A few days ago, I revised some innodb_stats_* parameters and ran mysqlcheck --analyze --all-databases on some Percona 5.5.38 servers. Now, whenever I execute any simple query in MySQL Workbench, it seems to execute a background query of SHOW INDEX ON t1; Needless to say, this caused some issues when it blocked updates to the table on production servers.
I have ascertained these things:
- it happens with any account with sufficient priviledges
- it doesn't happen from the command line
- it happens for other users
- it happens even after I upgrade to 6.2.4 and delete all the settings
- it DOESN'T happen against my Percona 5.6 database
- SHOW INDEX starts about 5 seconds AFTER my query
I haven't been able to find any explanation for this behavior on the net, so wanted to see if any experienced DBA's have run into this.
Current stats related settings:
- innodb_stats_auto_update 0
- innodb_stats_method nulls_equal
- innodb_stats_on_metadata ON
- innodb_stats_sample_pages 8
- innodb_stats_update_need_lock 1
- innodb_use_sys_stats_table OFF
Thanks for any pointers anyone can provide.
UPDATE After looking through the source code, it turns out that the SHOW INDEX happens for every query on a single table. It checks to see if the selected columns contain a primary key or other unique index, so it can decide whether you can edit the data.
The source code contains this comment:
// XXX this can be slow because of the I_S queries, depending on the server
I went to see why my INFORMATION_SCHEMA was slow, and found a helpful post from Percona
It turns out that
innodb_stats_on_metadata had gotten turned on during this process, which causes MySQL to recalculate the statistics every time INFORMATION_SCHEMA is accessed. So I just turned it back off.
What a nightmare.