> ## Documentation Index > Fetch the complete documentation index at: https://clickhouse.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Projections > Page expliquant ce que sont les projections, comment elles peuvent être utilisées pour améliorer les performances des requêtes et en quoi elles diffèrent des vues matérialisées. export const RunnableCode = ({children, run = false, showStats = true}) => { const [results, setResults] = useState(null); const [error, setError] = useState(null); const [loading, setLoading] = useState(false); const [showResults, setShowResults] = useState(false); const [stats, setStats] = useState(null); const [isDark, setIsDark] = useState(false); const [hoveredRow, setHoveredRow] = useState(-1); const codeRef = useRef(null); useEffect(() => { if (typeof window !== "undefined") { const check = () => setIsDark(document.documentElement.classList.contains("dark")); check(); const observer = new MutationObserver(check); observer.observe(document.documentElement, { attributes: true, attributeFilter: ["class"] }); return () => observer.disconnect(); } }, []); useEffect(() => { if (codeRef.current) { const block = codeRef.current.querySelector(".code-block"); if (block) { block.style.marginBottom = "0"; block.style.marginTop = "0"; block.style.borderBottomLeftRadius = "0"; block.style.borderBottomRightRadius = "0"; } } }); const getSqlText = () => { if (!codeRef.current) return ""; const code = codeRef.current.querySelector("code"); return (code || codeRef.current).textContent.trim(); }; const executeQuery = async () => { const sql = getSqlText(); if (!sql) return; setLoading(true); setError(null); setResults(null); setShowResults(true); try { const cleanQuery = sql.replace(/;$/, "").trim(); const params = new URLSearchParams({ query: cleanQuery, default_format: "JSONCompact", result_overflow_mode: "break", read_overflow_mode: "break", allow_experimental_analyzer: "1" }); const res = await fetch(`https://sql-clickhouse.clickhouse.com/?${params.toString()}`, { method: "POST", headers: { Authorization: `Basic ${btoa(`demo:`)}` } }); const text = await res.text(); if (!res.ok) { setError(text || `HTTP ${res.status}`); setLoading(false); return; } const json = JSON.parse(text); setResults(json); setStats(json.statistics || null); } catch (err) { setError(err.message || "Échec de l'exécution de la requête"); } setLoading(false); }; useEffect(() => { if (run) executeQuery(); }, []); const formatRows = n => { if (n >= 1e9) return `${(n / 1e9).toFixed(1)}B`; if (n >= 1e6) return `${(n / 1e6).toFixed(1)}M`; if (n >= 1e3) return `${(n / 1e3).toFixed(1)}K`; return String(n); }; const formatBytes = b => { if (b >= 1e9) return `${(b / 1e9).toFixed(2)} GB`; if (b >= 1e6) return `${(b / 1e6).toFixed(2)} MB`; if (b >= 1e3) return `${(b / 1e3).toFixed(2)} KB`; return `${b} B`; }; const isNumericType = type => { return (/^(UInt|Int|Float|Decimal)/).test(type); }; const isHyperlink = value => { return typeof value === "string" && (/^https?:\/\//).test(value); }; const computeColumnExtremes = (meta, data) => { const extremes = {}; for (let i = 0; i < meta.length; i++) { if (isNumericType(meta[i].type)) { let min = Infinity, max = -Infinity; for (const row of data) { const v = Number(row[i]); if (!isNaN(v)) { if (v < min) min = v; if (v > max) max = v; } } if (max > -Infinity) { extremes[i] = { min, max }; } } } return extremes; }; const computeColumnWidths = (meta, data) => { const lengths = meta.map((col, i) => { const headerLen = col.name.length + col.type.length + 1; let maxData = 0; for (const row of data) { const v = row[i]; const len = v === null ? 