Clearing history not reducing catalogue size or affecting speed of LR

[jjj]

I've been looking at ways of speeding up LR and thought to see if clearing 'History' helped.
Weirdly after clearing history from all my images the catalogue is still the same rather large 6.4GB. A restart of LR doesn't change anything either. LR didn't seem to take any time to do this job either which I thought was a bit odd, maybe because it didn't actually do anything.
So I have no history and no change in speed either. 
[I backed catalogue up first before testing BTW, so I can always restore history].

OSX 10.8.5 - OS is on an SSD and the LR catalogue is on a separate SSD all by itself in an attempt to speed things up
Interestingly I decided to clear all previews when starting with new SSD and very briefly LR was a bit snappier. Now I have a mere 4GB of previews and 2.3GB of smart previews and LR is back to Slowsville.

[john beardsworth]

Tried optimising the catalogue?

[jjj]

Do that regularly. Never makes any apparent difference John.
Ah! But I didn't after clearing history, so now trying that out.

[jjj]

Supposedly only a restart is needed after a history clearout, which is why I didn't think to try an optimisation.
But the fact that clearing history took zero time made me suspicious of what, if anything was going on.

[jjj]

Optimised and 2.0Gb lighter.
Been testing LR now and it certainly seems a lot snappier. Just like when I had zero previews, but hopefully this will be the fix I was looking for.  

BTW I use Snapshots and Virtual copies to record and edits that I think are worth keeping. So the loss of history is of no big deal to me.

[john beardsworth]

Optimisation should certainly reduce the db size as it's running a SQL Vacuum command. I usually find it speeds things up too.

Reopening should clear the space used by history, but in some bits of LR there's a delay. For example, if you delete previews through the UI, the actual files don't get trashed until about an hour later. It may be the same with history too. But that's a wild guess.

[jjj]

Optimising seemed to clear the deleted history straightaway. It had been 4 hours since I deleted history, LR was restarted several times and even computer had been rebooted since then to see if that had any effect.
But thanks for reminding me about optimising.