Support for STRUCT type

[destrex271]

This introduces a duckdb.struct type to postgres which can be used to read the STRUCT type in DuckDB.

Fixes #599

[JelteF]

We should really have a separate duckdb.struct type and not reuse the duckdb.row type (but you can copy and slightly modify the implementation of duckdb.row to create this new duckdb.struct type). We need this for one important reason (and possibly a few other less important ones): When duckdb.row is part of a select list, we replace it with a * before sending it to duckdb. We don't want that for this struct type. Apart from that they can basically behave the same, but we need two separate types to make the distinction between a type that is really a multiple columns under the hood (and which thus needs * expansion) versus a type that actually is a single column with a composite type in it.

[destrex271]

@JelteF do we also need to give the ability to create this type from postgres? like -

CREATE TABLE tester(a duckdb.struct);
INSERT INTO TABLE tester(a) VALUES('{a:12, b:12, c:Akshat}');

[destrex271]

Also correct me if I am wrong but we are handling the star exapnsion in pgduckdb_ddl::DuckdbHandleDDL, right?
As far as I understood, pgduckdb_target_list_contains_unresolved_type_or_row function helps us in this decision, right?

[destrex271]

So I have tried out the following for this:

1. Created a table on motherduck with a column of struct type and populated as follows:
   
   

2. From postgres I am able to perform the following operations:

-- Select entire content of table
postgres=# SELECT * FROM t1;
         s         
-------------------
 {'v': a, 'i': 42}
(1 row)

-- Use subscripts to access individual elements
postgres=# SELECT s['i'] FROM t1;
 s  
----
 42
(1 row)

-- Perform operations on selected values
postgres=# SELECT (s['i'] + 1) as sum FROM t1;
 sum 
-----
  43
(1 row)

As of now we do not have:

• Create new tables with struct type from postgres in duckdb
• Create duckdb.struct type in general

PS: For some reason the json_functions test are failing, will look into it as well
Fixed. Needed to update the tests to work with STRUCT type

[shah-nirmit]

Question: Where is this utility be used?

[destrex271]

thanks for pointing that out! we won't need that for duckdb.struct. I'll get rid of pgduckdb_var_is_duckdb_struct and pgduckdb_is_duckdb_struct functions

[JelteF]

Thanks for working on this. I made some relatively small improvements. The major one is to return the duckdb.struct type from the JSON functions that return a STRUCT. That way we actually have some tests for the subscript operator.

[JelteF]

This looks good to me now

[destrex271]

This looks good to me now

Thanks!