Misunderstanding of a date search in Oracle

Often in the data to retrieve the data occurred in the last month, so the use of the following statement select * from eventtable where eventdate> sysdate-30, when the amount of data is small can not see, if there is a large amount of data, you will find that the above statement is slow, but the eventdate Date field is also indexed. Why is it slow? It turns out that oracle constantly retrieves the ever-changing sysdate value while searching, instead of generating a conditional statement once we imagined and then searching. To speed up, We can first extract the date of the current day, convert it into a string, and then use the following statement for query. select * from eventtable where eventdate> to_date, yyyy-mm-dd ). The speed is actually dozens of times worse.