feat: add a custom ExprPlanner to support colon operator

[Samyak2]

Closes #26

• Add a custom ExprPlanner that handles : as an operator and re-writes it to variant_get.

[Samyak2]

TODO:

• This handles the 2-arg version. The 3-arg version (with cast) is yet to be converted
• Tests

[friendlymatthew]

Thanks @Samyak2

Can we add some tests as well please

[friendlymatthew]

Can we add some comments here that explain what this does

[Samyak2]

Done. Please take a look

[Samyak2]

There seems to a bug in the sql parser. These are failing:

select variant_pretty(json_to_variant(json_str):[1]) from json_data;

and

select variant_pretty(json_to_variant(json_str):['name']) from json_data;

fails with:

SQL error: ParserError("Expected: variant object key name, found: [ at Line: 1, Column: 49")

[Samyak2]

Another problem. Cast does not get parsed as a BinaryOperator. It's a separate variant of Expr: https://docs.rs/sqlparser/0.61.0/sqlparser/ast/enum.Expr.html#variant.Cast

ExprPlanner currently does not support custom parsing for cast.

So we won't be able to support 3-arg version like variant_col:name::string, etc.

[sdf-jkl]

There seems to a bug in the sql parser. These are failing:

select variant_pretty(json_to_variant(json_str):[1]) from json_data;

and

select variant_pretty(json_to_variant(json_str):['name']) from json_data;

fails with:

SQL error: ParserError("Expected: variant object key name, found: [ at Line: 1, Column: 49")

Is this a bug though? It seems like a Databricks dialect specifics.

Another problem. Cast does not get parsed as a BinaryOperator. It's a separate variant of Expr: https://docs.rs/sqlparser/0.61.0/sqlparser/ast/enum.Expr.html#variant.Cast

ExprPlanner currently does not support custom parsing for cast.

So we won't be able to support 3-arg version like variant_col:name::string, etc.

Another issue to work on in sqlparser-rs?