Introduce column cache per connection

[yahonda]

#1490 removed Oracle enhanced adapter own column cache feature
to use Rails db:schema:cache:dump. However, it caused slower performance
about metadata search, such as column information.

This commit restores Oracle enhanced adapter own column cache feature.
It used to be class level caching, it is connection level caching now
since it is much easier and cleaner than class level ones.

Related to #1720

[yahonda]

@koic I'd like you to review this pull request since this implementation just works, but does not look good.

https://github.com/yahonda/oracle-enhanced/blob/4a1ed1116b3027896c113531b9ce61764efdf891/lib/active_record/connection_adapters/oracle_enhanced/schema_statements.rb#L159-L170

        def columns(table_name)
          table_name = table_name.to_s
          if @columns_cache[table_name]
            @columns_cache[table_name]
          else
            @columns_cache[table_name] =
              column_definitions(table_name).map do |field|
              new_column_from_field(table_name, field)
            end
          end
          @columns_cache[table_name]
        end

Reasons:

• Copy and paste super method implementation https://github.com/rails/rails/blob/fee7fba8934ee6005b7e8eefc4baf7db48f03934/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L110-L116

• @columns_cache[table_name] appears many times.

Any feedbacks are welcomed.

[koic]

Sure thing. I will look it next week. Please wait a moment.

[koic]

How about using super to transfer to AR::ConnectionAdapters::SchemaStatements#columns(table_name) as follows?

         def columns(table_name)
           table_name = table_name.to_s
           if @columns_cache[table_name]
             @columns_cache[table_name]
           else
+            @columns_cache[table_name] = super(table_name)
-            @columns_cache[table_name] =
-              column_definitions(table_name).map do |field|
-              new_column_from_field(table_name, field)
-            end
           end
-          @columns_cache[table_name]
         end

There will be no duplication of implementation with super class.

[koic]

Copy and paste super method implementation

I wrote a comment on https://github.com/rsim/oracle-enhanced/pull/1744/files#r208188648.

@columns_cache[table_name] appears many times.

On the other hand. I don't have an idea to this dup codes...

[yahonda]

@koic Thanks for the review. Making use of super looks good. I have made a change as you suggested and squashed.

[koic]

I think that this explicit return value line is unnecessary. The return value is determined by L163 or L165.

[yahonda]

Right. Updated.

[koic]

Thanks for fixing pragmatic issue!