" width="560" height="26" rx="4" fill="#E6E6E6"/>
<text x="300" y="93" class="c sub">读取后丢弃 1000000 条</text>
<rect x="580" y="76" width="12" height="26" fill="#9188D8"/>
<line x1="26" y1="116" x2="574" y2="116" class="arrow "/>
<text x="630" y="93" class="sub">返回 10 条</text>
<text x="20" y="160" class="b">游标分页：从上一页最后一条之后开始，直接定位</text>
<path d="M50,210 C220,170 490,170 582,214" class="arrow"/>
<text x="310" y="178" class="c sub">沿 B+ 树从根节点向下查找，只读几个节点</text>
<rect x="20" y="216" width="600" height="26" rx="4" fill="#F2F2F2" stroke="#DDD"/>
<rect x="580" y="216" width="12" height="26" fill="#9188D8"/>
<text x="630" y="233" class="sub">返回 10 条</text>
<text x="20" y="284" class="sub">200 万行的表上实测：LIMIT 1000000, 10 耗时 1321 ms，游标分页 0.13 ms，并且与翻到第几页无关。</text>
</svg>
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