How can I use sequences in SQLite?

Question

I'm writing a PHP-based web application that should work with multiple database systems. The most important are MySQL and SQLite but PostgreSQL and others would also be nice. For this I try to use as portable SQL as possible. Where this isn't possible, I have defined some meta words in my queries that are handled by my DB layer and converted to platform-specific SQL commands.

I'm now trying to add sequences support. Every DBMS handles sequences differently, there is no common way to write them in SQL. I have read and understood the way PostgreSQL does it. I have found an interesting solution for MySQL that uses MyISAM tables to escape the isolation constraints of a transaction. After all, sequences are not rolled back with the transaction they're used in and that's exactly what I want. Sequences are supposed to be multi-user safe.

Now I haven't found a solution for SQLite. It lacks built-in sequence support. It doesn't provide ways to store data outside a running transaction. My current implementation is to lock the table far enough to do a SELECT MAX(...) and use that value. But I want to get rid of that entirely. In SQLite, this approach requires locking the whole database!

Does anybody know a solution for this with SQLite?

Answer 1

Just create a regular counter table. On creation of a sequence foo, do

create table foo(value int);
insert into foo(value) values(0);

Then, when asking for the next counter value, do

update foo set value=value+1;

While this gets rolled back when the transaction is aborted, it is multi-user safe: no two users will commit the same number. sqlite implements concurrency with a database lock, so the second writer will block anyway (not just because of the sequence update, but also because of the other changes it wants to make).