Spark: Allow overriding row-level operation mode (CoW/MoR) via session config

[ykourdi]

Feature Request / Improvement

Currently the row-level operation mode for DELETE/UPDATE/MERGE (copy-on-write vs
merge-on-read) is resolved only from the write.delete.mode / write.update.mode /
write.merge.mode table properties. There's no way to pick the mode for a single Spark
session without mutating the table.

Motivation / use case

I run a periodic batch job that works off historical snapshots: it creates a branch from
each one and rewrites rows there with a single MERGE INTO statement per historical snapshot.
Merge-on-read is much cheaper for that work. In addition, I don't want to change how the main branch is written, other
processes keep writing to it and should stay copy-on-write.

The mode is a table property, so it's global. Flipping write.merge.mode to merge-on-read
for my job flips it for those main branch writers too, so I'd have to lock the table for the
duration to keep them off the transient mode. A session-scoped override only affects my
job's writes and leaves everything else on the table's copy-on-write default.

Proposal

Add a session property that takes precedence over the write.delete.mode /
write.update.mode / write.merge.mode table properties:

• spark.sql.iceberg.row-level-mode (copy-on-write | merge-on-read)

Precedence is session config > table property > default, so behavior is unchanged when it's
unset. This follows spark.sql.iceberg.distribution-mode, which already overrides the
per-command write.*.distribution-mode properties with one session key.

The value is resolved through the normal config layering rather than reading the session
conf directly: a SparkWriteConf.rowLevelOperationMode(Command) accessor goes through the
existing SparkConfParser chain, and SparkRowLevelOperationBuilder calls it instead of
reading table.properties().

Willingness to contribute

Yes and I want to add support for Spark trees (spark/v3.4, v3.5,
v4.0, v4.1) with tests + docs, and will open a PR if there's agreement on the approach.

Query engine

Spark

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time