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"SHOW TABLES LIKE" slow
Posted by: Qiao Zhang
Date: December 13, 2022 10:46AM

We have a wordpress database with 494K tables.
After we upgraded database to MySQL 8, we encountered a performance issue.
"SHOW TABLES LIKE" interally query from information_schema.tables, and using the wrong index.
It should use the index on the TABLE_NAME, but actually it goes to TABLE_SCHEMA first.

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.30 |
+-----------+
1 row in set (0.00 sec)

mysql> SELECT TABLE_NAME AS name
-> FROM information_schema.tables
-> WHERE TABLE_NAME LIKE 'wp_28764_smartcrawl_redirects'
-> and TABLE_SCHEMA = 'wpress';
+-------------------------------+
| name |
+-------------------------------+
| wp_28764_smartcrawl_redirects |
+-------------------------------+
1 row in set (8.09 sec) <-- slow

mysql> SELECT TABLE_NAME AS name
-> FROM information_schema.tables
-> WHERE TABLE_NAME LIKE 'wp_28764_smartcrawl_redirects';
+-------------------------------+
| name |
+-------------------------------+
| wp_28764_smartcrawl_redirects |
+-------------------------------+
1 row in set (0.23 sec) <-- much faster without TABLE_SCHEMA

mysql> show tables;
......
| wp_wfStatus |
| wp_wfThrottleLog |
| wp_wfVulnScanners |
| wp_wiki_subscriptions |
+---------------------------------------------------+
494216 rows in set (8.64 sec) <-- almost the same elapse time of select from information_schema.tables

mysql> SELECT TABLE_NAME AS name
-> FROM information_schema.tables
-> WHERE TABLE_NAME LIKE 'wp_28764_smartcrawl_redirects'
-> and concat(TABLE_SCHEMA,'') = 'wpress';
+-------------------------------+
| name |
+-------------------------------+
| wp_28764_smartcrawl_redirects |
+-------------------------------+
1 row in set (0.21 sec) <-- disable index on TABLE_SCHEMA, it is fast.

mysql> explain
-> SELECT TABLE_NAME AS name
-> FROM information_schema.tables
-> WHERE TABLE_NAME LIKE 'wp_28764_smartcrawl_redirects'
-> and TABLE_SCHEMA = 'wpress';
+----+-------------+-------+------------+--------+--------------------+------------+---------+-------------------------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+--------------------+------------+---------+-------------------------+-------+----------+-------------+
| 1 | SIMPLE | cat | NULL | index | PRIMARY | name | 194 | NULL | 1 | 100.00 | Using index |
| 1 | SIMPLE | sch | NULL | eq_ref | PRIMARY,catalog_id | catalog_id | 202 | mysql.cat.id,const | 1 | 100.00 | Using index |
| 1 | SIMPLE | tbl | NULL | ref | schema_id | schema_id | 8 | mysql.sch.id | 56856 | 11.11 | Using where |
| 1 | SIMPLE | col | NULL | eq_ref | PRIMARY | PRIMARY | 8 | mysql.tbl.collation_id | 1 | 100.00 | Using index |
| 1 | SIMPLE | ts | NULL | eq_ref | PRIMARY | PRIMARY | 8 | mysql.tbl.tablespace_id | 1 | 100.00 | Using index |
| 1 | SIMPLE | stat | NULL | eq_ref | PRIMARY | PRIMARY | 388 | const,mysql.tbl.name | 1 | 100.00 | Using index |
+----+-------------+-------+------------+--------+--------------------+------------+---------+-------------------------+-------+----------+-------------+
6 rows in set, 1 warning (0.00 sec)

Any suggestion to fix this issue?
Is there anyting in MySQL 8 like Oracle SQL Profile to give hints to SQL?

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"SHOW TABLES LIKE" slow
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December 13, 2022 10:46AM


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