It is likely there are very few rows WHERE
AND ( wp_posts.post_date >= ‘2019-09-04 21:45:00’ )
there are likely many thousands of rows WHERE
AND ( wp_posts.post_date < ‘2019-09-04 21:45:00’ )
and a table scan is required. EXPLAIN SELECT SQL_NO_CACHE remainder of query for each and examine results carefully. Post the TEXT RESULTS of EACH EXPLAIN SELECT SQL_NO_CACHE remainder of your query for our assistance in understanding what is going on.
