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

[solracsf]

Summary

The system tag search joins compared filecache.fileid (bigint) against systemtag_object_mapping.objectid (varchar). On MySQL/MariaDB this implicitly coerces the objectid string column to a number, which makes any index on objectid unusable and turns the join into a full/BNL scan of the mapping table.

Comparing as strings instead (objectid = CAST(fileid AS CHAR)) keeps the index usable. On a 100k-row mapping table the systemtagmap access dropped from ~50k examined rows per outer row to 1, with identical results (objecttype is filtered to 'files', where objectid is always the decimal fileid). Applied to both the search query and the tag-usage count query.

Also, strip a lone to_char() string cast when parsing partitioned join conditions (the previous branch duplicated the int-cast check and was dead code) so sharded setups extract the join columns correctly on Oracle.

Before, only the objecttype prefix of the primary key is usable, so each of the 1,111 matched files scans
the entire mapping table. After, the (objecttype, objectid) prefix gives a direct lookup.

Metrics

systemtag_object_mapping access (the row that changes):

	type	key	key_len	ref	est. rows	actual rows read / probe
before	ref	PRIMARY	258	const	50,432	100,000.0
after	ref	PRIMARY	516	const,func	1	0.1

Before, only the objecttype prefix of the primary key is usable, so each of the 1,111 matched files scans
the entire mapping table. After, the (objecttype, objectid) prefix gives a direct lookup.

End-to-end (1,111 files, warm cache, best of 3 runs, identical 1,111 rows returned both ways):

before	after
~18,000 ms	~3 ms

Synthetic dataset (100k tag mappings) on a warm buffer pool, absolute timings will differ in production,
but the access-type / rows-examined improvement (from full mapping-table scan per file, to single index lookup)
is the durable result.

############################################################
## EXPLAIN — file search joining system tags (1111 files)
############################################################

>>> BEFORE (fileid = objectid : bigint vs varchar):
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+-------+-------+------------------------------------+
| id   | select_type | table        | type  | possible_keys                                                                                                    | key          | key_len | ref   | rows  | Extra                              |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+-------+-------+------------------------------------+
|    1 | SIMPLE      | filecache    | range | fs_storage_path_hash,fs_storage_mimetype,fs_storage_mimepart,fs_storage_size,fs_name_hash,fs_storage_path_prefix | fs_name_hash | 1003    | NULL  | 1111  | Using index condition; Using where |
|    1 | SIMPLE      | systemtagmap | ref   | PRIMARY,systag_by_objectid,systag_objecttype                                                                     | PRIMARY      | 258     | const | 50432 | Using where; Using index           |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+-------+-------+------------------------------------+

>>> AFTER (objectid = CAST(fileid AS CHAR)):
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+------------+------+------------------------------------+
| id   | select_type | table        | type  | possible_keys                                                                                                    | key          | key_len | ref        | rows | Extra                              |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+------------+------+------------------------------------+
|    1 | SIMPLE      | filecache    | range | fs_storage_path_hash,fs_storage_mimetype,fs_storage_mimepart,fs_storage_size,fs_name_hash,fs_storage_path_prefix | fs_name_hash | 1003    | NULL       | 1111 | Using index condition; Using where |
|    1 | SIMPLE      | systemtagmap | ref   | PRIMARY,systag_by_objectid,systag_objecttype                                                                     | PRIMARY      | 516     | const,func | 1    | Using where; Using index           |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+------------+------+------------------------------------+

############################################################
## ANALYZE — ACTUAL rows read (r_rows) on the same search
############################################################

>>> BEFORE:
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+-------+-------+-----------+----------+------------+------------------------------------+
| id   | select_type | table        | type  | possible_keys                                                                                                    | key          | key_len | ref   | rows  | r_rows    | filtered | r_filtered | Extra                              |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+-------+-------+-----------+----------+------------+------------------------------------+
|    1 | SIMPLE      | filecache    | range | fs_storage_path_hash,fs_storage_mimetype,fs_storage_mimepart,fs_storage_size,fs_name_hash,fs_storage_path_prefix | fs_name_hash | 1003    | NULL  | 1111  | 1111.00   |    50.00 |     100.00 | Using index condition; Using where |
|    1 | SIMPLE      | systemtagmap | ref   | PRIMARY,systag_by_objectid,systag_objecttype                                                                     | PRIMARY      | 258     | const | 50432 | 100000.00 |   100.00 |       0.00 | Using where; Using index           |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+-------+-------+-----------+----------+------------+------------------------------------+

>>> AFTER:
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+------------+------+---------+----------+------------+------------------------------------+
| id   | select_type | table        | type  | possible_keys                                                                                                    | key          | key_len | ref        | rows | r_rows  | filtered | r_filtered | Extra                              |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+------------+------+---------+----------+------------+------------------------------------+
|    1 | SIMPLE      | filecache    | range | fs_storage_path_hash,fs_storage_mimetype,fs_storage_mimepart,fs_storage_size,fs_name_hash,fs_storage_path_prefix | fs_name_hash | 1003    | NULL       | 1111 | 1111.00 |    50.00 |     100.00 | Using index condition; Using where |
|    1 | SIMPLE      | systemtagmap | ref   | PRIMARY,systag_by_objectid,systag_objecttype                                                                     | PRIMARY      | 516     | const,func | 1    | 0.10    |   100.00 |     100.00 | Using where; Using index           |
+------+-------------+--------------+-------+------------------------------------------------------------------------------------------------------------------+--------------+---------+------------+------+---------+----------+------------+------------------------------------+

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI