Add support for deserializing list-encoded JSON structs [#6558] #6643
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Currently, a StructArray can only be deserialized from a JSON object
(e.g. `{a: 1, b: "c"}`), but some services (e.g. Presto and Trino)
encode ROW types as JSON lists (e.g. `[1, "c"]`) because this is more
compact, and the schema is known.
This PR adds the ability to parse JSON lists into StructArrays, if the
StructParseMode is set to ListOnly. In ListOnly mode, object-encoded
structs raise an error. Setting to ObjectOnly (the default) has the
original parsing behavior.
Some notes/questions/points for discussion:
1. I've made a JsonParseMode struct instead of a bool flag for two
reasons. One is that it's self-descriptive (what would `true` be?),
and the other is that it allows a future Mixed mode that could
deserialize either. The latter isn't currently requested by anyone.
2. I kept the error messages as similar to the old messages as possible.
I considered having more specific error messages (like "Encountered a
'[' when parsing a Struct, but the StructParseMode is ObjectOnly" or
similar), but wanted to hear opinions before I went that route.
3. I'm not attached to any name/code-style/etc, so happy to modify to
fit local conventions.
Fixes apache#6558
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Sorry this is on my list to look into, but I've struggled to find time to look into it yet... I appreciate your patience and restraint from repeatedly tagging me (unlike some people are wont to do). |
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Which issue does this PR close?
Closes #6558.
Rationale for this change
Currently, a StructArray can only be deserialized from a JSON object (e.g.
{a: 1, b: "c"}), but some services (e.g. Presto and Trino) encode ROW types as JSON lists (e.g.[1, "c"]) because this is more compact, and the schema is known.Arrow-json cannot currently deserialize these.
What changes are included in this PR?
This PR adds the ability to parse JSON lists into StructArrays, if the StructParseMode is set to ListOnly. In ListOnly mode, object-encoded structs raise an error. Setting to ObjectOnly (the default) has the original parsing behavior.
Are there any user-facing changes?
Users may set the
StructParsingModeenum toListOnlyto parse list-style structs. The associated enum,variants, and method have been documented. I'm happy to update any other documentation.
Discussion topics
truebe?), and the other is that it allows a future Mixed mode that could deserialize either. The latter isn't currently requested by anyone.