doc: Describe potential footgun in MapArray::keys and values - #11145
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Noticed this in apache/datafusion#25543 |
Rich-T-kid
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Sep 20, 2026
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@Rich-T-kid Thanks for the suggestions! I made some improvements and additions based on your feedback. |
Rich-T-kid
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Sep 22, 2026
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I think this is very neat, thanks @neilconway
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| /// For example, given a `MapArray` holding the three maps `{a: 1, b: 2}`, | ||
| /// `{c: 3}` and `{d: 4, e: 5}`, the `entries` array holds five key-value pairs | ||
| /// and the offsets are `[0, 2, 3, 5]`. Calling `slice(1, 1)` yields a `MapArray` | ||
| /// holding the single map `{c: 3}`, but [`Self::keys`] still returns all five | ||
| /// keys `a, b, c, d, e` and [`Self::offsets`] is `[2, 3]`. Use the offsets, or | ||
| /// [`Self::value`], to find the entries belonging to each map. | ||
| /// | ||
| /// The same applies to any `MapArray` constructed with offsets that do not start | ||
| /// at `0` or do not extend to the end of `entries`. |
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nice I like that you provided an example
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| /// Note: The offsets may not start at `0` and may not cover all entries in | ||
| /// [`Self::entries`]. This can happen when the map array was sliced via | ||
| /// [`Self::slice`]. See documentation for [`Self`] for more details. |
Jefffrey
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Sep 23, 2026
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thanks @neilconway & @Rich-T-kid |
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Which issue does this PR close?
Rationale for this change
The behavior of the
keysandvaluesmethods of aMapArraycan be a bit surprising. This PR documents that behavior; consistent with the documentation forListArray::values, which behaves similarly.What changes are included in this PR?
Are these changes tested?
Existing tests pass.
Are there any user-facing changes?
No.