Dynamic and Self-Tuning
Merges are a way of self-tuning the performance of the system, and MarkLogic Server continuously assesses the state of each database to see if it would benefit from self-tuning through a merge. In most cases, the default merge settings and the dynamic nature of merges will keep the database tuned optimally at all times. Because merges can be resource intensive (both disk I/O and CPU), however, some DBAs might need to control when merges occur and/or when they do not occur. You can do that by setting your merge policy as appropriate for your environment, as described in Setting Merge Policy.
Dynamic and self-tuning, merges are a “good thing”; they not only reclaim disk space, but improve the query and search performance of the system. Databases are made up of one or more forests, and forests are made up of one or more stands. The more stands there are in a forest, the more time it takes to resolve a query. Merges reduce the number of stands in each forest in a database, thereby improving the time it takes to resolve queries.