GH-404: Clarify dictionary fallback encoding - #609
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The dictionary encoding docs currently say that writers fall back to PLAIN encoding when a dictionary gets too large. That is not always true. A writer can stop using dictionary encoding and write later data pages with another valid encoding. The actual encoding is already stored in each data page header, so readers should use that field instead of assuming the fallback is PLAIN. Update the wording to describe that behavior, and clarify that the dictionary data page layout only applies to dictionary-encoded pages.
d33bs
marked this pull request as ready for review
August 22, 2026 19:18
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Rationale for this change
The dictionary encoding docs currently say that writers fall back to PLAIN encoding when a dictionary gets too large.
That is too narrow. Writers can stop using dictionary encoding and write later data pages with another valid encoding. The actual encoding is already stored in each data page header, so readers should use that field instead of assuming that fallback pages are PLAIN.
What changes are included in this PR?
This PR updates
Encodings.mdto clarify that dictionary fallback encoding is chosen by the writer and recorded in the data page header.It also clarifies that the documented dictionary data page layout applies only to dictionary-encoded data pages.
Do these changes have PoC implementations?
I feel no PoC implementation is needed, but please let me know if you think otherwise. This is a documentation clarification for behavior that the format already supports.
Closes #404