Enable query cache on all connection pools

[eugeneius]

Followup to #26978.

Since the query cache no longer eagerly checks out a connection, we can enable it on all connection pools at the start of every request, and it will only take effect for requests that actually use those pools.

r? @matthewd

[eugeneius]

Fixes #17921.

[matthewd]

This is great, thank you! Sorry it fell off my list for so long.

[eugeneius]

No problem! Thanks for getting to it 😊

[rafaelfranca]

Don't we need to backport this?

[matthewd]

@rafaelfranca I don't think so; it's a new feature. We've never previously applied query caching to secondary connections.

[bogdan]

You can reduce the number of generated arrays by passing only the pools where caching was enabled into the complete hook:

    def self.run
      ActiveRecord::Base.connection_handler.connection_pool_list.select do |pool|
        unless pool.query_cache_enabled
           p.enable_query_cache!
        end
     end
    end

    def self.complete(pools)
      pools.each {|pool| pool.disable_query_cache! }
      ...
    end

[eugeneius]

You're right! That approach makes the code a bit clearer too.

As you already wrote the patch... do you want to open a pull request? 😄

[mainameiz]

Hello! I have a question about using query cache with slaves databases which are read-only.
I'm using active_record_slave to read some records from slave database. It uses special abstract model which connects to slave database using establish_connection and all reads go through this model by default.

My case (consider this happens inside http request/response cycle):

1. fetch some record - it will be read from slave and this query will be cached
2. delete that record - this query will go through AR::Base.connection_pool to master database and this will clear the cache for this connection pool but not for connection pool of the model for slave database.
3. fetch the same record from step №1 using the same query - this will return the record from cache, because all reads go to slave by default.

AR.uncached cannot be used because it works only for AR::Base.connection_pool.

As for me it will be nice to tell some connection pools not to use query cache.
I will be glad to hear your thoughts.

[eugeneius]

As of #35089, released in Rails 6.0, writes invalidate the query cache for all connections.

[mainameiz]

@eugeneius thanks a lot!