Most SQL based data sources such as SQL SERVER, Oracle, and Teradata that support SQL queries support query folding. The Power BI Incremental Refresh configuration window will notify you of a warning should it determine that query folding for the table cannot be achieved. If it cannot be achieved, the objective of incremental refresh is defeated. The mashup engine would then be required to retrieve all source rows, and then apply filters to determine incremental changes.
Generally, the following list describes transformations that can be query folded.
Renaming columns (SELECT column aliases).
Filtering rows, with static values or Power Query parameters (WHERE clause predicates).
Grouping and summarizing (GROUP BY clause).
Expanding record columns (source foreign key columns) to achieve a join of two source tables (JOIN clause).
Non-fuzzy merging of fold-able queries based on the same source (JOIN clause).
Appending fold-able queries based on the same source (UNION ALL operator).