Optout Merge

Overview

Preserve all columns by default when merging external datasets; only drop explicitly listed helper columns. Prefer opt-out over opt-in column lists to avoid accidental data loss.

Workflow

1. Normalize join keys and types on both sides.
2. Join without column selection on the right side.
3. Define exclude_cols for temporary/helper fields only, then drop.
4. If a canonical order is required, place canonical columns first and append the rest.

Patterns

Polars (LazyFrame or DataFrame)

joined = left.join(right, on=keys, how="left", suffix="_r")
if isinstance(joined, pl.LazyFrame):
    schema_names = joined.collect_schema().names()