The default Mastodon search only returns results for posts that have federated with your server and include a hashtag. Mastodon supports ElasticSearch/Full-text search. When enabled, users on a Mastodon server can search posts related to themselves. Examples:
- Posts they published
- Posts they favourited
- Posts where they were mentioned
- Their direct messages
This can be handy but requires a significant amount of extra server resources, so this feature is optional. On Masto.host ElasticSearch is available as a paid add-on.
Mastodon “does not allow searching for arbitrary strings in the entire database“. This means there is no option to search all posts on your server, whether you use the add-on or not.