I am having a strange issue with a slow query under certain conditions:
I have made a few tests and was able to isolate the issue to one single table called
products_description (all the tables are MyISAM).
At first I noticed that when this table is new (ie. just imported), the query always performs fast (~0.3s).
However, if I perform any of these operations on this specific table, at any time (even immediately after importing it):
REPAIR, the query suddenly slows down x10 times (takes ~4.5s) and keeps staying always slow.
Note that I forced no-caching when running the query to ensure the results are correct.
I am able to restore the performance only if I am then performing any of these operations on that table:
DROP the table and import it again.
ALTER any of the following of that table:
DELAY_KEY_WRITE. It then runs fast with the altered value and when reverting back to the old value, the performance remains fast.
Even then, if I perform any of the
REPAIR operations on that table, the query speed drops until I do either 1) or 2)
One more thing I tested:
Before performing any operation on the table, I backed up the table's files (
products_description.MYI), ran the query, it ran fast. Then I performed
CHECK on the table, ran the query, speed was x10 times slower, I copied the backed up files and overwrote the 3 files, ran the query again, slow speed again.
I have compressed the database in a ~5mb zip file (~80mb unzipped). If anyone wants to test the database in your own environment, please let me know and I will send you a download link. I could reproduce this on several different servers, on both MariaDB 10.1+ and MySQL 5.6+.
This is the SQL query that I am running and you should test with:
SELECT DISTINCT pav.products_options_values_id, pav.products_options_values_name, pav.products_options_values_sort_order FROM products_stock ps, products_options_values pav, (SELECT DISTINCT pa.products_id FROM products_attributes pa, products_options_values pov, (SELECT p.products_id, p.products_image, p.products_subimage1, pd.products_name, p.products_quantity, p.products_model, p.products_ordered, p.products_price, p.products_date_added, p.products_weight, p.products_length, p.products_width, p.products_height, p.products_tax_class_id, p.products_status, IF(s.status, s.specials_new_products_price, NULL) AS specials_new_products_price, IF(s.status, s.specials_new_products_price, p.products_price) AS final_price, IF(p.clearance_price < p.products_cost * 2.25, p.clearance_price, p.products_cost * 2.25) AS sorting_price FROM products p LEFT JOIN specials s ON p.products_id = s.products_id LEFT JOIN products_description pd ON p.products_id = pd.products_id WHERE /*FASTIDS*/ p.products_status = '1' AND Date_sub('2016-04-19', INTERVAL 7000 day) <= p.products_date_added ) m WHERE m.products_id = pa.products_id AND pa.options_id = 1 AND pa.options_values_id = pov.products_options_values_id AND pov.language_id = '1') q WHERE q.products_id = ps.products_id AND ps.products_stock_attributes = Concat('1-', pav.products_options_values_id) AND ps.products_stock_quantity > 0 ORDER BY pav.products_options_values_sort_order ASC
Here are the EXPLAIN EXTENDED results. Seems like the optimizer is working differently on both tables, but that doesn't really explain why that happens, as the copied database is supposedly identical.
These are the screenshots of the profiling of the query on the slow and fast database:
What can be the possible causes for this huge discrepancy and how can they be verified and fixed?
P.S. I posted this question on stackexchange and was advised to ask the experts here.