We have a MariaDB database running WordPress 4.8 and found a lot of transient named records in the wp_options table. The table was cleaned up with a Plugin and reduced from ~800K records down to ~20K records. Still getting slow query entries regarding the table:
# User@Host: wmnfdb[wmnfdb] @ localhost []
# Thread_id: 950 Schema: wmnf_www QC_hit: No
# Query_time: 34.284704 Lock_time: 0.000068 Rows_sent: 1010 Rows_examined: 13711
SET timestamp=1510330639;
SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes';
Found another post to create an index and did:
ALTER TABLE wp_options ADD INDEX (`autoload`);
That was taking too long and taking website offline. I found a lot of 'Waiting for table metadata lock' in the processlist. After canceling the ALTER TABLE, got all running again still with high loads and entries of course in the slow query log. I also tried creating the index with the web server offline and a clean processlist. Should it take so long if I try to create again tonight?
If you are deleting most of a table, it is better to create a new table, copy the desired rows over, then rename. The unfortunate aspect is that any added/modified rows during the steps would not get reflected in the copied table. (A plus: You could have had the new index already in place.)
In this, I give multiple ways to do big deletes.
What is probably hanging your system:
A big DELETE stashes away all the old values in case of a rollback -- which killing the DELETE invoked! It might have been faster to let it finish.
ALTER TABLE .. ADD INDEX -- If you are using MySQL 5.5 or older, that must copy the entire table over. Even if you are using a newer version (that can do ALGORITHM=INPLACE) there is still a metadata lock. How often is wp_options touched? (Sounds like too many times.)
Bottom line: If you recover from your attempts, but the delete is still to be done, pick the best approach in my link. After that, adding an index to only 20K rows should take some time, but not a deadly long time. And consider upgrading to 5.6 or newer.
If you need further discussion, please provide SHOW CREATE TABLE wp_options.
But wait! If autoload is a simple yes/no 'flag', the index might not be used. That is, it may be a waste to add the index! (For low cardinality, it is faster to do a table scan than to bounce back and forth between the index BTree and the data BTree.) Please provide a link to that post; I want to spit at them.