Oleg Lebedev contributed to the slick/slick repository by implementing advanced UNION support across projections, enabling complex unions of different table projections while preserving field order and table identity. He addressed schema construction challenges by introducing AST manipulation techniques in Scala, ensuring that duplicate fields were removed during StructType construction and that multi-column order was maintained in aggregates using distinctOn. His work improved the reliability and expressiveness of SQL queries, enhanced test coverage, and reduced unexpected results in database abstractions. Through focused compiler development and rigorous testing, Oleg delivered robust solutions that deepened the system’s support for sophisticated query patterns.