AutoDJ: fix GUI freeze when updating duration for many selected tracks

[ronso0]

fixes #12520

• move the duration collection to TrackDAO
• query the database duration column instead of inspecting each Track object

---

• move the duration collection to PlaylistTableModel
  (could also be in any of the classes that is derived from but... YAGNI?)
• query the duration column of each row instead of inspecting each Track object

Now updating the info for ~16k tracks takes ~5ms here.
With the 2.4 code, updating the info for 3 tracks already takes ~40ms 😬

[ronso0]

@uklotzde Do you have time to take a look?

[ronso0]

Hmm, no, this should better be a helper in PlaylistTableModel

[ronso0]

Hmm, no, this should better be a helper in PlaylistTableModel

Yes, and that's also much faster: ~5ms vs. 28ms for 16k tracks (maybe because no QHash and no second iteration?)

edit
I was curious if with SQL but interestingly querying the db for the total duration of certain playlist positions is slower.
This takes ~20ms for 16k tracks (regardless the number of selected rows of course):

SELECT 
  SUM(library.duration) 
FROM PlaylistTracks 
LEFT JOIN library 
  ON PlaylistTracks.track_id = library.id 
  AND PlaylistTracks.playlist_id = playlistId
  AND PlaylistTracks.position IN (selectedPositions)

[ywwg]

can you add a short explanatory comment, like:
// Derive total duration from the display widget instead of the database to speed up calculation.

[ronso0]

I'll add that in DlgAutoDJ where it is called, no need to mention that here IMO

[daschuer]

Suggested change

	for (const auto index : indices) {
	for (const auto& index : indices) {

[daschuer]

just on missing &

[ronso0]

Done.

[daschuer]

LGTM, thank you

[ronso0]

Oh, I just pushed the changes suggested by @uklotzde (it was a comment not a formal change request).
4240e90?notification_referrer_id=NT_kwDOAFqMd7I5MDIwODc3MjE3OjU5MzQxOTk

The update is #12537