[LRUG] Request for assistance with Rails performance oddity

[Andrew Stewart]

On 30 Oct 2010, at 10:07, John Winters wrote:
> Before loading events for a range of dates the code purges any existing
> events in that range.  It does this by the extraordinarily inefficient
> method of loading all events and destroying all those in the specified
> range.  (In my defence, there is a note just above that line of code
> saying it needs to be converted to a single SQL call.)
> 
> It seems that the remaining events (still 26858 of them) then hang
> around in memory and ActiveRecord takes a quick glance at each of them
> before creating each new event record.  Lots of quick glances add up to
> a load of CPU time.
> 
> By purging old events using a separate run of the script and then
> loading new events with a fresh run I can get back to the original fast
> speed.

If you use #delete_all then ActiveRecord will issue a SQL call without loading the individual records into memory.  If you use #destroy_all, ActiveRecord loads each one in turn into memory so it can fire before/after_destroy callbacks and cascade to associations.

Cheers,

Andy Stewart
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