Do Small Code Changes Merge Faster? A Multi-Language Empirical Investigation

Abstract: Code velocity, or the speed with which code changes are integrated into a production environment, plays a crucial role in Continuous Integration and Continuous Deployment. Many studies report factors influencing code velocity. However, solutions to increase code velocity are unclear. Meanwhile, the industry continues to issue guidelines on "ideal" code change size, believing it increases code velocity despite lacking evidence validating the practice. Surprisingly, this fundamental question has not been studied to date. This study investigates the practicality of improving code velocity by optimizing pull request size and composition (ratio of insertions, deletions, and modifications). We start with a hypothesis that a moderate correlation exists between pull request size and time-to-merge. We selected 100 most popular, actively developed projects from 10 programming languages on GitHub. We analyzed our dataset of 845,316 pull requests by size, composition, and context to explore its relationship to time-to-merge - a proxy to measure code velocity. Our study shows that pull request size and composition do not relate to time-to-merge. Regardless of the contextual factors that can influence pull request size or composition (e.g., programming language), the observation holds. Pull request data from two other platforms: Gerrit and Phabricator (401,790 code reviews) confirms the lack of relationship. This negative result as in "... eliminate useless hypotheses ..." challenges a widespread belief by showing that small code changes do not merge faster to increase code velocity.