Allow get_terms() to return slug&name fields

[ramiy]

When we use the ​get_terms() function, we can choose not to return the complete term objects but a smaller array with predefined fields (for performance).

it's done using fields argument:

$terms = get_terms( array(
	'taxonomy' => 'post_tag',
	'fields'   => 'all',
) );

Currently, the fields argument supports the following:

• all - returns an array of complete term objects. (DEFAULT)
• ids - returns an array of ids.
• names - returns an array of term names.
• count - returns the number of matching terms.
• id=>parent - returns an associative array with ids as keys, parent term IDs as values.
• id=>name - returns an associative array with ids as keys, term names as values.
• id=>slug - returns an associative array with ids as keys, term slugs as values.

The problem is that in a growing number of websites I need to return only term-slugs and term-names.

My patch adds another variation to the fields argument:

• slug=>name - returns an associative array with term slugs as keys, term names as values.

[boonebgorges]

Hi @ramiy - Thanks for the patch.

I'm against adding new values to fields. While there may be a narrow set of cases where selecting only slug and name will result in meaningful performance benefits, I think it's going to be exceedingly rare. In contrast, use of fields causes a lot of confusion with respect to caching: we currently keep separate copies of term queries in the cache for each potential value of fields. This pollutes the cache, by potentially multiplying the cache footprint of get_terms() by three or four times what it actually has to be.

A better strategy is for the main terms query to be for IDs only, and the term objects fetched individually from the 'terms' cache. This work was begun in #34239 for get_the_terms(), and the patches on #35381 will extend the strategy to get_terms().

If you're able to demonstrate that SELECT slug, name ... is more than a microoptimization over SELECT * in a real-world situation, we can reconsider the proposal.