Database cursors used in Tracker

A cursor on a query of a database is a finger pointing to the current row. Most databases do this without pulling the entire resultset into memory. It’s indeed much like a C/C++ pointer, except that a pointer can only point to memory in your process’ virtual memory. A cursor is a bit more abstract.

POSIX developers can compare a cursor with a pointer to a region in an mmap. For people who don’t know about mmap, mmap can be used to map a file into your process’ memory. You get a C/C++ pointer back, from which you can read the data as-if it’s in memory. With mmap, when you create a pagefault, the kernel will pull pages into your memory (from the file, or whichever resource is behind the mapping).

In Tracker all database operations used to be much like how using g_file_get_contents works: you read the entire thing into memory, and then you operate on that memory. Internally it used the database’s cursor API too, of course. The sqlite3_step is sqlite’s cursor API too.

First the database has filled up its pagecache with this data, then you copied it to your application’s memory, then you used it, then you freed it.

That’s kinda silly! Why not use it straight from the database’s caches instead? That’s what you use a DB cursor for.

The result is less copying of memory. This means less memory fragmentation and fewer memory operations to perform (which should result in a small performance improvement).

This effort is ongoing but a lot of Tracker’s internal loops over resultsets are now using a cursor instead of a in-memory result-set.