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Changes between Version 1 and Version 2 of Ticket #16910

-                      v1
+                      v2
 But, if you have many many countries (not a good example, I know), you waste a lot of time passing the IDs back and forth from PHP to SQL.
+Final query:
+{{{
+SELECT *
+FROM wp_posts
+WHERE post_type = 'city'
+AND post_parent IN (1, 2, 3, ...)
+}}}
 It would be a lot more scalable to put the first query into the second, directly, as a subquery.
 …
 ) );
 }}}
+Final query:
+{{{
+SELECT *
+FROM wp_posts
+WHERE post_type = 'city'
+AND post_parent IN (
+  SELECT ID
+  FROM wp_posts
+  WHERE post_type = 'country'
+  INNER JOIN wp_terms ...
+)
+}}}