OPEN_STRUCT: declared child defaultNullValue is honored only for keys absent from the whole segment

[Jackie-Jiang]

An OPEN_STRUCT child field spec can declare a custom defaultNullValue, but only one of the four per-key storage layouts honors it. With null handling disabled, the same query can therefore return different values for the same key depending only on which segment answers it.

Current behavior

A document that does not carry the key reads as:

Layout	Effective default	Resolved by
Key absent from the whole segment	the declared child spec's defaultNullValue	OpenStructDataSource#getValueFieldSpec
Materialized (dense) key	standard dimension default of the stored type	OpenStructColumnSplitter#writeDenseKeyColumn
Sparse key	standard dimension default of the stored type	SparseKeyDataSource
Consuming (mutable) key	standard dimension default of the stored type	MutableOpenStructIndex#allocateKeyColumn

So for a declared STRING child with "defaultNullValue": "N/A", a segment in which the key never occurs reads N/A for every row, while a segment in which the key occurs at least once reads null for every other row.

Desired

Pick one effective-default rule and apply it across all four layouts. The natural choice is the declared child field spec, which OpenStructDataSource#getValueFieldSpec already returns, since it is the only rule that reflects what was configured.

The sealed and consuming paths are coupled: MutableOpenStructIndex deliberately mirrors OpenStructColumnSplitter so that a document's resolved value does not change when a consuming segment is sealed. They have to move together.

MapFilterOperator#trySparseJsonIndex already refuses the sparse JSON fast path for the key's declared default, so it needs no change.

Code sites

• OpenStructColumnSplitter#writeDenseKeyColumn
• MutableOpenStructIndex#allocateKeyColumn
• SparseKeyDataSource

Each carries a TODO pointing at this issue.