Two WooCommerce tables grow steadily and are almost never looked at. On an older shop they are frequently the largest tables in the database.
Sessions
WooCommerce creates a session for every visitor who interacts with the basket – including bots that add things to baskets. Sessions are supposed to be cleaned up by a scheduled job. If that job is not running, they accumulate indefinitely, and we have seen this table reach hundreds of thousands of rows.
Every one of those rows is read and written as customers shop, so a bloated session table slows the whole shop, and it hurts most at exactly the busiest moments.
Scheduled actions
The actionscheduler tables keep a record of every background job, including completed and failed ones. WooCommerce prunes these, but on a busy shop it can fall behind, and a shop with an old backlog can carry hundreds of thousands of finished actions it will never need again.
Checking
In phpMyAdmin, sort the tables by size. If wc_sessions or the actionscheduler tables are near the top, that is your answer.
Fixing it
First make sure the background queue is actually running – if it is not, cleaning up now only buys time. See Action Scheduler: why background jobs stop running.
Expired sessions and completed actions can then be cleared. WooCommerce has tools for both under its status and scheduled actions screens, which is safer than deleting rows by hand.
Always back up first
These are live tables in a shop that is taking money. Take a database backup before any cleanup, and do it at a quiet time.
