albertlockett opened a new issue, #7610:
URL: https://github.com/apache/arrow-rs/issues/7610
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
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I'm working on a feature where we try to construct an array using the
smallest key type possible. As part of my logic, when we add a value to the
array, and receive a `DictionaryKeyOverflowError`, I'd like to then change to a
builder with a bigger key type.
**Describe the solution you'd like**
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I'd like this dictionary upgrade to be efficient and avoid unnecessary
copies.
I'd like to be able to construct a new builder that uses the same values &
other internal builder state, but a new builder for the keys.
**Describe alternatives you've considered**
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Constructing the dictionary array and copying everything into a new builder.
This seems less efficient:
```rs
let mut u8_keyed_dict_builder =
StringDictionaryBuilder::<UInt8Type>::new();
let _ = u8_keyed_dict_builder.append("val 0");
let _ = u8_keyed_dict_builder.append("val 1");
// ... append 254 more values
// ... eventually I realize the dict will overflow ...
// .. upgrade to builder keyed by UInt16 ...
let u8_keyed_dict = u8_keyed_dict_builder.finish_cloned();
let cast_result = cast(
&u8_keyed_dict,
&DataType::Dictionary(Box::new(DataType::UInt16),
Box::new(DataType::Utf8)),
)
.unwrap();
let casted_dict = cast_result
.as_any()
.downcast_ref::<DictionaryArray<UInt16Type>>()
.unwrap();
let typed_dict = casted_dict.downcast_dict::<StringArray>().unwrap();
let mut u16_keyed_dict_builder =
StringDictionaryBuilder::<UInt16Type>::new();
u16_keyed_dict_builder
// this copies values
.extend_dictionary(&typed_dict)
.unwrap();
```
**Additional context**
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