commit c3a5b6d031a3e7407c98d76ce772a7c24b7e26e6
Author: Atanas Georgiev <atanas@fmi.uni-sofia.bg>
Date:   Thu Sep 28 12:34:05 2023 +0300

    Alternative to UNION clause

diff --git a/category.php b/category.php
index 72538c8..8d51a36 100644
--- a/category.php
+++ b/category.php
@@ -3213,24 +3213,26 @@ class core_course_category implements renderable, cacheable_object, IteratorAggr
                 SELECT cc.". join(',cc.', $fields). ", $ctxselect
                   FROM {course_categories} cc
                   JOIN {context} ctx ON cc.id = ctx.instanceid AND ctx.contextlevel = :contextcoursecat1
-                  JOIN {role_assignments} ra ON ra.contextid = ctx.id
                  WHERE ctx.path LIKE :parentpath1
-                       AND ra.userid = :userid1
-            UNION
-                SELECT cc.". join(',cc.', $fields). ", $ctxselect
-                  FROM {course_categories} cc
-                  JOIN {context} ctx ON cc.id = ctx.instanceid AND ctx.contextlevel = :contextcoursecat2
-                  JOIN {role_capabilities} rc ON rc.contextid = ctx.id
-                  JOIN {role_assignments} rc_ra ON rc_ra.roleid = rc.roleid
-                  JOIN {context} rc_ra_ctx ON rc_ra_ctx.id = rc_ra.contextid
-                 WHERE ctx.path LIKE :parentpath2
-                       AND rc_ra.userid = :userid2
-                       AND (ctx.path = rc_ra_ctx.path OR ctx.path LIKE " . $DB->sql_concat("rc_ra_ctx.path", "'/%'") . ")
+					  AND (
+						(ctx.id IN (
+						  SELECT ra.contextid
+						  FROM {role_assignments} ra
+						  WHERE ra.userid = :userid1
+						))
+						OR
+						(ctx.id IN (
+						  SELECT rc.contextid
+						  FROM {role_capabilities} rc
+						  JOIN {role_assignments} rc_ra ON rc.roleid = rc_ra.roleid
+						  JOIN {context} rc_ra_ctx ON rc_ra_ctx.id = rc_ra.contextid
+						  WHERE rc_ra.userid = :userid2
+							AND (ctx.path = rc_ra_ctx.path OR ctx.path LIKE " . $DB->sql_concat("rc_ra_ctx.path", "'/%'") . ")
+						))
+					  )
         ", [
             'contextcoursecat1' => CONTEXT_COURSECAT,
-            'contextcoursecat2' => CONTEXT_COURSECAT,
             'parentpath1' => $parentcat->get_context()->path . '/%',
-            'parentpath2' => $parentcat->get_context()->path . '/%',
             'userid1' => $USER->id,
             'userid2' => $USER->id
         ]);