feat: recover struct column from exploded Series

[mattyopl]

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Fixes #357588049 internal 🦕

[GarrettWu]

it creates a local data copy in memory. It won't fit if the original DF is a large table in BQ. @chelsea-lin Do you have any suggestions?

[mattyopl]

just so I can learn: does struct.explode() [code pointer] create a local data copy in memory? @GarrettWu cc @tswast@TrevorBergeron as Tim introduced this change [PR] and Trevor reviewed

[mattyopl]

If explode() does create a local copy, I think we are okay to create local copy for struct, as the use case of this function is to unexplode a Dataframe back into a Series of structs (see b/357588049).

An alternative idea was to use DataFrame.loc but seems like that has some unnecessary overhead and also creates local copies through calling to_pandas() when Series is a BigFrames Series

[GarrettWu]

explode() doesn't. Column and Row aren't equivalent in BQ. Looping columns is OK since it can contain only up to 10k columns. But much larger size in rows. https://cloud.google.com/bigquery/quotas#standard_tables

You may want to add a STRUCT operator and apply to all the columns in the DF.

[chelsea-lin]

The majority of BigFrames operators are designed to be deferred. This means that we construct static expression tree representing your operations, and these trees won't be executed (causing compiler and data downloads) until you explicitly trigger it, typically within actions like to_pandas. Taken struct.explode() as example, it calls struct.field and series.rename, adhere to this deferred model as well. You can confirm this behavior by https://screenshot.googleplex.com/Pqf69h7ZKUTmHGP, where no BQ job for s.struct.explode().

Following Gerrett's suggestion, we can implement this operator by creating a new STRUCT operator. A similar implementation approach can be seen in this pull request, where a JSONExtract unary operation is defined. This operation takes a single argument, json_path, and its compiler rule (defined in the json_extract method) generates the SQL expression JSON_EXTRACT(json_obj, json_path).

[chelsea-lin]

The majority of BigFrames operators are designed to be deferred. This means that we construct static expression tree representing your operations, and these trees won't be executed (causing compiler and data downloads) until you explicitly trigger it, typically within actions like to_pandas. Taken struct.explode() as example, it calls struct.field and series.rename, adhere to this deferred model as well. You can confirm this behavior by https://screenshot.googleplex.com/Pqf69h7ZKUTmHGP, where no BQ job for s.struct.explode().

Following Gerrett's suggestion, we can implement this operator by creating a new STRUCT operator. A similar implementation approach can be seen in this pull request, where a JSONExtract unary operation is defined. This operation takes a single argument, json_path, and its compiler rule (defined in the json_extract method) generates the SQL expression JSON_EXTRACT(json_obj, json_path).

[chelsea-lin]

I am curious the following cases work for bbq.struct(df)

1. When the df has a struct type column.
2. When the df has a int column, which has a None element.
3. When the df has a array type column.
   If they're working, could you please add more tests for them?

[mattyopl]

done!

[chelsea-lin]

Thanks Matthew. LGTM!