[I] `convert_filters_to_predicate` negates a weakened AND, producing a predicate that drops matching rows [datafusion-iceberg]

["andygrove (via GitHub)" <gi...@apache.org>]

andygrove opened a new issue, #44:
URL: https://github.com/apache/datafusion-iceberg/issues/44

   ### Describe the bug
   
   When one side of an `AND` can't be converted, `to_iceberg_and_predicate` keeps only the other side ([expr_to_predicate.rs#L370-L382](https://github.com/apache/datafusion-iceberg/blob/a2bc9427d0659591b5f8122b90fec710fe2f5de6/crates/datafusion/src/physical_plan/expr_to_predicate.rs#L370-L382)). On its own that is fine for `Inexact` pushdown, because the result is weaker than the filter. `Expr::Not` then negates whatever its child produced ([expr_to_predicate.rs#L89-L105](https://github.com/apache/datafusion-iceberg/blob/a2bc9427d0659591b5f8122b90fec710fe2f5de6/crates/datafusion/src/physical_plan/expr_to_predicate.rs#L89-L105)). Negating a weaker predicate gives a stronger one, which rejects rows the original filter accepts.
   
   ### To Reproduce
   
   ```rust
   // schema: a INT, b STRING
   let expr = SessionContext::new().parse_sql_expr("NOT (a = 1 AND upper(b) = 'X')", &df_schema)?;
   let predicate = convert_filters_to_predicate(&[expr]);
   // Some(NOT (a = 1)): drops a = 1, b = 'y', which the filter keeps
   ```
   
   ### Expected behavior
   
   The result is `None`, or another predicate implied by the filter. One way is to track whether each converted sub-predicate is exact, and negate only exact ones.
   
   ### Additional context
   
   SQL doesn't trigger this today with the default optimizer. DataFusion's simplifier rewrites `NOT (A AND B)` to `NOT A OR NOT B` before pushdown, so nothing is pushed for this filter. `convert_filters_to_predicate` is public, though, so other callers can hit it, as can any plan whose filters reach the scan without simplification. `NOT (flag AND a = 1)` on a boolean column has the same problem.
   


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