根据CSV，每隔x分钟用PHP更新整个表格

Question

I have to update a big table (products) in a MySQL database, every 10 minutes with PHP. I have to run the PHP script with cron job, and I get the most up to date products from a CSV file. The table has currently ~18000 rows, and unfortunately I can not tell how much it will change in a 10 min period. The most important thing is of course I do not want the users to notice the update in the background.

These are my ideas and fears:

• Idea1: I know that there is a way to load a csv file into a table with MySQL, so maybe I can use a transaction to truncate the table, and import the CSV. But even if I use transactions, as long as the table is large, I'm afraid that there will be a little chance for some users to see the empty database.

• Idea2: I could compare the old and the new csv file with a library and only update/add/remove the changed rows. This way I think there it's not possible for a user to see an empty database, but I'm afraid this method will cost a lot of RAM and CPU, and I'm on a shared hosting.

So basically I would like to know which method is the most secure to update a table completely without the users noticing it.

Answer 1

Assuming InnoDB and default isolation level, you can start a transaction, delete all rows, insert your new rows, then commit. Before the commit completes, users will see the previous state.

While the transaction is open (after the deletes), updates will block, but SELECTs will not. Since it's a read only table for the user, it won't be an issue. They'll still be able to SELECT while the transaction is open.

You can learn the details by reading about MVCC . The gist of it is that any time someone performs a SELECT , MySQL uses the data in the database plus the rollback segment to fetch the previous state until the transaction is committed or rolled back.

From MySQL docs :

InnoDB uses the information in the rollback segment to perform the undo operations needed in a transaction rollback. It also uses the information to build earlier versions of a row for a consistent read.

Only after the commit completes will the users see the new data instead of the old data, and they won't see the new data until their current transaction is over.