《PostgreSQL自学的艺术》一书；我不明白横向连接是什么意思？

我猜在这个标签上很多人买了《PostgreSQL的艺术》一书。这本书的内容就是这个问题的背景。我自学这本书。我遇到了一些问题，所以我写了一封邮件给作者，也在这里问这个问题

第47页：

• 我完全不明白第27行

  limit:n

  是什么意思

• 我也不知道第34行

  ss（姓名、唱片ID、计数）对true的意义是什么
  
  

• 我有点明白了。但我仍然不确定是什么造成的 横向连接怎么办

  -- name: genre-top-n
   2 -- Get the N top tracks by genre
   3 select genre.name as genre,
   4 case when length(ss.name) > 15
   5 then substring(ss.name from 1 for 15) || '…'
   6 else ss.name
   7 end as track,
   8 artist.name as artist
   9 from genre
   10 left join lateral
   11 /*
   12 * the lateral left join implements a nested loop over
   13 * the genres and allows to fetch our Top-N tracks per
   Chapter 5 A Small Application | 47
   14 * genre, applying the order by desc limit n clause.
   15 *
   16 * here we choose to weight the tracks by how many
   17 * times they appear in a playlist, so we join against
   18 * the playlisttrack table and count appearances.
   19 */
   20 (
   21 select track.name, track.albumid, count(playlistid)
   22 from track
   23 left join playlisttrack using (trackid)
   24 where track.genreid = genre.genreid
   25 group by track.trackid
   26 order by count desc
   27 limit :n
   28 )
   29 /*
   30 * the join happens in the subquery's where clause, so
   31 * we don't need to add another one at the outer join
   32 * level, hence the "on true" spelling.
   33 */
   34 ss(name, albumid, count) on true
   35 join album using(albumid)
   36 join artist using(artistid)
   37 order by genre.name, ss.count desc;
  

请不要假设任何人手头有任何特定的书籍，并将您的问题保持独立，以便任何人回答。第34行是表格别名：