loop一般是用在电子音乐或者Hip-Hop这样的音乐里面的,往复循环的音色采样或者节奏型。比如你听一些说唱乐,尽管人声部分在不停的忙活,但是伴奏的鼓机的节奏或音色是不变的,这就是一段loop。here is an example:select * from xawhere a >= all(select e from xb)with the following plan:[-] select statement choose cost = 3802 (cost = 3802 cardinality = 95 bytes = 1140 ) [-] filter ( ) [ ] table access full sqlexps.xa analyzed (cost = 2 cardinality = 95 bytes = 1140 ) [ ] index full scan sqlexps.xb_inx analyzed (cost = 40 cardinality = 819 bytes = 5733 )the way oracle perform to search data is:performs a filter operation. it accepts row sources from xa full scan, eliminates rows from xb index full scan and returns the remaining rows from xa full scan to the user or application issuing the statement.nested loops is used to find matched rows from a table(normally full scan or index range scan) to index scan b table. there is no definite answer which is better, since normally it cannot replace each other unless you rewrite your sql in significant differently syntax.