Rails - Jul 2013 - Insights for SQL, cache, gem, regarding performance issues.

Hi there,
I would like to know some ideas from different people upon this performance 
issue I am going through.
I contribute to a gem named
impressionist<https://github.com/charlotte-ruby/impressionist/issues/94>,
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 in 
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 request_hash 
on that particular record saved.
That''s where it all started, It basically takes 13 seconds to do this
task
and then does something else, Obviously it is not suitable for production 
mode.
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 impression.
   - 
   
   I don''t know if this is viable, because Impressionist would have to
go
   through all the cached records and to see if there wasn''t any record
saved.
   
Thanks ever so much

Have fun, ABC( *Always Be Coding*)

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