perf(files): keep systemtag and filecache joins index-friendly

Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

Author: solracsf

lib/private/DB/QueryBuilder/Partitioned/JoinCondition.php

@@ -149,7 +149,7 @@ private static function clearConditionPart(string $part): string {
 		} elseif (str_starts_with($part, 'to_number(to_char(')) {
 			// oracle cast to int
 			$part = substr($part, strlen('to_number(to_char('), -2);
-		} elseif (str_starts_with($part, 'to_number(to_char(')) {
+		} elseif (str_starts_with($part, 'to_char(')) {
 			// oracle cast to string
 			$part = substr($part, strlen('to_char('), -1);
 		}

lib/private/Files/Cache/CacheQueryBuilder.php

@@ -33,8 +33,11 @@ public function selectTagUsage(): self {
 			->selectAlias($this->func()->count('filecache.fileid'), 'number_files')
 			->selectAlias($this->func()->max('filecache.fileid'), 'ref_file_id')
 			->from('filecache', 'filecache')
+			// Compare as strings (objectid = CAST(fileid AS CHAR)) so the systag_by_objectid
+			// index on the string column objectid stays usable; casting objectid to int (or
+			// the implicit string->number coercion against the bigint fileid) defeats it.
 			->leftJoin('filecache', 'systemtag_object_mapping', 'systemtagmap', $this->expr()->andX(
-				$this->expr()->eq('filecache.fileid', $this->expr()->castColumn('systemtagmap.objectid', IQueryBuilder::PARAM_INT)),
+				$this->expr()->eq('systemtagmap.objectid', $this->expr()->castColumn('filecache.fileid', IQueryBuilder::PARAM_STR)),
 				$this->expr()->eq('systemtagmap.objecttype', $this->createNamedParameter('files'))
 			))
 			->leftJoin('systemtagmap', 'systemtag', 'systemtag', $this->expr()->andX(

lib/private/Files/Cache/QuerySearchHelper.php

@@ -97,8 +97,13 @@ public function findUsedTagsInCaches(ISearchQuery $searchQuery, array $caches):
 	}
 
 	protected function equipQueryForSystemTags(CacheQueryBuilder $query, IUser $user): void {
+		// Compare the join as strings (objectid = CAST(fileid AS CHAR)) instead of
+		// casting objectid to an integer. objectid is a string column, so casting it
+		// (or letting MySQL implicitly coerce the string column to a number when comparing
+		// to the bigint fileid) makes the systag_by_objectid index unusable and turns this
+		// into a full/BNL join. Casting fileid instead keeps the index on objectid usable.
 		$query->leftJoin('file', 'systemtag_object_mapping', 'systemtagmap', $query->expr()->andX(
-			$query->expr()->eq('file.fileid', $query->expr()->castColumn('systemtagmap.objectid', IQueryBuilder::PARAM_INT)),
+			$query->expr()->eq('systemtagmap.objectid', $query->expr()->castColumn('file.fileid', IQueryBuilder::PARAM_STR)),
 			$query->expr()->eq('systemtagmap.objecttype', $query->createNamedParameter('files'))
 		));
 		$on = $query->expr()->andX($query->expr()->eq('systemtag.id', 'systemtagmap.systemtagid'));

tests/lib/DB/QueryBuilder/Partitioned/JoinConditionTest.php

@@ -72,4 +72,18 @@ public function testParseCastCondition(string $platform): void {
 		$this->assertEquals('f.fileid', $parsed->toColumn);
 		$this->assertEquals([], $parsed->fromConditions);
 	}
+
+	#[\PHPUnit\Framework\Attributes\DataProvider('platformProvider')]
+	public function testParseStringCastCondition(string $platform): void {
+		$query = $this->getBuilder($platform);
+
+		// Mirrors the systemtag<->filecache join: the string objectid column is compared
+		// to fileid cast to a string (so the index on objectid stays usable). The string
+		// cast must be stripped on every platform when parsing the join condition.
+		$condition = $query->expr()->eq('m.objectid', $query->expr()->castColumn('f.fileid', IQueryBuilder::PARAM_STR));
+		$parsed = JoinCondition::parse($condition, 'filecache', 'f', 'm');
+		$this->assertEquals('m.objectid', $parsed->fromColumn);
+		$this->assertEquals('f.fileid', $parsed->toColumn);
+		$this->assertEquals([], $parsed->fromConditions);
+	}
 }