What Happens During a Merge
A database consists of one or more forests, and each forest consists of one or more stands. Each stand consists of one or more fragments. When a document is updated, new versions of all of the fragments associated with the document update are created in a new stand. Any old versions of the fragment remain in the old stand with a system timestamp that lets MarkLogic Server know that they are old versions of the fragments. Similarly, when a document is deleted, its fragments remain in the old stand with a system timestamp that lets MarkLogic Server know that they are old versions of the fragments.
Merges occur to move any unchanged fragments from an old stand into a new stand, deleting any old versions of fragments (including deleted fragments), thereby freeing up disk space and compacting the usable fragments so they are all together on disk. Additionally, merges combine index data for all of the fragments in a stand, thereby optimizing the indexes. Merges are a normal part of database operation, and they ensure that the system continues to perform at its best as updates and deletes occur.
To summarize, as part of merging, the following occurs:
Multiple stands are combined into one for improved performance.
Disk space is reclaimed.
Indexes and lexicons are combined and re-optimized based on their new size.
The result is a database that is smaller and can resolve queries much faster than before the merge.