I just noticed something very strange. I had a query that was running quite slow on my database (MariaDB 10.1).

When I copied the same exact database to a new database and ran the same query on that database, it took 10% of the time. Both databases are supposedly identical and on the same SSD drive array.

Note that I forced no caching, I made sure the original database is optimized repaired. What can be the possible causes for this huge discrepancy and how can they be verified and fixed?

EDIT: The databases are using the MyISAM engine.

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:

EDIT 2:

I have made a few more tests and was able to isolate the issue to one single table called products_description. When this table is copied from the original database to the copied database, the query always performs fast. However, if I perform any of these operations on this specific table, at any time: CHECK, OPTIMIZE, ANALYZE or REPAIR, the query suddenly slows down x10 times.

I am able to restore the performance only if I am then performing any of these operations:

1. DROP the table and copy it again from the old database.

2. ALTER any of the following of that table: Collation or CHECKSUM or DELAY_KEY_WRITE. Tested with the altered values and when reverting back to the old value, the performance remains fast.

Even then, if I perform any of the CHECK, OPTIMIZE, ANALYZE or REPAIR operations, the speed drops until I do either 1) or 2)

One more thing I tested: After I copied the table from the old db to the new, before performing any operation on it, I backed up the table's files (products_description.frm, products_description.MYD, 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.

EDIT 3:

This the SQL query that I am running and you should test with is:

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): CHECK, OPTIMIZE, ANALYZE or 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:

1. DROP the table and import it again.

2. ALTER any of the following of that table: Collation or CHECKSUM or 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 CHECK, OPTIMIZE, ANALYZE or 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.frm, products_description.MYD, 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.

This is the SQL query that I am running and you should test with:

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?