Add option to disable ranking of results entirely when using pg_search_scope.

[undergroundwebdesigns]

This PR adds the option to set ignoring = :rank, which will cause pg_search to skip ordering the search results entirely, and to remove all calls to ts_rank from the query.

This change allows for much faster queries where ranking isn't important, especially where the search terms may match thousands of documents.

[shan98]

+1

[jeffcarbs]

+1 :+1:

I'm currently searching on a large table where the user chooses their own order, so the rank doesn't matter. I'm getting around this issue by clobbering the order via relation.reorder(:my_order) but reading this PR made me realize the DB is still calculating the rank.

[tamaloa]

+1 We have been using this pull request in production for some time now and would really like to see it merged into master.

[iwz]

Hi, @nertzy!

Looks like this has already been merged. I think this can be closed.

[nertzy]

Hi, @iwz!

I looked closer and I don't think there is a way to completely turn off ordering. It may be possible to do something like this, using except:

MyModel.my_search("my query").except(:order)

But I haven't tried it.

I'd consider merging this pull request if it is rebased and conflicts are fixed. I'd also want to change the API from ignoring: :rank to ranked_by: nil or ranked: false or something similar, because we already use ignoring: to trigger processing of the search query and document. I don't want the same key to also affect scope options.

[futuretap]

Even though it was merged, it has been overwritten and the option is no longer available. I resorted to use reorder to modify the ordering after the fact.