Re: MySQL query execution time differs between executions

[Rick James]

Please provide context for that EXPLAIN --
Show us the SELECT, and
SHOW CREATE TABLE for each table.

> then it deletes and updates in the original big table one by one accordingly

Look into multi-table DELETE and UPDATE. You can probably do the DELETE/UPDATE in a single SQL statement instead of many.

> after a DB restart, the same query takes 20 seconds or more

Possible cures:
* Don't restart the db.
* Make the query more efficient.