加入标签表 - 我应该加入PHP还是加入DB服务器？

Question

Using the Toxi solution , how should I select the tags for a certain "Bookmark" (to keep the Delicious theme):

I can either:

1) Join in a single query:

select bookmark.title, bookmark.url, 
(
  SELECT group_concat( tags.name ) as tagNames
  FROM taggings INNER JOIN tags
  ON taggings.tagId_fk=tags.tagId
  WHERE taggings.bookmarkId_fk = bookmarks.bookmarkId_fk
)
from bookmarks
where bookmarks.id=1 ;

^^ That gives
title    url               tagNames
A bkmrk  http://url.com    tag1,tag2,tag3

2) Use two queries : one to retrieve the bookmark id's to display, then another to retrieve the tags for those bookmarks. The results can then be merged in PHP.

So really this question is: In general efficiency/database load-wise is it better to do more joining in a single query or multiple queries?

How do you make that kind of decision? Or do you simply not think about it until load causes a problem?

Answer 1

Server side is more efficient.

In both cases, the server must read all of the tags.

If you bring them to PHP, then they must all travel over the wire and PHP has to fiddle with them.

If you do them on the server, the finished answer (smaller) comes over the wire ready for PHP to pass it up to the UI.