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docs(perf): clarify DASHBOARD_VIRTUALIZATION does not defer data fetch

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Virtualization only swaps a chart's visualization for a placeholder;
the chart component stays mounted and issues its data request
immediately regardless of scroll position. DASHBOARD_VIRTUALIZATION_DEFER_DATA
is the flag that actually skips the fetch for off-screen charts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
rusackas
2026-08-13 17:03:59 -07:00
co-authored by Claude Opus 4.8
parent ca885b2341
commit 36349da97c
1 changed files with 22 additions and 16 deletions
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docs/admin_docs/configuration/dashboard-performance.mdx
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@@ -58,9 +58,13 @@ charts hitting a fast cache behaves very differently from a dashboard of
## Lazy rendering — `DASHBOARD_VIRTUALIZATION`
Superset's dashboard layout is virtualized at the row level. Charts that
are far below the user's current scroll position are not rendered (and
therefore don't fetch data) until the user scrolls them into view, and they
are unmounted again if scrolled well past. This is on by default.
are far below the user's current scroll position render a placeholder
instead of their visualization until the user scrolls them into view, and
go back to a placeholder if scrolled well past. The chart component itself
stays mounted throughout — only the visualization is swapped for a
placeholder — so this alone does **not** reduce backend query load; see
[Deferred data fetch](#deferred-data-fetch--dashboard_virtualization_defer_data)
below for that. This is on by default.
**Feature flag**: `DASHBOARD_VIRTUALIZATION` (default: `True`)
@@ -70,25 +74,27 @@ operators can disable it if a specific layout misbehaves.
**Behavior** (from `superset-frontend/src/dashboard/components/gridComponents/Row/Row.tsx`):
- A chart is rendered when its row scrolls within **1 viewport height** of
the visible area.
- A chart is unmounted when its row scrolls more than **4 viewport
heights** away from the visible area.
- A chart's visualization is rendered when its row scrolls within **1
viewport height** of the visible area.
- A chart's visualization is swapped back for a placeholder when its row
scrolls more than **4 viewport heights** away from the visible area.
- Tabs that aren't currently selected don't render their content at all
(see below).
- The unmounting half is skipped in **embedded** mode (so an embedded
dashboard keeps its charts mounted once they've been seen, which avoids
re-fetching on scroll-up). Both halves are skipped for **headless /
bot** rendering (so screenshot / report jobs load every chart).
- The placeholder-swap-back is skipped in **embedded** mode (so an
embedded dashboard keeps its charts rendered once they've been seen,
which avoids re-rendering on scroll-up). Both halves are skipped for
**headless / bot** rendering (so screenshot / report jobs load every
chart).
## Deferred data fetch — `DASHBOARD_VIRTUALIZATION_DEFER_DATA`
By default, `DASHBOARD_VIRTUALIZATION` controls *rendering* — but charts
that don't render also don't fetch data, because Superset's chart
components issue their data request on mount. `DASHBOARD_VIRTUALIZATION_DEFER_DATA`
is a supplementary flag that further defers the data request itself, useful
By default, `DASHBOARD_VIRTUALIZATION` only controls whether a chart's
*visualization* is rendered — the chart component still mounts and issues
its data request immediately, regardless of scroll position.
`DASHBOARD_VIRTUALIZATION_DEFER_DATA` is a supplementary flag that skips
the data request itself for charts that aren't currently in view, useful
for backends where opening a connection or compiling a query is expensive
even if the result is later thrown away.
even if the result would be thrown away.
**Feature flag**: `DASHBOARD_VIRTUALIZATION_DEFER_DATA` (default: `False`)