MySQL选择最后一个ID与COUNT（*）

Question

SELECT id FROM store ORDER BY id DESC LIMIT 1
SELECT COUNT(*) FROM store

I have a table, ID is auto increment. I try to count the rows, since my id is auto increment from 1, so last id will be the same value of rows

I was wondering which query can have better performance SELECT last id or use COUNT

Answer 1

The first might perform better.

The second is accurate and you should use it. The id's are not guaranteed to have no holes. There will definitely be gaps if you have deletes.

Assuming id is a primary key, both can be satisfied using the index, which should be faster than reading the entire table.

Answer 2

If you're using MyISAM then go with COUNT for sure! That value is stored for each table anyway, so it doesn't even need to refer to the table content. But it's different for InnoDB. It has to actually count the rows. But still I would use COUNT. Not so often for InnoDB though...

Answer 3

I would go with count(*) since that's optimized in MySQL and it guarantees a correct answer. You can always delete rows so relying on the last id would not be a good idea.