fix: skip predicates on struct unnest in PushDownFilter

[akoshchiy]

Which issue does this PR close?

• Closes Unnested fields are not filterable when using subqueries. #16695.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Added new test case in push_down_filter.slt

Are there any user-facing changes?

No.

[alamb]

@adriangb is there any chance you have time to review this PR?

[adriangb]

My understanding is that we should not be pushing down filters that touch unnested columns. This seems to achieve that goal.

[adriangb]

@akoshchiy is there documentation anywhere on what "__unnest_placeholder" is? Is it a private implementation detail we are using to test, or something that users could / should use in their queries?

[akoshchiy]

As I can see, it's a result of struct unnest preprocessing in SqlToRel::try_process_unnest. unnest(struct_col) will be transformed into __unnest_placeholder(struct_col).field1, __unnest_placeholder(struct_col).field2 etc.
To be honest, I'm not sure, that it was supposed to be like that or not.

[adriangb]

I think a little more docs / context / comments are needed otherwise this is good to merge.

[adriangb]

I think a little more docs / context / comments are needed otherwise this is good to merge.

@akoshchiy could you add some comments explaining what's going on for future reference, and documenting what the __unnest_placeholder is? I realize that's been around and is used for other things already but I think it's important to improve documentation and understanding as we go along so that next time we have an easier time fixing things.

[akoshchiy]

I've added some comments to the docs.

btw, I've checked behaviour on duckdb, and it looks more clearly - there is no prefixes at all. Maybe we can do the same?

cat nested_2.ndjson 
{"metadata": {"product": "Product Name 1", "price": 1}}
{"metadata": {"product": "Product Name 2", "price": 2}}
{"metadata": {"product": "Product Name 3", "price": 3}}
{"metadata": {"product": "Product Name 4", "price": 4}}
{"metadata": {"product": "Product Name 5", "price": 5}}
{"metadata": {"product": "Product Name 6", "price": 6}}
{"metadata": {"product": "Product Name 7", "price": 7}}
{"metadata": {"product": "Product Name 8", "price": 8}}
{"metadata": {"product": "Product Name 9", "price": 9}}


duckdb 
DuckDB v1.3.2 (Ossivalis) 0b83e5d2f6
Enter ".help" for usage hints.
Connected to a transient in-memory database.
Use ".open FILENAME" to reopen on a persistent database.
D SELECT unnest(metadata) FROM 'nested_2.ndjson';
┌────────────────┬───────┐
│    product     │ price │
│    varchar     │ int64 │
├────────────────┼───────┤
│ Product Name 1 │     1 │
│ Product Name 2 │     2 │
│ Product Name 3 │     3 │
│ Product Name 4 │     4 │
│ Product Name 5 │     5 │
│ Product Name 6 │     6 │
│ Product Name 7 │     7 │
│ Product Name 8 │     8 │
│ Product Name 9 │     9 │
└────────────────┴───────┘