In order to use the Spark extensions, you will need to load them and then add them to Spark.

From Maven Repository: _wait until we upload an initial version, until then see Development

Add:

--conf spark.sql.extensions=io.lakefs.iceberg.extension.LakeFSSparkSessionExtensions \
--packages io.lakefs:spark-extensions:<VERSION>

to your spark-* command-line, or add this package to your spark.jars.packages configuration.

Run sbt package, then add

--conf spark.sql.extensions=io.lakefs.iceberg.extension.LakeFSSparkSessionExtensions \
--jars ./target/scala-2.12/lakefs-spark-extensions_2.12-0.1.0-SNAPSHOT.jar`

refs_data_diff is a Spark SQL table-valued function. The expression

refs_data_diff(PREFIX, FROM_SCHEMA, TO_SCHEMA, TABLE)

yields a relation that compares the "from" table PREFIX.FROM_SCHEMA.TABLE with the "to" table PREFIX.TO_SCHEMA.TABLE. Elements of "to" but not "from" are added and appear with lakefs_change='+', elements of "from" but not "to" are deleted and appear with lakefs_change='-'.

For instance,

SELECT lakefs_change, Player, COUNT(*) FROM refs_data_diff('lakefs', 'main~', 'main', 'db.allstar_games')
GROUP BY lakefs_change, Player;

uses lakeFS Iceberg support to compute how many rows were changed for each player in the last commit.

Internally this relation is exactly a SELECT expression. For instance, you can set up a view with it:

CREATE TEMPORARY VIEW diff_allstar_games_main_last_commit AS
    refs_data_diff('lakefs', 'main~', 'main', 'db.allstar_games');