Insights for SQL, cache, gem, regarding performance issues


Hi there,
I would like to know some ideas from different people upon this
issue I am going through.
I contribute to a gem named
It basically saves an ‘impression’ of a end user.
Everything works fine, I’m refactoring the code right now so if you
checkout the gem, don’t be scared.
An user has posted a Performance Issue on github whereas he has 220.000
rows on their db(pg) and impressionist is working very slow.
The reason is, In a app controller one can specify how impressions are
going to be saved, so one could choose to only log unique impressions.
Fair enough, However impressionist has to look through all the records
the db in order to save an unique impression using the following query:

(13684.8ms) SELECT DISTINCT COUNT(DISTINCT “impressions”.“request_hash”)
FROM “impressions” WHERE “impressions”.“impressionable_id” = $1 AND
“impressions”.“impressionable_type” = $2 [[“impressionable_id”, 60],
[“impressionable_type”, “Artist”]]

As you can see it goes through the db searching for a distinct
on that particular record saved.
That’s where it all started, It basically takes 13 seconds to do this
and then does something else, Obviously it is not suitable for
The solution I’ve got in mind is:

Use Ruby Threads(ThreadsWait standard ruby lib, written by Keiju
ISHITSUKA) and Mutex to prevent deadlocks

Also use Memcache

Problems are:

Using cache I’d have to tell impressionist to do a in memory
search(cached results) and upon that it may or may not save an

I don’t know if this is viable, because Impressionist would have to
through all the cached records and to see if there wasn’t any record

Thanks ever so much

Have fun, ABC( Always Be Coding)