Add 'post_date' as a possible orderby argument to get_terms()

[jchristopher]

There have been a couple of times where I needed to retrieve a list of taxonomy terms ordered by the post_date of their applicable posts. This patch considers post_date as a potential orderby argument value in get_terms().

[jchristopher]

Add orderby argument of post_date to get_terms()

[lpointet]

How does this query behaves when a same term is applied to multiple posts?
Because joining on posts may return multiple times the same terms for instance.

[jchristopher]

To prevent duplicate terms, SELECT DISTINCT when orderby = 'post_date'

[jchristopher]

Replying to lpointet:

How does this query behaves when a same term is applied to multiple posts?
Because joining on posts may return multiple times the same terms for instance.

You're absolutely right! 28439.2.patch ensures SELECT DISTINCT to avoid duplicate terms.

[jchristopher]

Here's an EXPLAIN from my test site which admittedly only has 4 Categories and 3 Posts:

EXPLAIN SELECT t.*, tt.* 
  FROM wpsvn_terms AS t 
INNER JOIN wpsvn_term_taxonomy AS tt 
  ON t.term_id = tt.term_id 
JOIN wpsvn_term_relationships AS tr 
  ON tr.term_taxonomy_id = tt.term_taxonomy_id 
JOIN wpsvn_posts AS p 
  ON p.ID = tr.object_id 
WHERE tt.taxonomy IN ('category') 
ORDER BY p.post_date DESC 

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	tt	ALL	PRIMARY,term_id_taxonomy,taxonomy	NULL	NULL	NULL	4	Using where; Using temporary; Using filesort
1	SIMPLE	tr	ref	PRIMARY,term_taxonomy_id	term_taxonomy_id	8	wpsvn.tt.term_taxonomy_id	1	Using index
1	SIMPLE	t	ALL	PRIMARY	NULL	NULL	NULL	4	Using where; Using join buffer
1	SIMPLE	p	eq_ref	PRIMARY	PRIMARY	8	wpsvn.tr.object_id	1

[boonebgorges]

See https://core.trac.wordpress.org/ticket/18106#comment:7. I'd make the same argument here: get_terms() seems like the wrong place for it, and it's hard to see how the abstraction would work in wp_get_object_terms().

[boonebgorges]

Gonna close this bad boy for the reasons outlined in #18106. You can finagle it by doing get_posts( array( 'tax_query' => ... ) ) to get the post order, then looping through results, grabbing term IDs (which will have been loaded into the cache, so no real overhead added), and then passing to get_terms( 'my_taxonomy', array( 'orderby' => 'include', 'include' => $term_ids ) ).