MySQL query with 3 left joins unexpectedly slow despite indexes - mysql

Question

This query takes 6+ minutes to run:
SELECT null as primary_id, t211.appln_id as appln_id, t212.pat_publn_id, t212.citn_id, citn_origin, cited_pat_publn_id, cited_appln_id, pat_citn_seq_nr, relevant_claim,
t212.cited_npl_publn_id, npl_citn_seq_nr, citn_gener_auth, npl_biblio, npl_type, publn_nr_original, t215.citn_categ, t215.CITN_REPLENISHED, npl_author,
npl_title1, npl_title2, npl_editor, npl_volume, npl_issue, npl_publn_date, npl_publn_end_date, npl_publisher, npl_page_first, npl_page_last, npl_abstract_nr, npl_doi, npl_isbn, npl_issn, online_availability, online_classification, online_search_date
FROM
tls211_pat_publn as t211
LEFT JOIN tls212_citation as t212 on t211.pat_publn_id = t212.pat_publn_id
LEFT JOIN tls214_npl_publn as t214 on t214.npl_publn_id = t212.cited_npl_publn_id
LEFT JOIN tls215_citn_categ as t215 on t215.pat_publn_id = t212.pat_publn_id and t215.citn_id = t212.citn_id
where t211.pat_publn_id=777;
This query takes 0 seconds to run:
SELECT null as primary_id, t211.appln_id as appln_id, t212.pat_publn_id, t212.citn_id, citn_origin, cited_pat_publn_id, cited_appln_id, pat_citn_seq_nr, relevant_claim,
t212.cited_npl_publn_id, npl_citn_seq_nr, citn_gener_auth, npl_biblio, npl_type, publn_nr_original, t215.citn_categ, t215.CITN_REPLENISHED, npl_author,
npl_title1, npl_title2, npl_editor, npl_volume, npl_issue, npl_publn_date, npl_publn_end_date, npl_publisher, npl_page_first, npl_page_last, npl_abstract_nr, npl_doi, npl_isbn, npl_issn, online_availability, online_classification, online_search_date
FROM
tls211_pat_publn as t211
LEFT JOIN tls212_citation as t212 on t211.pat_publn_id = t212.pat_publn_id
LEFT JOIN tls214_npl_publn as t214 on t214.npl_publn_id = t212.cited_npl_publn_id
LEFT JOIN tls215_citn_categ as t215 on t215.pat_publn_id = t212.pat_publn_id
where t211.pat_publn_id=777;
The only difference is adding: and t215.citn_id = t212.citn_id at the end of the last LEFT JOIN.
Both t212 and t215 have indexes for citn_id.
I've run "explain" on this an absurd number of times. Here's the explain on the slow one:
+----+-------------+-------+------------+--------+-------------------------+-------------+---------+-------------------------------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+-------------------------+-------------+---------+-------------------------------------------+------+----------+-------------+
| 1 | SIMPLE | t211 | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | t212 | NULL | ref | PRIMARY,ppi | ppi | 4 | const | 7 | 100.00 | NULL |
| 1 | SIMPLE | t214 | NULL | eq_ref | PRIMARY | PRIMARY | 4 | patstat_2019_fall.t212.cited_npl_publn_id | 1 | 100.00 | NULL |
| 1 | SIMPLE | t215 | NULL | ref | PRIMARY,idx_citn_id,ppi | idx_citn_id | 2 | patstat_2019_fall.t212.citn_id | 11 | 100.00 | Using where |
+----+-------------+-------+------------+--------+-------------------------+-------------+---------+-------------------------------------------+------+----------+-------------+
4 rows in set, 1 warning (0.00 sec)
Here's the explain on the fast one:
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------------------------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------------------------------------+------+----------+-------+
| 1 | SIMPLE | t211 | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | t212 | NULL | ref | PRIMARY,ppi | ppi | 4 | const | 7 | 100.00 | NULL |
| 1 | SIMPLE | t214 | NULL | eq_ref | PRIMARY | PRIMARY | 4 | patstat_2019_fall.t212.cited_npl_publn_id | 1 | 100.00 | NULL |
| 1 | SIMPLE | t215 | NULL | ref | PRIMARY,ppi | ppi | 4 | patstat_2019_fall.t212.pat_publn_id | 47 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------------------------------------+------+----------+-------+
4 rows in set, 1 warning (0.02 sec)
I see that the slow one says, "Using where" in the Extra column. This sounds ominous, but says only 11 rows.
My slow query log says:
Query_time: 269.658475 Lock_time: 0.000000 Rows_sent: 9 Rows_examined: 113106063
That's a lot of rows examined... but I have failed to understand WHY.
I'm confident this is obvious to someone, but I'm at the end of my rope.
Help?!

Answer 1

Only one index can be used for each join. If you have separate indexes for pat_publn_id and citn_id, it has to pick one of them to use, and then scan for the other column.
Since you're joining on two columns, to get the most effective indexing you need a multi-column index containing both columns.
ALTER TABLE tls212_citation ADD INDEX (pat_publn_id, citn_id);
ALTER TABLE tls215_citn_categ ADD INDEX (pat_publn_id, citn_id);

Answer 2

Does citn_id has single row entry in both t212 & t215? Or t212 has single but t215 has multiple entry. The second one cause speed issue. Even query can stop working.
You need to look on this, whether both table has single entry.
If multiple entry then left join won't work. You need to format the SQL in other way.