WP 4.3.1 ACF Pro 5.3.1
Given a field group with two relationship fields: One field is limited to a custom post type with <1500 entries. The other field is limited to a custom post type with >7,000 entries. All other settings for these custom post types are the same.
The Search filter is enabled for both of them.
On the entry screen, the search field returns results for the smaller group, but for any attempt to search the field with the greater number of posts, I get “No matches found” as a result.
I’ve eliminated as many other plugins as feasible (I saw on another thread that relevansi might be an issue, but I don’t have that plugin). And, I’ve tried various iterations and tests, including a test field with a small post type and another large post type, and get the same results.
This is a problem inherent in WP and MySQL. When searching WP does a
LIKE on all the post titles and post content. It’s probably the search on the content that is really slowing things down. I did find this article on searching by title only http://stackoverflow.com/questions/9468804/make-wordpress-search-only-in-post-title