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bcquality/microsoft/knowledge/performance/blob-fields-are-not-cached-prefer-media.md
Jesper Schulz-Wedde 800e266bfe Add seven performance knowledge articles from BC developer guidance 
Cover non-obvious platform behaviors a capable LLM reliably gets wrong:
hidden FlowFields still calculate, LockTable scopes to the whole table,
query objects bypass the primary-key cache, table-event subscribers
disable bulk ModifyAll/DeleteAll, Blob fields are uncached, OnCompanyOpen
subscribers block every session creation, and the test framework
disables bulk insert mode.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
2026-04-24 10:38:28 +02:00

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performance
blob
media
mediaset
cache
image
thumbnail
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Blob fields are never cached — prefer Media or MediaSet for images

Description

Blob field contents are not cached by the Business Central server or the client. Every read re-fetches the full payload from the database, even when the same blob was read moments earlier in the same session. For images displayed on a page, this turns into a database round-trip per render.

Media and MediaSet are purpose-built for this and behave differently in two ways that matter for performance. First, they are cached on the client, so subsequent renders of the same image do not re-hit the database. Second, the platform generates a thumbnail when the data is saved, so a list or card page can show the thumbnail immediately and lazy-load the full-resolution image — typically via a Page Background Task — only when needed.

Blob remains appropriate for non-image binary data that is written once and rarely read, or for data the platform does not need to render. For any field that is displayed repeatedly — profile pictures, item images, logos on documents — Media or MediaSet is the default.

Best Practice

Store images in Media or MediaSet fields. Bind the thumbnail to the page; load full-resolution data asynchronously when the user opens the full view. Reserve Blob for opaque payloads that are not rendered in the UI.

Anti Pattern

An Item Image field defined as Blob and shown directly on a list page. Every scroll re-fetches every image from SQL, the list page load time scales with row count and image size, and no client-side caching mitigates the cost.