4 : String(v).length; if (len > maxData) maxData = len; } return Math.max(headerLen, maxData); }); const total = lengths.reduce((s, l) => s + l, 0); return lengths.map(l => `${(l / total * 100).toFixed(1)}%`); }; const copyResultsAsTSV = () => { if (!results || !results.meta || !results.data) return; const header = results.meta.map(col => col.name).join("\t"); const rows = results.data.map(row => row.map(cell => cell === null ? "NULL" : String(cell)).join("\t")); const tsv = [header, ...rows].join("\n"); navigator.clipboard.writeText(tsv); }; const borderColor = isDark ? "rgba(255,255,255,0.15)" : "#e5e7eb"; const bgColor = isDark ? "rgba(255,255,255,0.05)" : "#f9fafb"; const headerBg = isDark ? "#2a2a2a" : "#f3f4f6"; const textColor = isDark ? "#e5e7eb" : "#1f2937"; const mutedColor = isDark ? "#d1d5db" : "#6b7280"; const accentColor = isDark ? "#FAFF69" : "#323232"; const accentTextColor = isDark ? "#000" : "#fff"; const barColor = isDark ? "#35372f" : "#d2d2d2"; const cellBg = isDark ? "#1f201b" : "#ffffff"; const cellBgHover = isDark ? "lch(15.8 0 0)" : "#f0f0f0"; const extremes = results && results.meta && results.data ? computeColumnExtremes(results.meta, results.data) : {}; const colWidths = results && results.meta && results.data ? computeColumnWidths(results.meta, results.data) : []; const getCellBarStyle = (cell, ci, ri) => { if (cell === null) return null; const colMeta = results.meta[ci]; if (!isNumericType(colMeta.type) || !extremes[ci] || results.data.length <= 1 || extremes[ci].max <= 0) return null; const ratio = 100 * Number(cell) / extremes[ci].max; const bg = ri === hoveredRow ? cellBgHover : cellBg; return { background: `linear-gradient(to right, ${barColor} 0%, ${barColor} ${ratio}%, ${bg} ${ratio}%, ${bg} 100%)` }; }; const renderCell = (cell, ci) => { if (cell === null) { return NULL; } const value = String(cell); if (isHyperlink(value)) { return {value} ; } return value; }; return
{}
{children}
{}
{results && } {showStats && stats && {formatRows(stats.rows_read)} lignes lues, {formatBytes(stats.bytes_read)} en {stats.elapsed.toFixed(3)}s }
{} {showResults &&
{loading &&
Exécution de la requête...
} {error &&
{error}
} {results && results.meta && results.data &&
{results.meta.map((col, i) =>
1 ? "right" : "left", backgroundColor: headerBg, borderBottom: `1px solid ${borderColor}`, color: textColor, fontWeight: 600, fontSize: "12px", whiteSpace: "nowrap", overflow: "hidden", textOverflow: "ellipsis" }}> {col.name} {col.type}
)} {results.data.map((row, ri) => row.map((cell, ci) =>
setHoveredRow(ri)} onMouseLeave={() => setHoveredRow(-1)} style={{ padding: "4px 12px", color: textColor, whiteSpace: "nowrap", overflow: "hidden", textOverflow: "ellipsis", textAlign: isNumericType(results.meta[ci].type) && results.meta.length > 1 ? "right" : "left", borderBottom: `1px solid ${borderColor}`, backgroundColor: ri === hoveredRow ? cellBgHover : ri % 2 === 0 ? "transparent" : bgColor, transition: "background-color 0.1s", ...getCellBarStyle(cell, ci, ri) }}> {renderCell(cell, ci)}
))}
} {results && results.data &&
{results.rows} ligne{results.rows !== 1 ? "s" : ""}
}
}
; }; export const Image = ({img, alt, size = "lg", background}) => { const normalizedSize = ["sm", "md", "lg"].includes(size) ? size : "lg"; const backgroundColor = background === "white" ? "white" : background === "black" ? "rgb(31 31 28)" : undefined; return
{alt}
; };
## Introduction
ClickHouse offre divers mécanismes pour accélérer les requêtes analytiques sur de grandes quantités de données dans des scénarios en temps réel. L’un de ces mécanismes pour accélérer vos requêtes consiste à utiliser des *projections*. Les projections permettent d’optimiser les requêtes en créant une réorganisation des données selon les attributs pertinents. Cela peut être : 1. Une réorganisation complète 2. Un sous-ensemble de la table d’origine avec un ordre différent 3. Une agrégation précalculée (semblable à une vue matérialisée), mais avec un ordre aligné sur l’agrégation.