[wp-trac] [WordPress Trac] #30262: Combine wp_terms and wp_term_taxonomy tables

[WordPress Trac]

#30262: Combine wp_terms and wp_term_taxonomy tables
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 Reporter:  boonebgorges  |      Owner:
     Type:  enhancement   |     Status:  new
 Priority:  low           |  Milestone:  Future Release
Component:  Taxonomy      |    Version:
 Severity:  normal        |   Keywords:
  Focuses:                |
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 The two database tables `wp_terms` and `wp_term_taxonomy` are designed to
 facilitate "shared terms": single "terms" that are shared by multiple
 "term-taxonomy" pairs. In 4.1 we've begun the process of phasing out
 shared terms (#5809, #21950), and at some point we'll be able to guarantee
 that items in `wp_terms` are in one-to-one correspondence with those in
 `wp_term_taxonomy` (#30261). Once this has happened, we have no more need
 for two tables. So let's get rid of one of them.

 The primary benefit of merging the two tables is that, over time, we can
 simplify the internals of much of our taxonomy API. No more JOINS will be
 necessary in `get_terms()`. There will be no need for
 `WP_Tax_Query::transform_query()`. And so forth. This will make the
 component easier to maintain, and will improve cachability and overall
 performance. A subsidiary benefit is that it makes the system a bit more
 sane for developers who are learning it for the first time.

 The main concern in the process of merging the two tables is backward
 compatibility. As [https://make.wordpress.org/core/2013/07/28/potential-
 roadmap-for-taxonomy-meta-and-post-relationships/ outlined by nacin in
 2013], we have a pretty good plan for how we can maintain close to 100%
 backward compat. Two parts:

 1. The table aliases `$wpdb->terms` and `$wpdb->term_taxonomy` should
 point to the same merged table, which will include all columns from both
 existng tables. This will provide support for anyone using these aliases
 to perform taxonomy queries.
 2. Create a [https://dev.mysql.com/doc/refman/5.0/en/views.html MySQL
 view] that replicates the deprecated table. This will provide support for
 anyone querying the table with syntax like `$wpdb->get_blog_prefix(
 $blog_id ) . 'terms'`.

 We can't do the migration for at least a version or two after #30261 is
 complete - we have to give time to all installations to get their shared
 terms cleared out before we can think about putting the tables together.

 A viable patch for this ticket will include: a schema change to introduce
 the view, the migration logic to move the old columns into the new
 combined table, and the necessary changes to `$wpdb`.

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Ticket URL: <https://core.trac.wordpress.org/ticket/30262>
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