Re: [Tracker] support for other database backends?

[Jussi Laako <jussi laako linux intel com>]

Hi Martyn,

On 11.5.2012 17:18, Martyn Russell wrote:
> they're faster. Last time I checked (a while back), MySQL was faster
> than PostgreSQL for querying. Just wondering why you've thought of
> PosgreSQL over MySQL really.

PostgreSQL is the only one that has LISTEN/NOTIFY system (especially 
nice in 9.x series) with C function callbacks. Extremely useful for 
notifying about updates in multi-client environment.

> I wasn't aware of the D-Bus ordering issue? Though I believe the store
> does collapse duplicate events. It also has been designed to work on
> embedded systems with less resources, so we have various optimisations
> to avoid spamming clients with events which are useless.

It still involves single-threaded central daemon that copies everything 
around. So everything goes through double-copy. This also messes up 
kernel scheduler because it hides inter-process scheduling dependencies.

> Indeed. With the store, we really needed to be sure data was written in
> cases where you had instant power outage (e.g. you drop your phone down
> the toilet). So while caches are good, syncing is needed to some extent.

In most cases, like Nokia N9, you cannot have sudden power removal, 
except if the device gets completely broken and in that case it doesn't 
matter anymore anyway. However, excessive syncing causes kernel to 
trigger filesystem barriers which causes severe filesystem stalls. It 
also causes flash wear.

> PostgreSQL is nicer to work with than MySQL from what I recall. We would
> gain a lot from a bigger database, but likely loose a lot on embedded
> systems and the efficiencies we made there. That's partly why we wanted
> to do some testing on this some months ago, to figure if the switch was
> worth it.

I was running postgre server for long time on my Nokia N900 proto and 
nobody could notice any negative impact due to it. It can be tuned to be 
fairly light. And it already consumes less CPU time than SQLite in many 
real use cases.

I think it should implemented such way that the database backend could 
be build-time selected to be either sqlite or postgre. Not being 
exclusively either one.