Hi @CamilaAraldi,
By definition in Jira, a JQL is “complex” when the scope of the JQL is not limited to a project or a set of projects or if uses a OR keyword without delimiting the search. Having OR without any parenthesis delimiting conditions or not searching among specific projects causes Jira to search in the entire site.
If you would like to share an example JQL that is causing issues we can point you in the right direction to make it “non-complex”, however those are the common scenarios that would cause them to search the entire site (thus making them complex).